We are diverted from true knowledge by many preconceptions which we have accumulated since birth. This is because we were born without speech, and we made various judgments about sensible things before our reason was fully developed. It seems that the only way we can free ourselves from these preconceptions is this: that just once in our lives, we should make a concerted effort to doubt every previous belief in which we find so much as the slightest hint of uncertainty.

2. That we should even regard beliefs which can be doubted as false.

It will even be useful to regard the beliefs we are going to put into doubt as false, so that we can discover all the more clearly what is most certain and readily knowable.

3. That, however, this process of doubt should not be extended to the conduct of life.

However, this process of doubt should be restricted to our considering what is true. For as far as the conduct of life is concerned, the moment for action would usually have passed long before we could resolve our doubts. We are often forced to opt for what is only probably right, and sometimes we even have to choose between two equally probable alternatives.

4. Why we can have doubts about sensible things.

So now let us embark on our enquiry into what is true (but only what is true). To begin with, it can be doubted whether any sensible or imaginable things exist. The first reason is that we sometimes notice that our senses deceive us, and it is wise never to put too much trust in what has let us down, even if on only one occasion. The second reason is that in our dreams we regularly seem to sense or imagine many things which are completely non-existent, and there are no obvious signs which would enable someone having such doubts to distinguish between sleeping and waking with any certainty.

5. Why we can even have doubts about mathematical proofs.

We shall even put into doubt everything else which we previously believed could not be more certain — even mathematical proofs, and the principles which up to now we considered self-evident. One reason is that we have sometimes observed people making mistakes in mathematics, and accepting things as absolutely certain and self-evident, which have seemed false to us. But the most important reason is that we have been told that there is an omnipotent God who created us. We do not know whether he might not perhaps have willed to make us of such a nature that we are always deceived, even over those matters which appear absolutely certain to us. This seems no less possible than that we are only sometimes deceived, as we have already observed to be the case. And if not to an omnipotent God, then let us imagine that we owe our existence to ourselves, or to some other being, since the less we attribute to the powerful author of our being, the more likely it is that we will be so imperfect that we are always deceived.

6. That our will is free to withhold assent on matters of doubt, and thus avoid error.

But whatever the ultimate source of our being, and however powerful or deceptive, we experience within ourselves a certain freedom, which enables us always to abstain from believing anything which is not obviously certain and established. Consequently we can avoid ever making any mistakes.

7. That we cannot doubt that we exist while we are doubting; and this is the first thing we know when philosophising in the right order.

So, if we reject everything we can doubt in any way, and even imagine it all to be false, we can readily suppose that there is no God, no sky, and no bodies — and even that we ourselves have no hands, no feet, and indeed no body at all. However, this does not allow us to suppose that we who are thinking such things are nothing, since it is a contradiction to believe that something which thinks does not exist at the very time when it is thinking. So the knowledge that I think therefore I am is the first and most certain of all items of knowledge which anyone will arrive at if they philosophise in the right order.

8. This enables us to recognise the distinction between soul and body, or between thinking thing and corporeal thing.

This is also the best approach for understanding the nature of mind, and its distinction from body. Let us introspect about who we are — we who are supposing that everything distinct from ourselves is illusory. It will be transparently obvious that our nature contains no extension, no shape, no motion, nor any such thing which could be ascribed to body. All we shall find is thought. Consequently, we know thought before, and more certainly than, we know any corporeal thing, since we have already perceived it, while still doubting about everything else.

9. What thought is.

By the word ‘thought’, I mean everything which happens in us while we are conscious, in so far as there is consciousness of it in us. So in this context, thinking includes sensing as well as understanding, willing, and imagining. If I say, ‘I see therefore I am,’ or ‘I walk therefore I am,’ and mean by that the seeing or walking which is performed by the body, the conclusion is not absolutely certain. After all, when I am asleep I can often think I am seeing or walking, but without opening my eyes or moving, — and perhaps even without my having any body at all. On the other hand, the conclusion is obviously certain if I mean the sensing itself, or the consciousness that I am seeing or walking, since the conclusion then refers to the mind. And it is only the mind which senses, or thinks about its seeing or walking.

10. That absolutely simple and self-evident things are made more obscure by logical definitions, and that they are not the sort of thing that can become known by academic study.

In this work, I shall not define the many other words I have already used, or which I shall use in what follows. In my opinion, their meanings are self-evident enough. I have often noticed that philosophers have made the mistake of trying to use logical definitions to explain things which are absolutely simple and self-evident, with the result that they have made them more obscure. So, I said above that the proposition ‘I think therefore I am’ is the first and most certain of all to be arrived at by anyone who philosophises in the right order. In saying this, I did not mean to deny that you already have to have concepts such as those of thought, existence, and certainty; and likewise that of what it is to be an impossibility, such as something which thinks without existing. However, I did not consider it appropriate to provide a list of them, since they are absolutely simple notions, which by themselves give no information about any particular existing thing.

11. The way in which our mind is known better than body.

We need to take note of the following if we are to know scientifically that the mind is known, not only before and more certainly than the body, but more evidently. It is absolutely obvious by the natural light that absence of affections or qualities is equivalent to absence of being.[n.1] Consequently, wherever we are aware of qualities, there must necessarily be a thing or substance which they belong to. And the more qualities we are aware of in the thing or substance, the more clearly we know it. But it is obvious that we know more things in our own minds than in anything else, because nothing gives us knowledge of anything outside the mind, without its leading us to a far more certain knowledge of our own minds. For example, if I judge that the earth exists from the fact that I can touch or see it, I ought to judge from the same fact, but with even more certainty, that my mind exists. For it could perhaps be the case that I judged that I was touching the earth, even though the earth did not exist at all; but it could not be the case that I came to that judgment, and that my mind which came to that judgment was non-existent. And the same goes for everything else.

12. Why it is not equally known to everyone.

The reason why those who have not philosophised in the right order think differently, is because they have never made a sharp enough distinction between mind and body. And even if they have believed that they themselves exist more certainly than anything else, they have failed to notice that, in this context, ‘they themselves’ should be taken as meaning just their minds. Instead, they meant just their bodies, which they saw with their eyes, and touched with their hands, and to which they wrongly attributed the power of sensing. And this distracted them from perceiving the nature of mind.

13. The sense in which knowledge of everything else depends on knowledge of God.

So far, the mind knows itself, but still doubts the existence of everything else. If it then searches everywhere in order to extend its knowledge further, it will first come across ideas of all sorts of things. These ideas cannot be deceptive, as long as the mind merely contemplates then, and does not either affirm or deny that there is anything similar to them outside the mind. It will also discover certain ‘common notions’, from which it can put together various demonstrations; and it will be absolutely convinced of the validity of these demonstrations as long as they are before the mind. So, for example, it has within itself ideas of numbers and shapes, and it also has among its common notions, the notion that if you add equals to equals, their sums will also be equal, and the such like. From these it is easily demonstrated that the angles of a triangle are equal to two right angles, etc. And so the mind is convinced that these and similar propositions are true, as long as it has before it the premises from which it deduced them. But it cannot attend to the premises all the time. So at a later time it may remember that it does not yet know scientifically whether or not it has been created with such a nature that it is mistaken even about the things which seem most obvious to it. If so, it will see that it is right to doubt such things, and that it cannot have any certain, scientific knowledge until it has identified the author of its being.

14. That the existence of God follows from the fact that necessary existence is contained in our concept of God.

Next, the mind considers the various ideas it has within itself; and one stands out far above the rest, namely that of a being which is totally intelligent, totally powerful, and totally perfect. It discerns that this idea includes existence — not merely possible and contingent existence (as in the ideas of all the other things of which it has a distinct conception), but unlimited necessary and eternal existence. The mind is absolutely compelled to conclude that a totally perfect being exists, solely from the fact that it perceives that necessary and eternal existence is contained in the idea of a totally perfect being. The conclusion follows in exactly the same way as, for example, the mind is absolutely convinced that a triangle has three angles equal to two right angles, solely from the fact that the idea of a triangle necessarily includes its three angles being equal to two right angles.

15. That necessary existence is not included in the concepts of other things in this way, but only contingent existence.

The mind will find this more credible if it considers that none of its ideas of anything else can be perceived to contain necessary existence in this way. And it will conclude from this that the idea of a totally perfect being has not been made up by the mind itself. It does not represent some chimerical nature, but one which is true and immutable, and which cannot fail to exist, since necessary existence is contained in it.

16. That the reason why not everyone clearly recognises the necessity of God’s existence is because their preconceptions get in the way.

As I say, our mind will find this easy to believe if it has first entirely purged itself of preconceptions. In the case of all other things, we are used to distinguishing existence from essence, and we even make up at will various ideas of things which do not exist, and never have existed. Consequently, it can easily happen that, if we contemplate the totally perfect being without complete concentration, we might wonder whether perhaps the idea of it is one of those which we made up at will, or at least one of those in which existence is separate from essence.

