Descartes on Mind
In Meditation II Descartes does not merely establish (or try to establish) that "I exist" is always true when it is thought. He also investigates what the "I" is in "I exist". He reaches the following conclusion about this "I" that is him:
"What about thinking? Here I make my discovery: thought exists; it alone cannot be separated from me. I am; I exist this is certain. But for how long? For as long as I am thinking; for perhaps it could also come to pass that if I were to cease all thinking I would then utterly cease to exist. At this time I admit nothing that is not necessarily true. I am therefore precisely nothing but a thinking thing; that is a mind, or intellect, or understanding, or reason words of whose meanings I was previously ignorant. Yet I am a true thing and am truly existing; but what kind of thing? I have said it already: a thinking thing." (p. 31)
"I know that I exist; I ask now who is this "I" whom I know? Most certainly, in the strict sense the knowledge of this "I" does not depend upon things whose existence I do not yet have any knowledge." (p. 31)
"But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wills, refuses, and that also imagines and senses." (p. 31)
Descartes here argues that the "I" in "I exist" is a thinking thing, or res cogitans. Now, there is at least one question that may be asked here, namely, is Descartes saying:
(a) I am only a thinking thing;
(b) I am at least a thinking thing, and perhaps something else as well.
Both claims entail that thinking is to be identified with him, such that, if one were to remove thinking, one would remove him. The first, however, goes further. It entails that nothing else is to identified with him. The second does not go so far.
Does Descartes have an argument for claim (a)? It might be that he believes that he has the following argument:
(1) I cannot doubt that I am a thing that thinks (a mind).
(2) I can doubt that I am an extended thing (a body).
(3) Whatever I cannot doubt that I am, I am.
(4) Whatever I can doubt that I am, I am not.
> I am not an extended thing (a body).
> I am a thinking thing (a mind).
The problem with this argument is that it premise (4) appears to be false. Even if I can doubt that I am something, it doesn't follow that I am not that something. I can doubt that I am someone's son, and still be his son. I can doubt that I am the winner, and still be the winner.
However, it may be that at this stage of the Meditations, Descartes is not actually advancing an argument for Cartesian Substance Dualism as such, but is merely sketching out the two different kinds of substance that will later be said to compose the world -- thinking things, or minds/souls, and extended things, or bodies.
One point should be made about the different kinds of thought that Descartes outlines:
(a) Doubts, understands, affirms, denies, wills, refuses
(b) Imagines and senses
Descartes does not say that 'imagines' and 'senses' are forms of thought exactly like the others. This is because these two forms of thought are the product of the union of the mind and the body, and not the product of the mind alone.
Descartes cannot yet be certain if there are any bodies in existence. Since one cannot 'sense' unless there is body present (otherwise it is a dream or a hallucination or a mirage or an illusion), and since to 'imagine' is actually to inspect an image produced by the brain, which is body, it is difficult at this stage of the Meditations to be positive about either sense perception or imagination. Therefore he is very careful to say that even if there are no bodies in existence, nevertheless, imagining and sensing are kinds of thinking. He 'slices off' the thinking component from imagining and sensing, and says that considered in this way, they are indeed forms of thinking even if there are no bodies in existence.
"But indeed it is also the same "I" who imagines; for although perhaps, as I supposed before, absolutely nothing that I imagined is true, still the very power of imagining really does exist, and constitutes a part of my thought. Finally, it is this same "I" who senses or who is cognizant of bodily things as if through the senses. For example, I now see a light, I hear a noise, feel heat. These things are false, since I am asleep. Yet I certainly do seem to see, hear, and feel warmth. This cannot be false. Properly speaking, this is what in me is called "sensing". But this, precisely taken, is nothing other than thinking." (p. 32)
So if we restrict sense perceptions and imaginations to just "seemings", then they are indeed thoughts.