"The Nature of the Human Mind: That It Is Better Known Than The Body"
Descartes begins Meditation II with the supposition from Meditation I:
"I suppose that everything I see is false. I believe that none of what my deceitful memory represents ever existed. I have no senses whatsoever. Body, shape, extension, movement, and place are all chimeras." (p. 30)
As I have already pointed out, it is not certain if this supposition is supposed to extend to the truths of arithmetic and geometry.
Descartes now argues that even if he supposes that all of his previously held beliefs are false (and not merely doubtful), it transpires that there is at least one belief that must be true.
"Is it then the case that I too do not exist?... there is some deceiver or other who is supremely powerful and supremely sly and who is always deliberately deceiving me. Then too there is no doubt that I exist, if he is deceiving me. And let him do his best at deception, he will never bring it about that I am nothing so long as I shall think that I am something. Thus, after everything has been most carefully weighed, it must finally be established that this pronouncement "I am, I exist" [ego sum, ego existo] is necessarily true every time I utter it or conceive it in my mind." (p. 30)
The Existence Argument may be put as follows:
(1) If I am not being deceived, then I exist.
(2) If I am being deceived, then I exist.
(3) So, if I am either being deceived or not being deceived, then I exist.
(4) I am either being deceived or not being deceived.
> I exist.
This argument is valid. The question remains whether it is sound, that is, whether all four premises are true. Premise (4) is simply a logical truth. Premise (3) is validly derived from premises (1) and (2), so if they are both true, then it is true also. Premise (1) would appear to be true.
The contentious premise, then, is premise (2).
(Note that premise (2) was not original to Descartes. St. Augustine (354-430) also argued "fallor ergo sum": "I am being deceived, therefore I exist". However, it is important to see that Descartes's premise is here presented as hypothetical: IF I am being deceived, THEN I exist.)
Here is perhaps an argument for premise (2).
(1) If I am being deceived, then I am thinking.
(2) If I am thinking, then I exist.
> If I am being deceived, then I exist.
Again, this argument is perfectly valid. The question that remains is whether it is sound. The possibly contentious premise here is (2).
Descartes, however, is fully confident that premise (2) is true. It cannot be the case, according to Descartes, that I can be thinking but not exist. In order for me to be thinking, I must exist.
(Note: this is not the same as: In order for me to exist, I must be thinking. It might be the case that I could exist and not be thinking. But it can't be the case that I am thinking and don't exist.)
However, premise (2) is merely a hypothetical claim. It merely says that IF I am thinking, THEN I exist. This makes it less contentious.
What is much more contentious is the following argument:
(1) If I am thinking, then I exist.
(2) I am thinking.
> I exist.
This argument, in premise (2) and its conclusion, contains the famous "I am thinking, therefore I am" (cogito, ergo sum) argument, which, however, is not stated in Meditation II (it can be found in the Replies to the Second Set of Objections: "ego cogito, ergo sum, sive existo" ("I am thinking, therefore I am, or exist")).
However, is Descartes entitled to premise (2), the claim that he is thinking? That is, is he entitled to the claim that there is an "I" that is thinking? How does he know this? Even if he is entitled to the claim that thinking is occurring, i.e. that a thought or thoughts exist (i.e. that some thoughts or other exist), how does he know that there is an "I" that is having these thoughts?
This is not the dogmatic argument that: There is no "I". It is simply the speculative argument: How does he know that there is an "I", as opposed to simply thoughts, but no "I" that has them?
If the claim is restricted to the claim that "There is a thought", or "A thought exists", or "A thought is occurring", the argument would be as follows:
(1) If a thought exists, then I exist.
(2) A thought exists.
> I exist.
But now the problem is that premise (1) is pretty obviously false. Just because some thought or other exists, nothing follows about my existence. If YOU have a thought, it doesn't follow that I exist.
Mark Rowlands, following Nietzsche (really the argument was given by another philosopher, Georg Lichtenberg, in the 18th century), argues: "Perhaps there are just thoughts, and no person to whom the thoughts attach." (p. 45) Perhaps, that is, there are 'ownerless' thoughts. This may be understood in at least two ways. First of all, there may be a bundle of thoughts, but no mind that has the bundle. Secondly, there may not even be a bundle of thoughts, but just 'free-floating' thoughts, and no mind. Note that the argument is not that this is the case. The argument is only that this might be the case. The argument is only that "Perhaps" there is no mind. That is all that Lichtenberg-Nietzsche-Rowlands needs to undermine Descartes's argument.
However, this argument can in turn be countered with the argument that it is simply not possible for thoughts to exist without minds (or souls) having them. Thoughts, on this argument, are the acts of minds. If there is no mind, there is no act of the mind, i.e. no thought. Hence we could argue as follows:
(1) If a thought exists, then a mind exists.
(2) A thought exists.
> A mind exists.
However, Descartes wants to argue not merely that some thought or other exists, and hence, that some mind or other exists. He wants to argue that the thought that exists is his thought, and hence, that his mind exists.
(1) A thought exists.
(2) If a thought exists, then a mind exists.
(3) The thought that exists is a thought of my mind.
> My mind exists.
This argument seems question-begging. If Descartes already knows that the thought that exists is his own thought, then he already knows that it is a thought of his own mind, and hence, he already knows that his own mind exists.
Descartes's argument, really, is as follows:
My mind exists (or I exist) because my thoughts exist.
The point is not that his claim that "My mind exists" (or his claim that "I exist") is false. The point is not that "my thoughts exist" is false either. These claims are, more than likely, true. The point is that it is trivial that if the existing thoughts are my thoughts, then my mind exists. What Descartes needs to prove is that the existing thoughts are my thoughts. He cannot prove that "my thoughts exist" is true. If he simply assumes that "my thoughts exist" is true, then he hasn't proven anything. He hasn't proven that "I exist" is true.