(Relevant to Meditation II, VI)

From Objection IV (Arnauld): (Body a Machine, animals)

I had got as far as this in my comments, and was intending to show how the author's principles, which I thought I had managed to gather from his method of philosophizing, would enable the immortality of the soul to be inferred very easily from the real distinction between the mind and the body. But at this point, a little study composed by our illustrious author was sent to me, which apart from shedding much light on the work as a whole, puts forward the same solution to the point at issue which I was on the point of proposing.

As far as the souls of the brutes are concerned, M. Descartes elsewhere suggests clearly enough that they have none. All they have is a body which is constructed in a particular manner, made up of various organs in such a way that all the operations which we observe can be produced in it and by means of it.

But I fear that this view will not succeed in finding acceptance in people's minds unless it is supported by very solid arguments. For at first sight it seems incredible that it can come about, without the assistance of any soul, that the light reflected from the body of a wolf onto the eyes of a sheep should move the minute fibres of the optic nerves, and that on reaching the brain this motion should spread the animal spirits throughout the nerves in the manner necessary to precipitate the sheep's flight.

Descartes’s Response:

I will not answer my critic's further observations regarding the immortality of the soul, because they do not conflict with my views. As far as the souls of the brutes are concerned, this is not the place to examine the subject, and, short of giving an account of the whole of physics, I cannot add to the explanatory remarks I made in Part 5 of the Discourse on the Method. But to avoid passing over the topic in silence, I will say that I think the most important point is that, both in our bodies and those of the brutes, no movements can occur without the presence of all the organs or instruments which would enable the same movements to be produced in a machine. So even in our own case the mind does not directly move the external limbs, but simply controls the animal spirits which flow from the heart via the brain into the muscles, and sets up certain motions in them; for the spirits are by their nature adapted with equal facility to a great variety of actions. Now a very large number of the motions occurring inside us do not depend in any way on the mind. These include heartbeat, digestion, nutrition, respiration when we are asleep, and also such waking actions as walking, singing and the like, when these occur without the mind attending to them. When people take a fall, and stick out their hands so as to protect their head, it is not reason that instructs them to do this; it is simply that the sight of the impending fall reaches the brain and sends the animal spirits into the nerves in the manner necessary to produce this movement even without any mental volition, just as it would be produced in a machine. And since our own experience reliably informs us that this is so, why should we be so amazed that the 'light reflected from the body of a wolf onto the eyes of a sheep' should equally be capable of arousing the movements of flight in the sheep?

But if we wish to determine by the use of reason whether any of the movements of the brutes are similar to those which are performed in us with the help of the mind, or whether they resemble those which depend merely on the flow of the animal spirits and the disposition of the organs, then we should consider the differences that can be found between men and beasts. I mean the differences which I set out in Part 5 of the Discourse on the Method, for I think these are the only differences to be found. If we do this, it will readily be apparent that all the actions of the brutes resemble only those which occur in us without any assistance from the mind. And we shall be forced to conclude from this that we know of absolutely no principle of movement in animals apart from the disposition of their organs and the continual flow of the spirits which are produced by the heat of the heart as it rarefies the blood. We shall also see that there was no excuse for our imagining that any other principle of motion was to be found in the brutes. We made this mistake because we failed to distinguish the two principles of motion just described; and on seeing that the principle depending solely on the animal spirits and organs exists in the brutes just as it does in us, we jumped to the conclusion that the other principle, which consists in mind or thought, also exists in them. Things which we have become convinced of since our earliest years, even though they have subsequently been shown by rational arguments to be false, cannot easily be eradicated from our beliefs unless we give the relevant arguments our long and frequent attention.

From Objection IV (Arnauld): (Argument from Knowledge; substance)

This is certainly very acute. But someone is going to bring up the objection which the author raises against himself: the fact that I have doubts about the body, or deny that it exists, does not bring it about that no body exists. 'Yet may it not perhaps be the case that these very things which I am supposing to be nothing, because they are unknown to me, are in reality identical with the "I" of which I am aware? I do not know,' he says 'and for the moment I shall not argue the point. I know that I exist; the question is, what is this "I" that I know? If the "I" is understood strictly as we have been taking it, then it is quite certain that knowledge of it does not depend on things of whose existence I am as yet unaware.'