17. That the greater the objective perfection of any of our ideas, the greater must be its cause.

Considering more deeply the ideas we have in us, we shall see that there is not much difference between them in so far as they are just various modes of thought. But they are utterly different in so far as they represent different things; and the more objective perfection they contain, the more perfect their cause must be. If someone has in themselves the idea of some extremely complicated machine, it can reasonably be asked what cause they got it from. Did they at some time see such a machine which had been made by someone else? Or have they studied engineering so meticulously, or are they just so naturally clever, that they were able to think it up by themselves, without ever having seen one anywhere? All the complexity which is contained in the idea only objectively, or, as it were, in a mental picture, must exist in its cause. And whatever sort of cause it turns out to be, it must contain the complexity, not merely objectively or representatively (at least in the case of the first and supreme cause), but in actual fact, whether formally or eminently.

18. That this provides an additional proof of the existence of God.

So, since we have within ourselves the idea of God or a supreme being, we are justified in examining what cause we have got the idea from. We find so much immensity in it, that because of this we are absolutely certain that it can only have been implanted in us by a thing which actually contains the totality of all perfections, that is, only by a really existing God. For it is as obvious as could be by the natural light, not only that nothing comes from nothing; but also that something more perfect cannot be produced by something less perfect as its total and efficient cause; and also that there cannot be in us an idea or mental picture of anything without the existence of some archetype, whether in ourselves or outside us, which actually contains all its perfections. But in no way do we find in ourselves the supreme perfections which we have the idea of. Therefore we rightly conclude from this very fact, that they are in something which is distinct from ourselves, namely in God — or at least that they were once in God, from which it most certainly follows that they still are in God.

19. That even though we cannot comprehend the nature of God, we know his perfections more clearly than anything else.

This is certain and obvious enough to anyone who is used to contemplating the idea of God, and attending to his supreme perfections. We cannot comprehend them, since it is naturally impossible for finite beings such as ourselves to comprehend the infinite. Nevertheless, we can understand them more clearly and distinctly than any corporeal things, because they occupy our thinking more; they are simpler; and they are not made obscure by any limitations.

20. That God exists because we were created by God, and not by ourselves.

However, not everyone is aware of this. Moreover, whereas people who have the idea of a complex machine usually know where they got the idea from, we do not remember the idea of God having come from God at any time, because we have always had it. So we still need to investigate where we ourselves come from — we who have within ourselves the idea of the supreme perfections of God. Now it is certainly as obvious as could be by the natural light, that a thing which knows something more perfect than itself is not the cause of its own existence, since it would have given itself all the perfections which it has an idea of within itself. So it must have come from something else which has all those perfections within itself, namely God.

21. That the continuation of our existence is sufficient to prove the existence of God.

Nothing can obscure the obviousness of this demonstration, provided we concentrate on the nature of time, or the continued existence of things. Time is such that its parts do not depend on each other, and never co-exist. So from the fact that we exist now, it does not follow that we will still exist at the immediately succeeding instant, unless some cause (namely the same cause as first created us) continuously re-creates us, as it were, or conserves us. For we can easily see that there is in us no power of conserving ourselves. And he who has enough power to conserve us as beings distinct from himself, is all the more able to conserve himself as well (or rather, he does not need any conserving) — and, finally, that being is God.

22. That because of the way we know God’s existence, we simultaneously know such of his attributes as are knowable by virtue of our natural intelligence.

This way of proving God’s existence (that is, through the idea we have of him) has the great advantage that we simultaneously recognise what he is like, as far as the feebleness of our nature allows. In other words, if we inspect the innate idea we have of him, we see that he is eternal, omniscient, omnipotent, the fount of all goodness and truth, the creator of all things, and, in short, that he has within himself everything in which we can clearly perceive some infinite perfection — that is, a perfection which is not limited by any imperfection.

23. That God is not corporeal; nor does he have sense perceptions like ours; nor does he will the wickedness of sin.

Obviously there are many things which we recognise as containing some perfection, yet we also detect in them some imperfection or limitation. Consequently they cannot belong to God. So it is certain that God is not body, because the nature of body includes divisibility as well as extension in space, and it is an imperfection to be divisible. Again, although in us our ability to sense is a sort of perfection, yet all sensation involves an element of passivity, and passivity means dependence on something else. Consequently, it must not be thought that God has any form of sensation, but only that he understands and wills. Moreover, even his understanding and will are not the same as in us. In us, they are to a certain extent distinct mental acts, whereas God understands, wills, and does everything all at once through a single action which is always the same and utterly simple. And when I say ‘everything’, I mean all things. He does not will the wickedness of sin, since it is not a thing.

24. That to move from knowledge of God to knowledge of the created world, it must be remembered that he is infinite, and we are finite.

We have so far established that God alone is the true cause of everything that exists, or that could possibly exist. Now it is obvious that the best way of proceeding in our philosophising is to try and deduce from our knowledge of God himself an account of the things created by him. This way we shall acquire the most complete scientific knowledge, which consists in deriving knowledge of effects from knowledge of causes. But in order to set out in reasonable safety and without danger of error, we must be careful always to have at the forefront of our minds that God the author of things is infinite, whereas we are finite in every respect.

25. That whatever has been revealed by God must be believed, even if it transcends our understanding.

So if it happens that God reveals to us something about himself or other things, which transcends the natural capacity of our wits (as he has already revealed the mysteries of the incarnation and the trinity), we shall not refuse to believe them, even though we do not understand them clearly. There is no way we should be surprised that there are many things which are beyond our grasp, whether in his unfathomable nature, or even in the things he has created.

26. That we should never quibble about the nature of infinity. Instead we should regard things in which we observe no limits as indefinite — for example the extension of the world, the divisibility of the parts of matter, the number of the stars, etc.

So we should never exhaust ourselves with arguing about the nature of infinity. Since we are finite, it would obviously be absurd for us to determine anything about it, and so to try and set limits to it, as it were, and comprehend it. Therefore we shall not bother to give any answer to people who ask whether half an infinite line is also infinite, or whether an infinite number is odd or even, and the such like. It seems that only those who believe their minds are infinite should think about these things. In every case where we can find no limit to some aspect of a thing, we shall not assert that it is infinite, but we shall regard it as indefinite. For example, we cannot imagine an extension so large that we cannot understand the possibility of an even larger one. So we shall say that the size of possible things is indefinite. Again, a body cannot be divided into so many parts, that we cannot understand that each of these parts is divisible still further. So we shall make it our opinion that quantity is indefinitely divisible. Again, we cannot imagine that the number of stars is so great, that we could not believe that God would have been able to create still more. So we shall suppose that their number is also indefinite. And similarly with everything else.

27. The difference between the indefinite and the infinite.

There are two reasons for calling these things ‘indefinite’ rather than ‘infinite’. The first is in order to reserve the word ‘infinite’ for God alone, because in him alone and in every respect, we do not merely fail to recognise any limits, but we also understand positively that there are none. The second is that, in the case of everything else, we do not have the same sort of positive understanding of their lacking limits in some respect. We merely, in a negative way, admit that we cannot discover their limits — if they have any.

28. That we should not consider the final causes of created things, but only their efficient causes.

In short, when we are investigating things in the natural world, we should never draw our explanations from the purposes which God or nature had in creating them. We should not be so presumptuous as to think that we are privy to his plans. Instead, we should treat him as the efficient cause of everything. He has endowed us with the natural light, which shows us what conclusions we should draw. Treating him this way, we shall see what we should conclude from those of his attributes which he has willed us to have some knowledge of, to such effects he has brought about which are revealed to our senses. However, we should always bear in mind (as I have already said), that we should trust in this natural light only as long as the contrary has not been revealed by God himself.

29. That God is not the cause of errors.

The first attribute of God which comes under consideration here is that he is totally truthful and the source of all illumination. So much so, that it would obviously be a contradiction in terms if he deceived us. To put it another way, we know from experience that we are liable to make mistakes; but the positive cause of these mistakes cannot lie within God himself. Among us humans, the ability to deceive might seem to be a significant indication of intelligence. But deliberate deception can only arise from malice or fear and weakness — so it cannot be attributed to God.

30. That it follows from this that everything which we conceive clearly is true, and that our earlier doubts are resolved.

From this it follows that the light of nature (the faculty of knowing, which God has given us) can never illuminate any object which is not true, in so far as the object is illuminated by it — that is, in so far as it is conceived clearly and distinctly. For he would rightly be described as a deceiver if he had given it to us turned upside down, so that it took the false for the true. This removes the main doubt, which we found in the fact that we did not know whether we might not be of such a nature that we were mistaken even about things which seemed most obviously true. The same principle also easily disposes of all the other reasons for doubt which I raised earlier. Mathematical truths should no longer be under suspicion, since they are utterly transparent. And if we concentrate on what is clear and distinct in our sensations (whether we are awake or asleep), and distinguish it from what is confused and obscure, we shall easily recognise what is to be held as true in any thing. There is no need to expand on these matters here, since I have already dealt with them as best I could in my Metaphysical Meditations, and a more detailed account would presuppose knowledge of what I am going to say later.

