Principles of Philosophy
Similar to Discourse, the cogito statement appears twice (in B1 and B2) in Principles:
B1: (Art. VII) In rejecting […] everything which we can in any way doubt, it is easy for us to suppose that there is no God and no heaven, and that there are no bodies, and even that we ourselves have no hands or feet, or indeed any body at all. But we cannot for all that suppose that we, who are having such thoughts, are nothing. For it is a contradiction to suppose that what thinks does not, at the very time when it is thinking, exist. Accordingly, this piece of knowledge ‘I am thinking, therefore I exist’ is the first and most certain of all to occur to anyone who philosophizes in an orderly way. (Art. VIII) This is the best way to discover the nature of the mind and the distinction between the mind and the body. For if we, who are supposing that everything which is distinct from us is false, examine what we are, we see very clearly that neither extension nor shape nor local motion, nor anything of this kind which is attributable to a body, belongs to our nature, but that thought alone belongs to it. (P.Ph., AT-VIII, 7; CSM-I, 195)
B2: (Art. X) And when I said that the proposition ‘I am thinking, therefore I exist’ is the first and most certain of all to occur to anyone who philosophizes in an orderly way, I did not in saying that deny that one must first know what thought, existence and certainty are, and that it is impossible that that which thinks should not exist, and so forth. But because these are very simple notions, and ones which on their own provide us with no knowledge of anything that exists, I did not think they needed to be listed. (ibid., AT-VIII, 8; CSM-I, 196)
The construction of the cogito argument in Principles is the same as the one in Discourse, which I have examined above. In B1 Descartes acquires ‘the piece of knowledge “I am thinking, therefore I exist”’ (art. VII), and determines the ‘I’ as that which the thought alone belongs to (art. VIII); that is, B1 is fully sufficient to explain the essence of the cogito (that even if one can doubt everything, one cannot doubt the existence of the ‘I’, who doubts everything, and that the ‘I’ is a substance whose nature is simply to think and does not depend on any material things). The problem, then, is how we should read B2.
We must note that the present tense is generally used in Principles, but the past tense is used in B2. It is in his reply to the Sixth Objection that Descartes ‘did not […] deny that one must first know what thought, existence and certainty are, and that it is impossible that that which thinks should not exist, and so forth’. The Sixth Objection states:
[F]rom 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. (6ae Obj., AT-VII, 413; CSM-II, 278)