But the author admits that in the argument set out in the Discourse on the Method the proof excluding anything corporeal from the nature of the mind was not put forward 'in an order corresponding to the actual truth of the matter' but merely in an order corresponding to his 'own perception'. So the sense of the passage was that he was aware of nothing at all which he knew belonged to his essence except that he was a thinking thing. From this answer it is clear that the objection still stands in precisely the same form as it did before, and that the question he promised to answer still remains outstanding: How does it follow, from the fact that he is aware of nothing else belonging to his essence, that nothing else does in fact belong to it? I must confess that I am somewhat slow, but I have been unable to find anywhere in the Second Meditation an answer to this question. As far as I can gather, however, the author does attempt a proof of this claim in the Sixth Meditation, since he takes it to depend on his having clear knowledge of God, which he had not yet arrived at in the Second Meditation. . . .
Suppose someone knows for certain that the angle in a semi-circle is a right angle, and hence that the triangle formed by this angle and the diameter of the circle is right-angled. In spite of this, he may doubt, or not yet have grasped for certain, that the square on the hypotenuse is equal to the squares on the other two sides; indeed he may even deny this if he is misled by some fallacy. But now, if he uses the same argument as that proposed by our illustrious author, he may appear to have confirmation of his false belief, as follows: 'I clearly and distinctly perceive', he may say, 'that the triangle is right-angled; but I doubt that the square on the hypotenuse is equal to the squares on the other two sides; therefore it does not belong to the essence of the triangle that the square on its hypotenuse is equal to the squares on the other sides.'

Descartes’s Response:

But I will begin by pointing out where it was that I embarked on proving 'how, from the fact that I am aware of nothing else belonging to my essence (that is, the essence of the mind alone) apart from the fact that I am a thinking thing, it follows that nothing else does in fact belong to it'. The relevant passage is the one where I proved that God exists - a God who can bring about everything that I clearly and distinctly recognize as possible.

Now it may be that there is much within me of which I am not yet aware (for example, in this passage I was in fact supposing that I was not yet aware that the mind possessed the power of moving the body, or that it was substantially united to it). Yet since that of which I am aware is sufficient to enable me to subsist with it and it alone, I am certain that I could have been created by God without having these other attributes of which I am unaware, and hence that these other attributes do not belong to the essence of the mind.

For if something can exist without some attribute, then it seems to me that that attribute is not included in its essence. And although mind is part of the essence of man, being united to a human body is not strictly speaking part of the essence of mind.

I must also explain what I meant by saying that 'a real distinction cannot be inferred from the fact that one thing is conceived apart from another by an abstraction of the intellect which conceives the thing inadequately. It can be inferred only if we understand one thing apart from another completely, or as a complete thing.'

I do not, as M. Arnauld assumes, think that adequate knowledge of a thing is required here. Indeed, the difference between complete and adequate knowledge is that if a piece of knowledge is to be adequate it must contain absolutely all the properties which are in the thing which is the object of knowledge. Hence only God can know that he has adequate knowledge of all things.

A created intellect, by contrast, though perhaps it may in fact possess adequate knowledge of many things, can never know it has such knowledge unless God grants it a special revelation of the fact. In order to have adequate knowledge of a thing all that is required is that the power of knowing possessed by the intellect is adequate for the thing in question, and this can easily occur. But in order for the intellect to know it has such knowledge, or that God put nothing in the thing beyond what it is aware of, its power of knowing would have to equal the infinite power of God, and this plainly could not happen on pain of contradiction.

Now in order for us to recognize a real distinction between two things it cannot be required that our knowledge of them be adequate if it is impossible for us to know that it is adequate. And since, as has just been explained, we can never know this, it follows that it is not necessary for our knowledge to be adequate.

Hence when I said that 'it does not suffice for a real distinction that one thing is understood apart from another by an abstraction of the intellect which conceives the thing inadequately', I did not think this would be taken to imply that adequate knowledge was required to establish a real distinction. All I meant was that we need the sort of knowledge that we have not ourselves made inadequate by an abstraction of the intellect.

There is a great difference between, on the one hand, some item of knowledge being wholly adequate, which we can never know with certainty to be the case unless it is revealed by God, and, on the other hand, its being adequate enough to enable us to perceive that we have not rendered it inadequate by an abstraction of the intellect.

In the same way, when I said that a thing must be understood completely, I did not mean that my understanding must be adequate, but merely that I must understand the thing well enough to know that my understanding is complete.