31. That our errors are negations if attributed to God, and defects if attributed to us.

Despite the fact that God is not a deceiver, it nevertheless often happens that we make mistakes. So we need to investigate the source and cause of our errors, in order to learn how to avoid them. It is to be observed that they do not depend on the intellect so much as on the will, and that they are not things, [n.2] which require the substantive co-operation of God to bring them into being. In so far as they are attributable to him, they are mere negations; and in so far as they are attributable to us, they are defects. [n.3]

32. That there are in us only two modes of thinking, namely intellectual perception and the exercise of the will.

Indeed, all the modes of thinking which we experience in ourselves can be reduced to two kinds. One is perception, or the exercise of the intellect; and the other is volition, or the exercise of the will. For sensing, imagining, and pure understanding are just different modes of perceiving; and wanting, disliking, affirming, denying, and doubting are different modes of willing. [n.4]

33. That we err only when we make a judgment about something which we have not perceived adequately.

When we perceive something without affirming or denying anything at all about it, it is obvious that we cannot be in error. Nor can we be in error if we confine ourselves to affirming or denying only what we clearly and distinctly perceive as to be affirmed or denied. We only lapse into error when (as happens) we make a judgment about something without perceiving it correctly.

34. That judgment requires the will as well as the intellect.

Judgment certainly involves the understanding, because we cannot make any judgment about something unless we perceive at least some aspect of it. But given that some aspect of a thing is perceived, the will is also required, so as to provide assent to it. However, in order to make a judgment (or at least some sort of a judgment), it is not necessary for us to have a complete perception of a thing in all its aspects. For we can assent to many things which we know only very obscurely and confusedly.

35. That the will has a wider scope than the intellect, and that this is the source of our errors.

Intellectual perception extends only to the few things that are revealed to it, and it is always strictly finite. But the will can be described as infinite in a certain sense, since we have never been aware of anything which can be the object of some other will (even of the fathomless will which is in God), but which cannot also be the object of our own will. So it is easy for us to extend our will beyond what we perceive clearly; and when we do this, it is hardly surprising if we happen to fall into error.

36. That it is impossible to attribute our errors to God.

In no way can we imagine God to be the source of our errors, on the grounds that he failed to provide us with an omniscient understanding. A created understanding is by its very nature finite; and a finite understanding by its very nature does not extend to everything.

37. That the greatest human perfection is the freedom of the will, and that this makes humans worthy of praise or blame.

On the other hand, it belongs to the very nature of the will that it should have the widest possible scope. It is one of the main perfections in humans that they act through their will — that is, freely. In their own special way, they are the originators of their own actions, and deserve praise for them. We do not praise robots for precisely executing all the motions they were constructed for, since they execute them necessarily. Rather, we praise the technicians for constructing them so precisely, because they constructed them freely, not necessarily. For the same reason, we obviously deserve more credit for embracing the truth (when we do embrace it) through an act of will, than we would if we could not help embracing it.

38. That error is a defect in our behaviour, not in our nature; and that although superiors can often be blamed for the faults of their subordinates, God can never be blamed for our errors.

The fact that we lapse into error is a defect in our behaviour, or in our exercising of our freedom; and it is not a defect in our nature. The reason is because our nature is the same when we make incorrect judgments as when we make correct ones. And although God could have made our understanding so penetrating that we never made mistakes, we have no right to demand this of him. It might be argued that, since we call a person the cause of some evil if they have the authority to prevent it, but fail to do so; similarly, God should be considered the cause of our errors, because he could have brought it about that we never made mistakes. But this would be wrong, since the authority which we humans have over one another was given to us so that we could use it for saving others from evil; whereas the authority which God has over us all is completely absolute and free. Consequently, we owe him our utmost gratitude for the good things he has bestowed on us; but we can have no right to complain about his not having bestowed on us everything we know he could have bestowed on us.

39. That the freedom of the will is self-evident.

That there is freedom in our will, and that there are many things which we can choose either to believe or not to believe, is so evident that it must be numbered among the primary and absolutely common notions which are innate to us. This was made especially obvious above, when we tried to doubt everything, and got as far as imagining that some very powerful author of our being was trying to deceive us in every way. Even so, we were still aware that we had this freedom to withhold our belief in anything which was not obviously certain and established. And nothing can be more self-evident and transparently obvious than whatever still seemed indubitable at that stage in the argument.

40. That it is also certain that everything has been predetermined by God.

Now that we acknowledge the existence of God, we can see that his power is so immense, that we consider it blasphemous to believe that we could ever do anything without its already having been preordained by him. But we can easily involve ourselves in major difficulties if we try to bring this predetermination together [n.5] with the freedom of our will, and to understand both of them simultaneously.

41. How the freedom of our will and the predetermination of God are to be brought together at the same time.

We shall get ourselves out of these difficulties if we remember that our mind is finite, whereas God has infinite power, by which he not merely foresaw from eternity, but also willed and predetermined everything that exists or could possibly exist. So we have made sufficient contact with this power to perceive clearly and distinctly that it is in God; but we have not grasped it fully enough to see how it leaves free human actions undetermined. On the other hand, we are so aware of the freedom and indifference [n.6] which is within us, that there is nothing we can grasp more evidently or more completely. It would be absurd to doubt something which we are intimately familiar with and which we experience within ourselves, simply because there is something else which we do not understand, and which we know from its very nature must be incomprehensible to us.

42. How our errors are due to our will, even though we do not wish to err.

Now that we know that all our errors depend on our will, it might seem strange that we ever lapse into error, since no-one wishes to be wrong. But wishing to be wrong is utterly different from being willing to assent to things in which there happens to be an error. In fact no-one consciously wishes to be wrong; yet there is hardly anyone who does not often willingly assent to things which contain an error they are unaware of. Besides, the very desire to pursue the truth frequently means that people who do not have a correct and scientific knowledge of the method by which it should be pursued, come to a judgment about things they do not perceive, and therefore lapse into error.

43. That we never err as long as we assent only to what is clearly and distinctly perceived.

However, it is certain that we shall never take anything false to be true, as long as we give assent only to what we perceive clearly and distinctly. I say ‘certain’, because God is not a deceiver, and the faculty of perceiving which he has given us cannot lead to falsehood. Nor can the faculty of assenting, as long as it restricts itself to what is perceived clearly. And even if this were not proved by reasoning, nature has impressed it on everyone’s minds, so that whenever we perceive something clearly, we spontaneously assent to it, and cannot doubt its truth in any respect.

44. That we always misjudge when we assent to things which we have not perceived clearly, even if we chance on the truth; and that this happens because we suppose that we had previously scrutinised them thoroughly enough.

It is also certain that, when we assent to some object of reason which we do not perceive, then either we are wrong, or we happen on the truth only by chance, since we cannot prove that we are not mistaken. Of course, it rarely happens that we assent to things which we know we have not perceived, since the light of nature tells us that we should only make judgments about things we know. But we very often go wrong here, since there are many things which we think we once perceived, but which we never in fact perceived; and we assent to them as having been perceived absolutely, even though they exist only in our memory.

45. What is a clear perception, and what is a distinct one.

Throughout their whole lives, very many people never perceive anything correctly enough to make a sure judgment about it. For if a perception is to support a sure and indubitable judgment, it must not only be clear, but also distinct. I call an idea ‘clear’ when it is present and fully revealed to the mind attending to it, just as we say we see something clearly when it is present to the observing eye, and affects it strongly and fully enough. I call an idea ‘distinct’ when, as well as being clear, it is so separated and demarcated from all other ideas, that it contains in itself absolutely nothing which is not clear.

46. Using the example of pain, it is shown that a perception can be clear without being distinct, but not distinct without being clear.

While someone is suffering from great pain, there is in them a very clear perception of pain. However, it is not always distinct, because people commonly think that there is something in the painful part which is similar to the sensation of pain; and they confuse their obscure judgment about the nature of the former with the sensation, which is all that they perceive clearly. This is how a perception can be clear without being distinct, although it can never be distinct without being clear.

47. That in order to correct the preconceptions of childhood, we must consider the simple notions, and what is clear in each.

In childhood, the mind is so immersed in the body that, although it perceives many things clearly, it never perceives anything distinctly. All the same, it makes judgments about many things, and thus supplies us with a large number of preconceptions, which most people never subsequently abandon. So that we can free ourselves of them, I shall here give a summary list of all the simple notions which our thoughts are put together from, and in each I shall distinguish what is clear from what is obscure, or which we can be mistaken about.

48. That everything we perceive is considered either as things or their affections, or as eternal truths. A list of things.

Whatever may be perceived by us, we consider either as things or their various affections, or as eternal truths having no existence outside our thought. Of those which we consider as things, the most general are substance, duration, order, number, and any others of the same sort which apply to all genera of things. However, I recognise only two highest genera of things. One is the genus of things pertaining to the understanding or thought — that is, to the mind, or thinking substance. The other is the genus of material things, or things pertaining to extended substance, or body. Perception, volition, and all the modes of perceiving and willing, are attributed to thinking substance; and to extended substance, size (or extension itself in length, breadth, and depth), shape, motion, position, the divisibility of its parts, and the such like. But we also experience within ourselves certain other things, which should not be attributed to the mind alone, or to the body alone, and which (as I shall show below in the appropriate places) arise from the close and intimate union of our mind with the body. These are the appetites of hunger, thirst, etc.; and likewise the arousing of emotions (or passions of the soul), which do not consist in thinking alone, such as arousal to anger, happiness, sadness, love, etc.; and finally all sensations, such as of pain, pleasure, light and colour, sounds, smells, tastes, heat, hardness, and other tactile qualities.

