From Objection II (Mersenne): (Cogito, not an inference)Thirdly, you are not yet certain of the existence of God, and you say that you are not certain of anything, and cannot know anything clearly and distinctly until you have achieved clear and certain knowledge of the existence of God. It follows from this that you do not yet clearly and distinctly know that you are a thinking thing, since, on your own admission, that knowledge depends on the clear knowledge of an existing God; and this you have not yet proved in the passage where you draw the conclusion that you clearly know what you are.

Descartes’s Response:

Thirdly, when I said that we can know nothing for certain until we are aware that God exists, I expressly declared that I was speaking only of knowledge of those conclusions which can be recalled when we are no longer attending to the arguments by means of which we deduced them. Now awareness of first principles is not normally called 'knowledge' by dialectitians. And when we become aware that we are thinking things, this is a primary notion which is not derived by means of any syllogism. When someone says 'I am thinking, therefore I am, or I exist', he does not deduce existence from thought by means of a syllogism, but recognizes it as something self-evident by a simple intuition of the mind. This is clear from the fact that if he were deducing it by means of a syllogism, he would have to have had previous knowledge of the major premiss 'Everything which thinks is, or exists'; yet in fact he learns it from experiencing in his own case that it is impossible that he should think without existing. It is in the nature of our mind to construct general propositions on the basis of our knowledge of particular ones.

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 V (Gassendi): (Cogito, only thought implies existence))

Turning to the Second Meditation, I see that you still persist with your elaborate pretence of deception, but you go on to recognize at least that you, who are the subject of this deception, exist. And thus you conclude that this proposition, I am, I exist, is true whenever it is put forward by you or conceived in your mind. But I do not see that you needed all this apparatus, when on other grounds you were certain, and it was true, that you existed. You could have made the same inference from any one of your other actions, since it is known by the natural light that whatever acts exists.

Descartes’s Response:
Again, what reason have you for saying that I 'did not need all this apparatus' to prove I existed? These very words of yours surely show that I have the best reason to think that I have not used enough apparatus, since I have not yet managed to make you understand the matter correctly. When you say that I 'could have made the same inference from any one of my other actions' you are far from the truth, since I am not wholly certain of any of my actions, with the sole exception of thought (in using the word 'certain' I am referring to metaphysical certainty, which is the sole issue at this point). I may not, for example, make the inference 'I am walking, therefore I exist', except in so far as the awareness of walking is a thought. The inference is certain only if applied to this awareness, and not to the movement of the body which sometimes - in the case of dreams - is not occurring at all, despite the fact that I seem to myself to be walking. Hence from the fact that I think I am walking I can very well infer the existence of a mind which has this thought, but not the existence of a body that walks. And the same applies in other cases.

From Objection VI (Mersenne): (Cogito, infinite regress of representations)

The first point is that from the fact that we are thinking it does not seem to be entirely certain that we exist. For in order to be certain that you are thinking you must know what thought or thinking is, and what your existence is; but since you do not yet know what these things are, how can you know that you are thinking or that you exist? Thus neither when you say 'I am thinking' nor when you add 'therefore, I exist' do you really know what you are saying. Indeed, you do not even know that you are saying or thinking anything, since this seems to require that you should know that you know what you are saying; and this in turn requires that you be aware of knowing that you know what you are saying, and so on ad infinitum. Hence it is clear that you cannot know whether you exist or even whether you are thinking.

Descartes’s Response:
1. It is true that no one can be certain that he is thinking or that he exists unless he knows what thought is and what existence is. But this does not require reflective knowledge, or the kind of knowledge that is acquired by means of demonstrations; still less does it require knowledge of reflective knowledge, i.e. knowing that we know, and knowing that we know that we know, and so on ad infinitum. This kind of knowledge cannot possibly be obtained about anything. It is quite sufficient that we should know it by that internal awareness which always precedes reflective knowledge. This inner awareness of one's thought and existence is so innate in all men that, although we may pretend that we do not have it if we are overwhelmed by preconceived opinions and pay more attention to words than to their meanings, we cannot in fact fail to have it. Thus when anyone notices that he is thinking and that it follows from this that he exists, even though he may never before have asked what thought is or what existence is, he still cannot fail to have sufficient knowledge of them both to satisfy himself in this regard.