I thought I had made this clear from what I had said just before and just after the passage in question. For a little earlier I had distinguished between 'incomplete' and 'complete' entities, and I had said that for there to be a real distinction between a number of things, each of them must be understood as 'an entity in its own right which is different from everything else'.

And later on, after saying that I had 'a complete understanding of what a body is', I immediately added that I also 'understood the mind to be a complete thing'. The meaning of these two phrases was identical; that is, I took 'a complete understanding of something' and 'understanding something to be a complete thing' as having one and the same meaning.

But here you may justly ask what I mean by a 'complete thing', and how I prove that for establishing a real distinction it is sufficient that two things can be understood as 'complete' and that each one can be understood apart from the other.

My answer to the first question is that by a 'complete thing' I simply mean a substance endowed with the forms or attributes which enable me to recognize that it is a substance.

We do not have immediate knowledge of substances, as I have noted elsewhere. We know them only by perceiving certain forms or attributes which must inhere in something if they are to exist; and we call the thing in which they inhere a 'substance'.

But if we subsequently wanted to strip the substance of the attributes through which we know it, we would be destroying our entire knowledge of it. We might be able to apply various words to it, but we could not have a clear and distinct perception of what we meant by these words.

I am aware that certain substances are commonly called 'incomplete'. But if the reason for calling them incomplete is that they are unable to exist on their own, then I confess I find it self-contradictory that they should be substances, that is, things which subsist on their own, and at the same time incomplete, that is, not possessing the power to subsist on their own. It is also possible to call a substance incomplete in the sense that, although it has nothing incomplete about it qua substance, it is incomplete in so far as it is referred to some other substance in conjunction with which it forms something which is a unity in its own right.

Thus a hand is an incomplete substance when it is referred to the whole body of which it is a part; but it is a complete substance when it is considered on its own. And in just the same way the mind and the body are incomplete substances when they are referred to a human being which together they make up. But if they are considered on their own, they are complete.

For just as being extended and divisible and having shape etc. are forms or attributes by which I recognize the substance called body, so understanding, willing, doubting etc. are forms by which I recognize the substance which is called mind. And I understand a thinking substance to be just as much a complete thing as an extended substance.

From Objection III (Hobbes): (Thinking Substance)

On the Second Meditation ('The nature of the human mind')

I am a thinking thing.

Correct. For from the fact that I think, or have an image (whether I am awake or dreaming), it can be inferred that I am thinking; for 'I think' and 'I am thinking' mean the same thing. And from the fact that I am thinking it follows that I exist, since that which thinks is not nothing. But when the author adds 'that is, I am a mind, or intelligence, or intellect or reason', a doubt arises. It does not seem to be a valid argument to say 'I am thinking, therefore I am thought' or 'I am using my intellect, hence I am an intellect.' I might just as well say 'I am walking, therefore I am a walk.' M. Descartes is identifying the thing which understands with intellection, which is an act of that which understands. Or at least he is identifying the thing which understands with the intellect, which is a power of that which understands. Yet all philosophers make a distinction between a subject and its faculties and acts, i.e. between a subject and its properties and its essences: an entity is one thing, its essence is another. Hence it may be that the thing that thinks is the subject to which mind, reason or intellect belong; and this subject may thus be something corporeal. The contrary is assumed, not proved. Yet this inference is the basis of the conclusion which M. Descartes seems to want to establish.

In the same passage we find the following: 'I know I exist; the question is, what is this "I" that I know. If the "I" is understood strictly as we have been taking it, then it is quite certain that knowledge of it does not depend on things of whose existence I am as yet unaware.'

It is quite certain that the knowledge of the proposition 'I exist' depends on the proposition 'I am thinking' as the author himself has explained to us. But how do we know the proposition 'I am thinking'? It can only be from our inability to conceive an act without its subject. We cannot conceive of jumping without a jumper, of knowing without a knower, or of thinking without a thinker.

It seems to follow from this that a thinking thing is something corporeal. For it seems that the subject of any act can be understood only in terms of something corporeal or in terms of matter, as the author himself shows later on his example of the wax: the wax, despite the changes in its colour, hardness, shape and other acts, is still understood to be the same thing, that is, the same matter that is the subject of all these changes. Moreover, I do not infer that I am thinking by means of another thought. For although someone may think that he was thinking (for this thought is simply an act of remembering), it is quite impossible for him to think that he is thinking, or to know that he is knowing. For then an infinite chain of questions would arise: 'How do you know that you know that you know . . . ?'