49. That eternal truths cannot be listed in the same way, but that there is no need.

We consider all the above as things, or qualities or modes of things. However, when we recognise that it is impossible for anything to come into being out of nothing, we consider the proposition ‘Nothing comes from nothing,’ not as some existing thing, nor even as a mode of a thing, but as an eternal truth, which has its seat in our mind. It is also called a common notion, or an axiom. Examples of this genus are ‘It is impossible for one and the same thing to be and not to be simultaneously;’ ‘What has been done cannot have been not done;’ ‘Someone who is thinking cannot fail to exist while thinking’ — and countless others. Obviously I cannot list them all, but they cannot fail to be recognised when we have occasion to think about them without being blinded by preconceptions.

50. That they are perceived clearly, but not all of them by everyone, because of preconceptions.

As for these common notions, they are rightly described as ‘common’, because it is certainly possible for anyone to perceive them clearly and distinctly. However, some of them are not actually perceived so clearly and distinctly by everyone, so it is less appropriate to describe these last as ‘common’ to everyone. However, in my opinion, this is not because one person has the ability to know a wider range of things than another, but because some people happen to have preconceived opinions which contradict those common notions, and hence make it difficult for these people to grasp them. On the other hand, there are plenty of other people who are free of such preconceptions, and perceive those notions as absolutely obvious.

51. What substance is, and that this term is applied to God and created beings in different senses.

As for what we view as things or modes of things, it is worthwhile considering each one separately. The only meaning we can give to ‘substance’ is that it is a thing which exists in such a way that it does not need anything else in order for it to exist. There is only one substance we can make sense of as existing independently of absolutely everything else, namely God. By contrast, we perceive that all other substances can exist only through the co-operation of God. Consequently, the term ‘substance’ cannot be applied to God and other things univocally (in the usual scholastic terminology) — in other words, it is impossible to have a distinct understanding of any sense of the term which is common both to God and to created beings.

52. That the term ‘substance’ applies to mind and body in the same sense, and how substance is known.

However, we can use a unitary concept of substance for understanding created bodily substance and created mind (or thinking substance), in that they are things which need nothing other than God in order to exist. However, we cannot initially recognise that something is a substance simply from the fact that it is an existing thing, because this in itself alone does not affect us. However, we can easily recognise it from any of its attributes, by virtue of the common notion that absence of attributes (or properties or qualities) is equivalent to absence of being. So from the fact that we perceive the presence of an attribute, we co

nclude that there must necessarily also be present some existing thing, or substance, to which it can be attributed.

53. That each substance has one distinctive attribute — that of mind is thought, and that of body is extension.

Although the presence of substance can be recognised through any attribute, each substance has just one distinctive property, [n.7] which constitutes its nature and essence, and which is the foundation of all its other properties. So, extension in length, breadth, and depth, constitutes the nature of bodily substance; and thought constitutes the nature of thinking substance. And everything else which can be attributed to body presupposes extension, and is only a mode of that which is extended; [n.8] similarly, all the contents of our minds are merely different modes of thinking. Thus, for example, we can only make sense of shape in that which is extended, or of motion in extended space; and we can only make sense of imagination, or sensation, or willing in a thinking thing. Whereas we can make sense of extension without shape or motion, and of thought without imagination or sensation, and so on. This should be obvious to anyone who considers it carefully.

54. How we can have clear and distinct notions of thinking and bodily substance, and of God.

So we can certainly have two clear and distinct notions or ideas: one of created thinking substance, and one of bodily substance. The way to achieve this is by carefully separating all the attributes of thought from the attributes of extension. In the same way, we can also have a clear and distinct idea of uncreated and independent thinking substance, namely of God. However we must not suppose that it adequately reveals to us everything that there is in God; nor should we pretend that it contains anything which we are not aware of as actually being included in it, and which we do not vividly perceive as belonging to the nature of a totally perfect being. Nobody can deny that we have such an idea of God within ourselves, unless they judge that there is no notion whatever of God in human minds.

55. How duration, order, and number are also understood distinctly.

We shall also have a perfectly distinct understanding of duration, order, and number, as long as we refrain from attaching the concept of substance to them. For example, we should think of the duration of each thing as nothing other than the mode through which our conception of that thing is limited to its continuing existence. Similarly, order and number are not separate from ordered and numbered things, but are merely modes through which we consider those things.

56. What modes, qualities, and attributes are.

Here, by ‘modes’, I obviously mean the same as what I have elsewhere called ‘attributes’, or ‘qualities’. However, they should be called ‘modes’ when we consider a substance to be affected or variegated [n.9] by them; they should be called ‘qualities’ when this variegation is the basis for giving them a name; and they should be called ‘attributes’ when we view them more generally, and only in so far as they exist in the substance. So, strictly speaking, we say that there are no modes or qualities in God, but only attributes, because it is incomprehensible how he could be subject to any variegation. And even among created things, we should use the term ‘attribute’ rather than ‘quality’ or ‘mode’ for anything which is in things only in one and the same way [n.10] — for example, existence and duration in that which exists and continues to exist.

57. That some attributes are in things, and others in thought. And what duration and time are.

Some attributes are in the things themselves, of which they are said to be the attributes or modes; whereas others are only in our thought. For example, when we distinguish time from duration in general, and say that time is the measure [n.11] of motion, [n.12] it is only a mode of thought. For obviously we do not understand there to be one duration in motion, and a different duration in things which are not moving. This is made clear by the fact that if two bodies move for an hour, one slowly and the other quickly, we will not count a greater amount of time in the one than in the other, even though the quantity of motion in it is much greater. Instead, whenever we measure the duration of anything, we compare its duration with the duration of those immense and most regular motions which give rise to years and days; and we call this duration ‘time’. So this adds nothing to duration in general apart from a mode of thought.

58. That number and all other universals are only modes of thinking.

So, when number is considered, not as in any created things, but only in abstract or in general, it is merely a mode of thinking. The same goes for everything else which we call universals.

59. How universals arise; and what the five standard ones are: genus, species, specific difference, property, and accident.

These universals arise simply because we use one and the same idea for thinking about all individuals which are similar to each other. We also apply the same name to all the things represented by that idea; and the name is universal. So, when we see two stones, and consider, not their nature, but only the fact that there are two of them, we form the idea of the number which we call the number ‘two’. And when we subsequently see two birds or two trees, and again consider, not their nature, but only the fact that there are two of them, we go back to the same idea as before, which is why it is universal. And we call this number by the same universal name of ‘two’. In the same way, when look at a figure bounded by three lines, we form a certain idea, which we call the idea of a triangle; and we subsequently use the same idea as a universal one for presenting to our minds all other figures which are bounded by three lines. And when we notice that some triangles have one right-angle, and others not, we form the universal idea of a right-angled triangle; and since its relationship to the former is one of less generality, it is called a ‘species’ of it. And the rectangularity is the universal ‘specific difference’ by which all right-angled triangles are differentiated from other triangles. And the fact that in these triangles the square of the hypotenuse is equal to the sum of the squares of the other two sides is a ‘property’, which belongs to all right-angled triangles and to none other. Finally, if we suppose that some right-angled triangles move, and others do not, this will be a universal ‘accident’ in them. This is the rationale for the standard list of five universals: ‘genus’, ‘species’, ‘specific difference’, ‘property’, and ‘accident’.

60. On distinctions; and firstly, real distinction.

Now number, as applied to things themselves, arises from their being distinguished from one another; and there are three sorts of distinction, namely ‘real’, ‘modal’, and ‘of reason’. Strictly speaking, a ‘real’ distinction is only one between two or more substances. And we perceive that they are really distinct from each other only by virtue of the fact that we can understand the one clearly and distinctly without the other. Acknowledging God, we are certain that he can bring about whatever we understand distinctly. So much so, that, for example, we are certain that it is possible for extended or bodily substance to exist, even though we do not yet know for certain that any such thing actually exists, simply from the fact that we already have an idea of it. Furthermore, if it exists, each and every part of it as defined by our thought is really distinct from the other parts of the same substance. Likewise, it is certain that each one of us is really distinct from every other thinking substance, and from every bodily substance, simply by virtue of the fact that each one of us understands that we are a thinking thing, and can in thought shut out from ourselves every other substance, whether thinking or extended. And even if we suppose that God has joined a bodily substance to such a thinking substance so closely that they could not be joined more closely, and thus welded together something unitary out of these two, nevertheless, they remain really distinct, because, however closely he might have united them, he could not have divested himself of the power he previously had of separating them, or of keeping one of them in existence without the other. And things which God can separate, or preserve separately, are really distinct.

61. On modal distinction.

There are two sorts of modal distinction. One is the distinction between a mode in the strict sense, and the substance of which it is a mode; and the other is the distinction between two modes of the same substance. The first can be recognised from the fact that we can have a clear perception of the substance independently of the mode we say differs from it, but we cannot, conversely, understand the mode independently of the substance. For example, shape and motion are modally distinct from the bodily substance they exist in; and affirmation and memory are modally distinct from the mind. The other sort can be recognised from the fact that one mode can be known independently of the other, and vice versa; but neither can be known independently of the substance which they both exist in. For example, if a stone moves and is square, I can understand its square shape independently of its motion; and, conversely, its motion independently of its square shape; but I cannot understand the motion or the shape independently of the substance of the stone. However, when a mode of one substance differs from another substance, or from a mode of another substance (as the motion of one body differs from another body, or from mind; and as motion differs from duration [n.13]), it seems more appropriate to call the distinction real rather than modal. This is because these modes are not clearly understood independently of the really distinct substances of which they are modes.

