system
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human
Is "Cogito ergo sum" a logical fallacy?

Hi, this is my first question here, hope I don't break any rules.

I think I've heard that Descartes' famous "Cogito ergo sum" ("I think, therefore I am") was recognised as a logical fallacy already in his own day. I believe the problem is supposed to be that it's circular to some extent.

What are your views on this? What problems does the claim have, and is there any way to save it? If so, how?

Personally and at first glance, I think it seems fairly clear that there must be some kind of "I"-identity that thinks, but the whole thing also feels a bit circular in some vague way.

Thank you.
gpt
It’s not on the face of it fallacious. Moreover circular arguments are always valid. They’re just unhelpful and unpersuasive because you have to already endorse the conclusion to accept all the premises. 

There have been objections on the grounds of circularity (some says he goes to far to assume that it is him doing the thinking and that rather we should argue something like “there is thinking going on so something exists”) but that wouldn’t amount to a fallacious use of logic. The charge here is that he’s already assuming that the self exists by stating that the self is thinking. If he was doubtful of his own existence he would instead only posit something like “there is thinking going on”.