The knowledge of the proposition 'I exist' thus depends on the knowledge of the proposition 'I am thinking'; and knowledge of the latter proposition depends on our inability to separate thought from the matter that is thinking. So it seems that the correct inference is that the thinking thing is material rather than immaterial.

Descartes’s Response:

When I said 'that is, I am a mind, or intelligence, or intellect or reason', what I meant by these terms was not mere faculties, but things endowed with the faculty of thought. This is what the first two terms are commonly taken to mean by everyone; and the second two are often understood in this sense. I stated this point so explicitly, and in so many places, that it seems to me there was no room for doubt.

There is no comparison here between 'a walk' and 'thought'. 'A walk' is usually taken to refer simply to the act of walking, whereas 'thought' is sometimes taken to refer to the act, sometimes to the faculty, and sometimes to the thing which possesses the faculty.

I do not say that the thing which understands is the same as intellection. Nor, indeed, do I identify the thing which understands with the intellect, if 'the intellect' is taken to refer to a faculty; they are identical only if 'the intellect' is taken to refer to the thing which understands. Now I freely admit that I used the most abstract terms I could in order to refer to the thing or substance in question, because I wanted to strip away from it everything that did not belong to it. This philosopher, by contrast, uses absolutely concrete words, namely 'subject', 'matter' and 'body', to refer to this thinking thing, because he wants to prevent its being separated from the body.

But I am not afraid that anyone will think my opponent's method is better suited to the discovery of the truth than my own; for his method lumps together a large number of different items, whereas I aim to distinguish each individual item as far as I can. But let us stop talking about terminology and discuss the issue itself.

'It may be', he says, 'that the thing that thinks is something corporeal. The contrary is assumed, not proved.' But I certainly did not assume the contrary, nor did I use it as the 'basis' of my argument. I left it quite undecided until the Sixth Meditation, where it is proved.

He is quite right in saying that 'we cannot conceive of an act without its subject'. We cannot conceive of thought without a thinking thing, since that which thinks is not nothing. But he then goes on to say, quite without any reason, and in violation of all usage and all logic: 'It seems to follow from this that a thinking thing is something corporeal.' It may be that the subject of any act can be understood only in terms of a substance (or even, if he insists, in terms of 'matter', i.e. metaphysical matter); but it does not follow that it must be understood in terms of a body.

Logicians, and people in general, normally say that some substances are spiritual and some are corporeal. All that I proved with the example of the wax was that colour, hardness and shape do not belong to the formal concept of the wax itself. I was not dealing in that passage with the formal concept of the mind or even with that of the body.

It is irrelevant for the philosopher to say that one thought cannot be the subject of another thought. For who, apart from him, ever supposed that it could be? If I may briefly explain the point at issue: it is certain that a thought cannot exist without a thing that is thinking; and in general no act or accident can exist without a substance for it to belong to. But we do not come to know a substance immediately, through being aware of the substance itself; we come to know it only through its being the subject of certain acts. Hence it is perfectly reasonable, and indeed sanctioned by usage, for us to use different names for substances which we recognize as being the subjects of quite different acts or accidents. And it is reasonable for us to leave until later the examination of whether these different names signify different things or one and the same thing. Now there are certain acts that we call 'corporeal', such as size, shape, motion and all others that cannot be thought of apart from local extension; and we use the term 'body' to refer to the substance in which they inhere. It cannot be supposed that one substance is the subject of shape, and another substance is the subject of local motion etc., since all these acts fall under the common concept of extension. There are other acts which we call 'acts of thought', such as understanding, willing, imagining, having sensory perceptions, and so on: these all fall under the common concept of thought or perception or consciousness, and we call the substance in which they inhere a 'thinking thing' or a 'mind'. We can use any other term you like, provided we do not confuse this substance with corporeal substance. For acts of thought have nothing in common with corporeal acts, and thought, which is the common concept under which they fall, is different in kind from extension, which is the common concept of corporeal acts. Once we have formed two distinct concepts of these two substances, it is easy, on the basis of what is said in the Sixth Meditation, to establish whether they are one and the same or different.