62. On distinction of reason.

Finally, a distinction of reason is that between a substance and one of its attributes, without which the substance cannot be understood; or between two such attributes of one and the same substance. It is recognised from the fact that we cannot form a clear and distinct idea of the substance if we exclude the attribute from it; or, in the second case, that we cannot perceive clearly the idea of the one attribute if we separate it from the other. For example, there is only a distinction of reason between any substance whatever and its duration, since it ceases to exist at all if it ceases to have duration. Again, all the modes of thinking which we consider as existing in objects differ only in reason, both from the objects they are thought of as being in, and from each other in one and the same object. I remember that I once conflated this sort of distinction with the modal sort, at the end of my reply to the first set of objections to my Meditations on First Philosophy. But in that passage there was no need to distinguish them precisely, and it was enough for my purpose to distinguish both of them from a real distinction.

63. How thought and extension can be distinctly known as constituting the nature of mind and body.

Thought and extension can be considered as constituting the natures of intelligent and bodily substance respectively. Given this, they should not be conceived as anything other than thinking substance and extended substance themselves, that is, as mind and body. Not only is this the clearest and most distinct way of conceiving them, but it is also easier to form a conception of extended substance or of thinking substance than of substance alone, leaving out the fact that it thinks or that it is extended. For there is no little difficulty in abstracting the notion of substance from the notions of thought or extension, since they differ from it only through a distinction of reason. A concept does not become more distinct by virtue of our including less in it, but only in so far as we carefully distinguish whatever we include in it from everything else.

64. How they can also be known as modes of substance.

Thought and extension can also be taken as modes of substance, that is in so far as one and the same mind can have many different thoughts; and one and the same body, while retaining its same quantity, can be extended in many different ways — now longer, but less wide or deep, and now wider, but shorter. In that case, they are modally different from substance. They can be understood just as clearly and distinctly as substance, provided they are considered not as substances (i.e. things separate from other things), but only as modes of things. For by the very fact that we consider them as in the substances of which they are modes, we distinguish them from those substances, and recognise them for what they really are. But if, on the other hand, we wanted to consider them independently of the substances they exist in, we would thereby be considering them as subsistent things, and thus we would be confusing the ideas of mode and substance.

65. How their modes are also to be known.

Thought and extension themselves have various modes in their turn; and these are best perceived if we consider them only as modes of the things they are in. Examples of modes of thought are understanding, imagination, memory, volition, etc.; and examples of modes of extension (or modes spread over extension) are all shapes, and the position and motion of parts. In the case of motion, it is best perceived if we think only of motion from one place to another, and ignore the force which initiates it (however, I shall try and explain it in the appropriate place).

66. How sensations, emotions, and appetites are clearly known, even though we often make wrong judgments about them.

It remains to discuss sensations, emotions, and appetites. These too can also be perceived clearly, provided we scrupulously avoid making any judgments about them which go in any way beyond what is included in our perceptions, and what we are intimately conscious of. But it is extremely difficult to stick to this, at least in the case of sensation. Right from the earliest age, every one of us has judged that all the things we sensed were things existing outside our minds, and that they exactly resembled our sensations (that is, the perceptions we had of them). So much so, that seeing a colour, for example, we thought we were seeing some sort of thing positioned outside us, and exactly similar to the idea of colour which we were experiencing within ourselves at the time. And because of our habit of judging in this way, we seemed to ourselves to see this so clearly and distinctly, that we took it for certain and indubitable.

67. That we often make wrong judgments even about pain.

And the same is obviously true of everything else we sense, including pleasure and pain. Although we do not think that these exist outside ourselves, we do not usually consider them as solely in the mind (or in our perception), but as in the hand or foot, or some other part of body. When, for example, we feel a pain as being in the foot, it is clearly no more certain that it is something outside our mind and existing in the foot, than it is that, when we see light as being in the sun, the light exists outside ourselves in the sun. As will be made clear below, these are both preconceptions deriving from our earliest years.

68. How, in these cases, we should distinguish between what we know clearly from what we can be wrong about.

In order to distinguish here between what is clear from what is obscure, we must note with the greatest care that pain and colour, and all other things of the same sort, are perceived clearly and distinctly only when they are considered as sensations or thoughts. When they are judged to be things existing outside our minds, it is quite impossible to understand what sort of thing they might be. For someone to say that they see colour in some physical object, or feel pain in some part of their body, is exactly the same as to say that they see or feel in it something of which they are completely ignorant — in other words, that they do not know what they are seeing or feeling. Through lack of attention, they might easily persuade themselves that they have quite considerable knowledge of it, by assuming that it is something similar to the sensation of colour or pain which they experience in themselves. But if they ask themselves what it is which the sense of colour or pain represents as existing in the coloured object or in the painful part of the body, they will certainly realise that the do not know.

69. That size, shape, etc. are known very differently from colours, pains, etc.

This will be especially clear to them if they consider that there are two completely different ways of knowing things. As I have said, knowledge of colour, or pain, or smell, or taste, and so on, is due to our sensations. But I said earlier that in the same visible objects, we have a clear perception of what size is, or figure, or motion, or position, or duration, or number, and so on. (When I say ‘motion’, I mean motion from one place to another. Philosophers have made the nature of motion less intelligible for themselves by inventing various other types of motion.) When we see a particular object, we are no more certain that it exists in so far as it appears to us as having a shape, than we are in so far as it appears to us as being coloured. But we recognise far more clearly in it what having a shape consist in, than what being coloured consists in.

70. That there are two ways of arriving at judgments about objects of sensation, one of which keeps us from error, and the other of which leads us into error.

It is therefore obvious that to say we perceive colours in objects is really the same as to say we perceive something in objects, we know not what, but which causes in ourselves a certain sensation which is completely evident and lucid, and which is called the sensation of colour. But when we are in the mode of making judgments, there are two very different kinds of judgment we can make. One is to judge that there is something in objects (that is, in the things, whatever they might be, which our sensation comes to us from), but that we do not know what. As long as we confine ourselves to this, far from lapsing into error, we shall instead save ourselves from error, because we are less inclined to make rash judgments about something we know we are ignorant about. The other kind of judgment is when we think we perceive colours in objects, even if we do not in fact know what it is we are giving the name of colour to, and cannot understand how there can be any similarity between the colour we suppose to be in objects, and the colour we experience in sensation. But we do not notice this fact, and note instead that there are many other things, such as size, shape, number, etc., which we clearly perceive to be sensed or understood by us exactly as they are, or at least can be, in objects. We therefore naturally fall into the error of judging that what we call colour in objects is something similar in every respect to the colour we sense, and hence judge that we clearly perceive something which in fact we cannot perceive in any way at all.

71. That the main cause of errors lies in the preconceptions of infancy.

And in this we can recognise the first and main cause of all errors. In childhood, our mind was so tightly bound to the body that it had no room [n.14] for any thoughts other than those through which it sensed what was happening to the body. It did not yet attribute them to anything assumed as external to itself. It merely felt pain when something harmful happened to the body, and felt pleasure when something beneficial happened to it. And when the body was affected by something without much harm or benefit, depending on which parts were affected and in what way, the mind had a variety of sensations, namely the ones we call sensations of tastes, smells, sounds, warmth, cold, light, colours, and other similar ones, which do not represent anything assumed as existing outside thought. At the same time, the mind also perceived sizes, shapes, motions, and the suchlike, which were presented to it not as sensations, but as things of some sort (or modes of things) existing outside thought (or at least capable of doing so), even if it had not yet noticed this difference between them. Now nature has constructed the machinery of the body in such a way that it can move in various ways under its own power. So at some later time, the random motions of the body in all directions chanced to bring about something beneficial, or avoid something harmful. [n.15] Consequently, the mind which was attached to this body began to notice that what was brought about or avoided in this way was external to itself; and it attributed to it, not only sizes, shapes, motions, and the suchlike (which it perceived as things or modes of things), but also tastes, smells, and the rest (which it noticed it had sensations of, caused by the external object). And since it related everything to the interests of the body which it was it was immersed in, it thought that the extent to which each object it was affected by was a thing was a function of the extent to which it was affected by it. Consequently, it thought there was much more substance or bodiliness in rocks or metals than in water or air, because it had a greater sensation of hardness and heaviness in the former. Indeed, it concluded that air was nothing at all, as long as it had not yet experienced any wind, or cold, or heat in it. And since the stars shone on it with no more light than the tiny flames of lamps, it did not represent the stars to itself as any larger than the flames of lamps. And since it did not observe that the earth revolved, or that its surface was curved spherically, it was all the more prone to think both that it was stationary, and that it had a flat surface. From earliest infancy, our mind was ingrained with a thousand other preconceptions of this sort. Then in childhood it forgot that it had acquired them without sufficient investigation, and it accepted them as completely true and certain, as if they were known through sensation, or implanted by nature.

