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Question On 'Cogito Ergo Sum' If this statement means that the only thing I can know to be true is that I exist, then that means I don't know if the reasoning used to deduce this statement is logically sound. What evidence do we have that our reasoning is to be believed? The only reason that we trust our reasoning is because have reasoned that it is trustworthy. We trust our reasoning because we trust our reasoning. I know that I came to this conclusion with the same human logic as cogito ergo sum, so this conclusion must be equally invalid. Humans are imperfect->humans 'invented' logic-> logic is not necessarily perfect. "I do not know if I know anything." Please fix any broken logic I have, or point me in the direction of relevant articles on how my thinking was outdone hundreds of years ago. Thanks

Accepted:
January 30, 2014

Comments

I think I could do little better than to point you to what Descartes himself says about this. It's quite right that, in the First Meditation, he does seem to bring even logical (or mathematical) reasoning into the scope of his sceptical doubt: "What is more, since sometimes I believe that others go astray in cases where they think they have the most perfect knowledge, may I not similarly go wrong every time I add two and three or count the sides of a square, or in some even simpler matter, if that is imaginable?" (CSM II, 14; AT VII, 21). And then, at the start of the Second Meditation, we get the Cogito which, on the face of it, certainly seems to have the form of a logical deduction of existence from thought. So is he really entitled to it at all? Couldn't he similarly go wrong there too?

Well, Descartes would say no, and the clue to his solution is to be found in the Second Replies: "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 recognises it as something self-evident by a simple intuition of the mind." (CSM II, 100; AT VII, 140). If the Cogito were a complex of two different elements ("I think", "I am"), perhaps linked by some general principle ("Everything that thinks, is"), then maybe it could be disrupted in the transition between those two elements, in just the same way as that arithmetical calculation could apparently be disrupted in the transition between "2+3" and "5". But, the grammatical structure of the sentence notwithstanding, he denies that the thought itself actually has any such complex form. It's not really a matter of reasoning at all, but rather just one simple truth of which I am immediately and incorrigibly aware: I am thinking. And, because there's no complexity there, no connections to undermine, there's nowhere that an evil demon (or whatever) could swoop in and knock things off course.

I don't mean to criticize Prof. Reid's excellent scholarly response on behalf of Descartes. But it's worth pointing out that the reasoning from the Second Replies that he attributes to Descartes is more complex and dubitable than the inference from 'I think' to 'I exist' is to begin with. In the quoted passage, Descartes seems to make a universal generalization about human psychology based on a single known case, his own. That generalization can't be more reliable than inferring 'I exist' from 'I think', or else we wouldn't need an empirical science of psychology. Likewise, the psychological claim that a two-step inference is always less reliable than a one-step thought is a claim that's got to be more dubitable than inferring 'I exist' from 'I think' or calculating the sum of 2 and 3. It would have been better had Descartes denied the very intelligibility of doubting the simplest inferences we make -- or at least had he challenged a skeptical opponent to make sense of such doubt.