From Objection VI (Mersenne): (Mind cannot be matter)

To come to the second difficulty, when you say you are thinking and that you exist, someone might maintain that you are mistaken, and are not thinking but are merely in motion, and that you are nothing else but corporeal motion. For no one has yet been able to grasp that demonstration of yours by which you think you have proved that what you call thought cannot be a kind of corporeal motion. Have you used your method of analysis to separate off all the motions of that rarefied matter of yours? Is this what makes you so certain? And can you therefore show us (for we will give our closest attention and our powers of perception are, we think, reasonably keen) that it is self-contradictory that our thoughts should be reducible to these corporeal motions?

Descartes’s Response:

2. When someone notices that he is thinking, then, given that he understands what motion is, it is quite impossible that he should believe that he is mistaken and is 'not thinking but merely in motion'. Since the idea or notion which he has of thought is quite different from his idea of corporeal motion, he must necessarily understand the one as different from the other. Because, however, he is accustomed to attribute many different properties to one and the same subject without being aware of any connection between them, he may possibly be inclined to doubt, or may even affirm, that he is one and the same being who thinks and who moves from place to place. Notice that if we have different ideas of two things, there are two ways in which they can be taken to be one and the same thing: either in virtue of the unity or identity of their nature, or else merely in respect of unity of composition. For example, the ideas which we have of shape and of motion are not the same, nor are our ideas of understanding and volition, nor are those of bones and flesh, nor are those of thought and of an extended thing. But nevertheless we clearly perceive that the same substance which is such that it is capable of taking on a shape is also such that it is capable of being moved, and hence that that which has shape and that which is mobile are one and the same in virtue of a unity of nature. Similarly, the thing that understands and the thing that wills are one and the same in virtue of a unity of nature. But our perception is different in the case of the thing that we consider under the form of bone and that which we consider under the form of flesh; and hence we cannot take them as one and the same thing in virtue of a unity of nature but can regard them as the same only in respect of unity of composition - i.e. in so far as it is one and the same animal which has bones and flesh. But now the question is whether we perceive that a thinking thing and an extended thing are one and the same by a unity of nature. That is to say, do we find between thought and extension the same kind of affinity or connection that we find between shape and motion, or understanding and volition? Alternatively, when they are said to be 'one and the same' is this not rather in respect of unity of composition, in so far as they are found in the same man, just as bones and flesh are found in the same animal? The latter view is the one I maintain, since I observe a distinction or difference in every respect between the nature of an extended thing and that of a thinking thing, which is no less than that to be found between bones and flesh.

However, you go on to say that no one has been able to grasp this demonstration of mine. In case this appeal to authority may prejudice the truth, I am compelled to reply that even though not many people have yet examined the demonstration, there are nevertheless several who affirm that they understand it. One witness who has sailed to America and says that he has seen the antipodes deserves more credence than a thousand others who deny their existence merely because they have no knowledge of them. And similarly, those who give due consideration to the true force of an argument will have more respect for the authority of one person who says that he has understood a proof correctly, than they will accord to a thousand others who claim, without providing any argument to back up their case, that it cannot be understood by anyone. For the fact that such people fail to understand the argument themselves does not prevent anyone else's understanding it; indeed, the very fact that they infer its general unintelligibility from their own failure to understand it shows that their reasoning is careless, and that they do not deserve to have their views accepted.

Lastly, my critics ask whether I have used my method of analysis to separate off all the motions of that rarefied matter of mine. Is this (they ask) what makes me certain? And can I therefore show my critics, who are most attentive and (they think) reasonably perceptive men, that it is self-contradictory that our thought should be reduced to corporeal motions? By 'reduced' I take it that they mean that our thought and corporeal motions are one and the same. My reply is that I am very certain of this point, but I cannot guarantee that others can be convinced of it, however attentive they may be, and however keen, in their own judgement, their powers of perception may be. I cannot guarantee that they will be persuaded, at least so long as they focus their attention not on things which are objects of pure understanding but only on things which can be imagined. This mistake has obviously been made by those who have imagined that the distinction between thought and motion is to be understood by making divisions within some kind of rarefied matter. The only way of understanding the distinction is to realize that the notions of a thinking thing and an extended or mobile thing are completely different, and independent of each other; and it is self-contradictory to suppose that things that we clearly understand as different and independent could not be separated, at least by God. Thus, however often we find them in one and the same subject - e.g. when we find thought and corporeal motion in the same man - we should not therefore think that they are one and the same in virtue of a unity of nature, but should regard them as the same only in respect of unity of composition.