72. That another cause of error is that we cannot forget our preconceptions.

Now that we have reached maturity, the mind is no longer wholly devoted to the body. It no longer relates everything to the body, but it also inquires about the truth of things considered as they are in themselves. It thus becomes aware that very many of its previous judgments were false. However, it is not so easy for it to eliminate them from its memory; and as long as they remain there, they can be the cause of various errors. For example, from our earliest years we imagined that the stars were very tiny, even though arguments from astronomy clearly show us that they are extremely large. However, that preconceived opinion still has so much force with us, that it is very difficult indeed for us to imagine them any differently from before.

73. That a third cause is that we get tired attending to things which are not present to the senses, and so we are accustomed to making judgments about them which are based on a preconceived opinion rather than on a present perception.

Besides, our mind cannot pay attention to anything without some difficulty and tiredness, and it has most difficulty attending to things which are not present either to the senses or, indeed, to the imagination. This might be because it is a natural consequence of its union with the body, or it might be because it was exclusively concerned with the senses and the imagination in its early years, and became more practised and skilful at thinking about these than about other things. This is why many people still have no understanding of substance except as imaginable, bodily, and even sensible. They do not know that the only imaginable things are those which consist of extension, motion, and shape, whereas many other things are intelligible. And they think that nothing can subsist unless it is a body, and that all bodies are sensible. But in fact we do not perceive anything as it is in itself by sense alone, as I shall clearly show below. Consequently, it happens that most people never perceive anything except confusedly for the whole of their lives.

74. That a fourth cause is that we associate our concepts with words which do not correspond precisely to things.

Finally, in order to make speech possible, we associate all our concepts with words to express them with; and we can only commit our concepts to memory together with their associated words. Subsequently we find it easier to recall the words than the things, and so we hardly ever have a concept of a thing which is so distinct that we can keep it completely separate from our concept of the words. Almost everybody’s thoughts involve words more than things — so much so, that they very often assent to words they have not understood, because they think they once understood them, or received them from others who understood them correctly. I cannot go into any further detail here, because I have not yet given an account of the nature of the human body, and I have not yet even proved that any bodies exist. Nevertheless, all this seems intelligible enough to help distinguish clear and distinct concepts from obscure and confused ones.

75. A summary of what is to be observed in order to philosophise correctly.

In order to philosophise seriously, and to discover the truth about all things that are knowable, we must do the following: First, we must lay aside all preconceptions — in other words, we must strictly avoid relying on any of our previously held opinions, until we have re-examined them, and found them to be true. Next, we must scrutinise, in proper order, the notions which we have within ourselves. We must judge to be true all those and only those which we know clearly and distinctly during this scrutiny. In the course of this, we shall first of all observe that we exist in so far as it is our nature to think. At the same time, we shall also observe that God exists; that we depend on him; and that we can discover the truth about the other things by considering his attributes, since he is their cause. Finally, we shall observe that, apart from the notions of God and of our mind, there is also in us a notion of many eternally true propositions, such as that nothing comes from nothing, etc.. Similarly, there is a notion of a certain bodily nature (that is, extended, divisible, capable of moving, etc,). And again, there is a notion of various sensations which affect us, such as of pain, colours, tastes, etc., even though we do not yet know the cause of their affecting us in this way. And by comparing these with our previously rather confused thoughts, we shall acquire the habit of forming clear and distinct concepts of all knowable things. And it seems to me that this brief list covers the main principles of human knowledge.

76. That divine authority takes precedence over our perceptions; but othwerise it is inappropriate for a philosopher to accept anything which has not been perceived.

However, we must fix in our memories as the highest rule above all others, that everything which God has revealed to us must be believed as the most certain of all. Even if the light of reason, which is so absolutely clear and evident, occasionally seems to suggest something different to us, we must trust divine authority alone rather than our own judgment. But in matters which divine faith tells us nothing about, it is quite improper for mortal philosophers to take something as true without ever having perceived it as true, and to put more trust in the senses (that is, in childish judgments made without thought), than in their mature reason.

1. On what grounds we know with certainty that material things exist.

Everybody is satisfied that material things exist. However, we must now investigate the grounds on which their existence can be known with certainty, since I cast doubt on it a little earlier, and I included it among the preconceptions of our early years. It is beyond doubt that all our sensations come from a thing of some sort, which is distinct from our mind. This is because it is not in our power to bring it about that we have a sensation of one thing rather than of another; but it obviously depends on the things which affect our senses. However, it can be asked whether this thing is God, or whether it is distinct from God. Now, rather than saying simply that we have sensations, we should say that, under compulsion from our sensations, we clearly and distinctly perceive a certain matter, extended in length, breadth, and depth, of which its various parts are characterised by various shapes, and move with various motions, and also bring it about that we have various sensations of colours, smells, pain, etc. Given all this, let us suppose that God revealed this idea of extended matter to our mind through his own immediate agency. Or, alternatively, let us suppose merely that he brought it about that the idea was revealed to us by some sort of thing, but one which had no extension, or shape, or motion. Either way, it is impossible to think of any reason why he should not be considered a deceiver. For we clearly understand it as being a thing which is utterly distinct both from God and from ourselves (i.e. our minds). Moreover, we also seem to see clearly that its idea comes from things situated outside us, and to which it is similar in every respect. But it is absolutely inconsistent with God’s nature that he should be a deceiver, as I have already observed. Therefore we must here conclude quite generally that there exists some sort of thing which is extended in length, breadth, and depth, and which has all the properties which we clearly perceive as belonging to that which is extended. And this extended thing is what we call body or matter.

2. By which it is also known that the human body is closely joined to the mind.

The same argument proves that one body is more closely united to our mind than other bodies are. The reason is because we observe that pains and other sensations come to us unexpectedly, and the mind is aware that they do not come from itself alone. Nor can they belong to it solely by virtue of its being a thinking thing, but only by virtue of the fact that it is joined to some other thing which is extended and capable of motion, and which is called the human body. But this is not the place to go into greater detail on the subject.

3. That perceptions of the senses tell us not what is really in things, but what is good or bad for the human compound.

It will be enough to note that perceptions of the senses relate only to this compound of the human body and the mind. They usually reveal to us only what good or harm can be done to it by external bodies; and only occasionally and by accident do they tell us what they are like in themselves. This will make it easy for us to abandon the preconceptions of the senses; and in what follows, we shall use the understanding alone for carefully scrutinising the ideas which nature has endowed it with.

4. That the nature of body does not consist in weight, hardness, colour, or the such like, but in extension alone.

Following this procedure, we shall perceive that the nature of matter (or body in general) does not consist in the fact that it is a thing which is hard, or heavy, or coloured, or affecting the senses in some other way, but only in the fact that it is a thing which is extended in length, breadth, and depth. For as far as hardness is concerned, all that our sensations tell us about it is that the parts of hard bodies resist the motion of our hands when we hit them. But if, every time our hands moved against some part, all the bodies existing there moved backwards at the same speed as our hands were moving forwards, we would never have any sensation of hardness. And it is quite incomprehensible why these bodies should lose the nature of body just because they were moving backwards. Consequently, the nature of body does not consist in hardness. By the same reasoning, it can be shown that weight, colour, and all other such qualities which are sensed in bodily matter, can be taken away from it, while leaving the matter itself intact. From which it follows that its nature depends on none of them.

I would not have added anything more to this Part IV of the Principles of Philosophy, if (as was my previous intention) I were going to write two more parts, namely Part V on living things (animals and plants), and Part VI on human beings. But I have not yet fully worked out what I wanted to deal with in them, and I do not know whether I shall ever have enough time to finish them. So, in order not to delay publication of the first four parts any longer, nor to leave out anything relevant which I might have kept back for the other parts, I shall here add a few remarks about the objects of our sensations. So far I have described the Earth, and hence the whole visible world, as like a machine, taking account only of the shapes and motions in it. Yet our sensations reveal to us many other aspects of it, such as colours, smells, sounds, and the such like. If I were to say nothing at all about these, I would seem to have left out the main part of any account of natural things.

189. What a sensation is, and how it occurs.

It needs to be borne in mind that, even though the human soul informs the whole body, it has its main seat in the brain. This is the only place where it not only understands and imagines, but also has sensations. It has sensations by means of the nerves, which extend like threads from the brain to all other parts of the body. They are connected to them in such a way that hardly any part of the human body can be touched, without the touch moving the ends of some of the nerves distributed throughout it. Their motions are transferred to the other ends of the nerves, which come together in the brain around the seat of the soul (as I have explained in sufficient detail in Chapter 4 of the Dioptrics). Now the motions which the nerves thus set up in the brain affect the soul or mind which is intimately joined to the brain in different ways, depending on the differences in the motions. And these different affections of the mind (or thoughts) which are the immediate consequences of these motions are what we call ‘perceptions of the senses’, or simply ‘sensations’ in ordinary language.

190. On the distinction between sensations; and first on internal sensations, that is, on the emotions of the mind, and on the natural appetites.

The diversity of these sensations depends, first on the diversity of the nerves themselves, and secondly on the diversity of motions which take place in individual nerves. However, it is not the case that each individual nerve brings about a unique sensation which is different from all the others. In fact it is possible to distinguish only seven species [n.16] of sensation, of which two are species of internal sensations, and the other five of external sensations. So, the nerves which go to the stomach, gullet, throat, and other internal organs responsible for satisfying our natural wants, constitute one of the internal senses, which is called ‘natural appetite’. But the little nerves which go to the heart and chest, despite being very small, constitute another internal sense, which is made up of all the emotions (or passions) and affections of the soul, such as happiness, sadness, love, hate, and the such like. For example, the blood might be in a condition in which it is liable to expand more than usual in the heart. If so, it will stretch and move the little nerves distributed around its orifices so that another motion is transmitted from there to the brain, which will affect the mind with a natural sensation of joy. Also, if any other causes whatever move these nerves in the same way, they will provide the same sensation of joy. Thus if you imagine enjoying something nice, the imagination itself does not include the actual sensation of joy. Rather, it sends spirits from the brain to the muscles surrounding these same nerves, and the spirits expand the orifices of the heart. Then the little nerves of the heart are subjected to precisely the sort of motion from which the sensation of joy must follow. Similarly, on hearing some good news, initially the mind comes to a judgment about it, and rejoices with the intellectual rejoicing which takes place without any bodily emotion (and this is why the Stoics said that the sage could experience it [n.17]). Then, when an image is formed of the event, spirits flow from the brain to the muscles of the chest, and move the little nerves there; and by their means, the spirits set up another motion in the brain, which affects the mind with a sensation of spiritual [n.18] joy. The same type of explanation works for cases when the blood is too thick. It flows sluggishly into the ventricles of the heart, and since it does not expand enough in them, it sets up a different sort of motion in the same little nerves of the chest. When this motion is transmitted to the brain, it conjures up a sensation of sadness in the mind, even though the mind might not know why it should be sad [n.19] — and many other causes can produce the same effect. Other motions of these little nerves bring about different affections, such as those of love, hate, fear, anger, etc., in so far as they are only affections (or passions of the soul); that is, in so far as they are a confused type of thought, which the mind does not generate by itself, but which it has because something has happened to the body it is intimately united with. I say ‘in so far as,’ because these affections belong to a completely different category from the distinct thoughts we have about what ought to be espoused, or desired, or avoided, etc. The same argument applies to the natural appetites (hunger, thirst, etc.), which depend on the nerves of the stomach, throat, etc. They are completely distinct from willing to eat, drink, etc.; but they are called ‘appetites’ because they are nearly always accompanied by such a volition, and ‘appetition’ [n.20] is another term for ‘volition’.

191. On the external senses, and firstly on touch.

As for the external senses, they are popularly supposed to be five in number, since there are five different genera of objects setting in motion the nerves which serve them, and there are the same number of genera of confused thoughts excited in the soul by these motions. Firstly, the nerves which end in the skin of the whole body can be touched by any physical object whatever through the medium of the skin, and set in motion by any of their qualities [n.21] — in one way by their hardness, in another way by their weight, in another way by their heat, in another way by their moistness, etc. They excite as many different sensations in the mind as there are ways in which they are moved, or are prevented from moving as they would otherwise have done; [n.22] and on the basis of these different sensations we give names to different kinds of tactile quality. When these nerves are stimulated more forcefully than usual, but without any harmful consequences to the body, this gives rise to an additional sensation of pleasure, because the mind is naturally pleased to receive evidence of the strength of the body to which it is closely united. If, on the other hand, the body is injured, the additional sensation is one of pain. From this it is obvious why bodily pleasure and pain are so little different from each other in the object, even though they are opposite sensations. [n.23]

192. On taste.

Secondly, other nerves dotted around the tongue and the areas next to it are set in motion in different ways by scattered particles [n.24] of the same bodies. These particles are carried around the mouth by the saliva, and their different shapes bring about sensations of different tastes.

193. On smell.

Thirdly, there are also two nerves (or appendages of the brain, since they do not protrude beyond the skull) which are set in motion by separated particles of the same bodies flying around in the air. These are not just any particles, but only those which are rarefied and forceful [n.25] enough to penetrate as far as these nerves through the passages of the mucous membrane after being drawn into the nostrils. Their different motions are responsible for the different sensations of smell.

194. On hearing.

Fourthly, two other nerves, hidden in the innermost cavities of the ears, receive the quivering and vibrating motions of all the surrounding air. For when the air beats against the little membrane of the eardrum, it simultaneously shakes the little chain of three tiny bones which are connected to it, and which the nerves are attached to. And sensations of different sounds arise from the differences in these motions.

195. On vision.

Finally, the ends of the optic nerves, which form the membrane in the eyes called the retina, are not moved there by the air or by any macroscopic bodies, but only by globules of the second element. [n.26] This is how we get our sensations of light and colours, as I have already explained more fully in my Dioptrics and my Meteorology.

196. That the soul has sensations only in so far as it is in the brain.

The soul senses what happens to individual parts of the body by means of the nerves, and there are a number of conclusive proofs that it does so, not in so far as it is in these individual parts of the body, but only in so far as it is in the brain. Firstly, it is proved by the fact that various diseases which affect only the brain, obliterate or distort all sensation. Similarly even sleep, which is only in the brain, daily suspends most of our capacity for sensation, though it is restored when we wake up. The second proof is that, even if the brain is undamaged, we lose the sensation of parts of the body external to it, if there is merely an obstruction of the routes by which the nerves connect them to it. The third proof is that we sometimes feel pain as in a certain part of the body, even though there is no cause of pain in that part itself, but only in other parts through which the nerves pass en route to the brain. This last can be established by many empirical facts; but one example is enough here. There was a girl who had a seriously diseased hand. Whenever the doctor called, they blindfolded her so that she would not be upset by the sight of his surgical equipment. After a few days, her arm was cut off at the elbow, because of advancing gangrene. They then faked up the missing arm with bandages, so that she was completely unaware of her loss. However, she went on complaining that she could feel various pains in different fingers of her amputated hand. Clearly, this can only be explained by the fact that the nerves which previously went all the way from the brain to the hand, but which now terminated at the elbow, were set in motion in the same way at the elbow, as they must previously have been set in motion in the hand, in order to impress on the soul residing in the brain the sensation of this or that finger being in pain.

197. That mind is of such a nature that its various sensations can be stimulated in it by bodily motion alone. [n.27]

Next it is proved that the nature of our mind is such that it can be forced into thoughts of any kind whatever simply because of the occurrence of certain motions in the body, even though these thoughts include no representation of the motions — in particular, the confused thoughts which we call ‘sensations’. [n.28] For example, we see that words (whether spoken or merely [n.29] written) can arouse absolutely any kind of thought or emotion in our minds. If the tip of a pen is guided over the surface of a piece of paper in a particular way, it will form letters which arouse in the mind of the reader thoughts of battles, storms, or the Furies, and affections of indignation and sadness. But if the same pen is moved over the same paper with the same ink in a very slightly different way, it will bring about utterly different thoughts, such as of tranquillity, peace, or beauty, and the completely opposite affections of love and joy. [n.30] Perhaps someone will reply that writing or speech do not directly excite emotions in the mind, nor images of things other than themselves, but only a variety of intellections, on the occasion of which the mind itself then forms within itself images of the various things. [n.31] But what will be said about the sensations of pain and pleasure? A sword strikes against our body, and cuts it. Pain follows from this alone, and the pain is no less different from the motion of the sword, or of the body which is cut, than colour, or sound, or smell, or taste. This clearly shows that the sensation of pain is aroused in us simply by virtue of the fact that certain parts of our body are set in motion by contact with some other body. Consequently, we can draw the conclusion that our mind is of such a nature that it can also be subjected to the affections of all the other senses by some kind of motion or other.

198. That our sensations reveal nothing to us in external objects apart from their shapes, sizes, and motions.

Furthermore, we do not observe any differences of kind among the nerves, such as would allow us to conclude that something qualitatively different reached the brain from the external sense organs through different kinds of nerve. Indeed, we have no basis for concluding that anything at all reaches the brain apart from the motion of the nerves themselves. And we see that this motion presents us with the sensation of light and sounds as well as of pleasure and pain. If you are hit in the eye in such a way that the vibration from the blow penetrates as far as the retina, this alone is enough to make you see many sparks of bright light, even though the light will not exist outside your eye. And if you stick your fingers in your ears, you will hear a sort of fluctuating muffled noise, which comes from nothing other than the motion of the air trapped in the ears. Finally, let us take the example of heat, or of any other sensible qualities in so far as they are in objects themselves — and this also includes the forms of purely material things, such as the form of fire. [n.32] We often observe that they are caused by the motions of certain bodies, and that they in their turn cause other motions in other bodies. We have a perfect understanding of how the various motions in one body are caused by the various sizes, shapes, and motions of the particles of another body. But there is no way we can understand how size, shape, and motion can bring about something of a completely different nature from themselves — for example, those ‘substantial forms’ and ‘real qualities’ which many people suppose to exist in things. Nor can we understand how these qualities or forms could subsequently have the power to cause motions in other bodies. So add to all this our scientific knowledge that the nature of our soul is such that all its sensations can be stimulated by the variety of motions alone. Add also the observational evidence that in fact these various sensations are stimulated in the mind without our being able to detect anything passing from the external sense organs to the brain, apart from motions of the same sort. We can then draw the general conclusion that we also have no basis for asserting that the things in external objects which we superstitiously reify [n.33] with the names of light, colour, smell, taste, sound, heat, cold, and other tactile qualities (or even substantial forms), are anything other than the various dispositions of these objects, which bring it about that our nerves can move in various ways.

199. That no phenomena of nature have been omitted in this treatise.

A simple checklist will show that I have omitted no phenomena of nature in this treatise. Nothing is to be counted among the phenomena of nature unless it is revealed in sensation. I have explained what size, shape, and motion are in each and every body. Apart from these, we have no sensations of anything situated outside ourselves, except for those of light, colour, smell, taste, sound, and the tactile qualities. And I have already proved that, in the objects themselves, these are nothing other than (or at least cannot appear to us as anything other than) various dispositions consisting in size, shape, and motion.

200. That in this treatise I have used no principles which are not accepted by everyone; and that this philosophy is not new, but extremely ancient and established.

I should also wish it to be noted that, in this treatise, I have tried to explain the complete nature of material things without using any principle at all which was not accepted by Aristotle, and by all other philosophers of all historical periods — so much so, that this philosophy is not new, but the most ancient and established of all. [n.34] That is, I have taken into account the shapes, motions, and sizes of bodies, and I have explored what must follow from collisions between them in accordance with the laws of mechanics, reinforced by specific [n.35] everyday observations. And who ever doubted that bodies move, and have various shapes and sizes, and that they also have different motions depending on their different shapes and sizes? Or that when bodies collide, bigger ones are broken up into many smaller ones, and change shape? We detect this, [n.36] not just through one sense, but through many — sight, touch, and hearing — and we also have distinct imagery and understanding of it. The same cannot be said of the rest (such as colours, sounds, etc.), which are perceived not by means of many senses, but only one. In our thinking, the images of them are always confused, and we do not know scientifically what they are.

201. That there exist imperceptible particles of bodies.

I take account of the many particles in individual bodies, which are not perceived by means of any sensation. Here I am perhaps in disagreement with those who make their sensations the criterion of what is knowable. But who can doubt that there are many bodies which are so minute that we cannot detect them by means of any sensation? One only needs to consider what is added each hour to things which slowly get larger, or what is lost to things which get smaller. A tree grows every day; but it is only through understanding that a body is added to it, that we can understand its being made larger than it was before. But who ever detects through their sensations what specific corpuscles are added to a growing tree in one day? At least those who recognise that any quantity is infinitely divisible must admit that its parts can be made so small that they cannot be perceived by any sensation. In fact it should not be surprising that we cannot sense very small bodies. The reason is that our nerves, which must be set in motion by objects if we are to have a sensation of them, are not themselves so very small. They are like fine ropes, made up of many particles smaller than themselves; so they cannot be set in motion by the smallest of bodies. I do not think anyone who uses their reason will deny that we should make judgments about the characteristics [n.37] of minute corpuscles by analogy with the characteristics we perceive in macroscopic bodies through our sensations, since the only reason why we have no sensations of the former is because they are too small. This is far better than explaining them by thinking up incomprehensible entities which have nothing in common with the things which we do have sensations of. [n.38]

202. That the philosophy of Democritus is no less different from ours than it is from scholastic philosophy.

Democritus also imagined there to be corpuscles of a certain sort, which were characterised by their various shapes, sizes, and motions, and which gave rise to all sensible bodies through their accumulations and collisions with each other — yet the basis of his philosophy is usually rejected by everyone. However, nobody ever rejected it because it includes bodies which are too small to be sensed, and which are said to have various sizes, shapes, and motions. As I have just shown, nobody can doubt that there are in fact many such bodies. No, the reasons why it has been rejected are as follows. First, he supposed that these corpuscles were indivisible — and on this account I too reject his philosophy. Second, he invented a vacuum surrounding them, whereas I have proved that a vacuum is impossible. Third, he endowed them with weight, whereas my understanding is that weight is not a characteristic of any body considered in itself, but only in so far as the body depends on the position and motion of other bodies and stands in relation to them. Fourth, he failed to show how individual things arose from collisions of corpuscles alone. Or if he showed how in some cases, not all of his arguments were mutually inconsistent — at least as far as one can judge from the extant reports of his opinions. I leave it to others to judge whether my philosophical writings so far are sufficiently consistent.

203. How we know the shapes and motions of insensible particles.

I attribute determinate shapes, sizes, and motions to the insensible particles of bodies, just as if I had seen them — yet I admit that they are insensible. Consequently, many people will perhaps ask how I know what they are like. My reply is as follows. Starting out from the knowledge of the simplest and most evident principles which nature has endowed our minds with, I worked out, in general terms, what the main differences could be between the sizes, shapes, and positions of bodies which are insensible merely because of their smallness; and what sensible effects would follow from their various collisions. Then, since I noticed some similar effects among sensible things, I reckoned that they had arisen from similar collisions of insensible bodies, especially since it seemed impossible to think of any other way of explaining these effects. In this, I was greatly helped by considering human artefacts — and I should say that the only distinction I recognise between artefacts and natural bodies is that, for the most part, the functions of artefacts are carried out by mechanisms big enough to be easily perceived by sensations. This is necessary, otherwise human beings would not be able to construct them. By contrast, natural effects nearly always depend on mechanisms [n.39] too small to be sensed. Certainly there are no explanations in mechanics which do not apply also to physics, since physics is a part or species of mechanics. [n.40] It is no less natural for a clock to tell the time because of the various wheels it is made of, than it is for a tree to produce the type of fruit it does because of the seed it was grown from. When expert engineers know the function of a machine and have sight of some of its parts, they can easily extrapolate from them how the parts they cannot see were made. It is in the same way that I have tried to investigate the insensible causes and particles of natural bodies on the basis of their sensible effects and parts.

204. That it is sufficient for me to have explained what insensible things might possibly be like, even if perhaps they might not actually be like that.

Although I trust I have given an intelligible account of how all natural things could have come into being, it should not be concluded that they actually came into being in this way. To give an analogy. One and the same clockmaker can make two clocks which tell the time equally well, and are exactly identical from the outside; but internally they consist of a completely different set of wheels. Similarly, there is no doubt that the supreme creator of things could have brought about everything we see in many different ways. I personally have no difficulty over admitting this to be the case. I think I shall have achieved enough if what I have written is at least perfectly consistent with all the phenomena of nature. Besides, this is all that is needed for the conduct of life, since medicine, mechanics, and all the other arts which can be brought to perfection by means of physics, are directed only towards sensible things — and sensible things are the phenomena of nature. And just in case anyone thinks that Aristotle achieved more, or wanted to do so, he himself makes an explicit statement at the beginning of the seventh chapter of the first book of the Meteorologica. In discussing things which are not revealed to the senses, he says he thinks he has adduced sufficient arguments and proofs, provided only that he has shown that it is possible for them to happen in accordance with his account.

205. That my explanations seem to be at least morally certain.

In case anyone thinks I am somehow fiddling with the truth here, it should be noted that some things are uncertain in relation to God’s absolute power, but are held to be morally certain — that is, they are as certain as is necessary for the conduct of life. For example, suppose someone is trying to read a letter written in Latin characters, but in code. They might guess that A should be read as B, B as C, and similarly that every character should be replaced by the immediately following character. By doing so, they might find that the characters form Latin words, and they will have no doubt that the true meaning of the letter is contained in these words. However, this is only a guess, and it could perhaps be the case that the writer replaced the real characters, not with the immediately following ones, but with other ones. In this way, the writer could have hidden a different meaning in the letter. But this is so improbable that it does not seem believable. [n.41] Some people might think that I have picked on various principles at random and without any rationale, as the fewest principles from which I could deduce so many things about magnetism, fire, and the construction of the whole world in this treatise. But perhaps they overlook the fact that it would hardly be possible for so many factors to be mutually consistent if they were false.

206. Indeed, more than morally certain.

Besides, there are some things, even in the realm of nature, which we regard as absolutely certain, and more than merely morally certain. This is built on the metaphysical foundation that God is completely good and utterly undeceptive. It follows that the faculty he gave us for distinguishing the true from the false cannot lead us into error as long as we use it correctly, and perceive something distinctly by its means. Examples are mathematical proofs; the recognition that material things exist; and all conclusive reasonings we make about them. Perhaps readers will also include the contents of this treatise as more than morally certain, if they consider how they have been deduced in a continuous sequence from primary and absolutely simple principles of human knowledge. Especially so if they have a sufficient understanding of the fact that we cannot sense external objects unless they stimulate some sort of motion in our nerves. The fixed stars, which are a very long way from us, cannot stimulate such a motion unless there is also a motion both in them and in the whole space between them and us. If this is accepted, everything else seems hardly intelligible except as I have explained it — at least the more general aspects of the universe and the earth which I written about.

207. But that I submit everything I have written to the authority of the church.

Nevertheless, I am aware of my weakness, and I affirm nothing. I submit everything I have written, both to the authority of the Catholic Church, and to the judgment of people who are wiser than myself. Nor would I wish anyone to believe anything which they are not convinced of by obvious and irrefutable reasoning

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