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Does thinking imply existing? Descartes argues yes: it is impossible for anything to think which does not exist. Does existing imply thinking? Most people would say no. Most would say that a rock exists, and the rock does not think. Therefore it is possible for something to exist which doesn't think. As such, the basic claim doesn't go both ways. ...


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Perhaps what would work is cogito ⇔ cogito sum, that is, "I think if and only if I think I am". Although someone who thinks may not think of asking themselves whether they are or not, if someone asked them whether they were or not I imagine they would not deny that they are. Having a biconditional is not what makes a tautology. All one needs is a ...


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There's a modern version of the argument, from Kripke in Naming and Necessity. The idea is that identity is a necessary relation, so that if x = y, then necessarily x = y. Put in the language of possible worlds, we can say that in every possible world, x exists if and only if y exists, and in every possible world where they both exist they must have exactly ...


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If you say "I exist", then you are thinking. It is that thinking that makes you sure you exist. That was Descartes point. Changing the order of the words of his thoughts doesn't make your thoughts any less thought-ish.


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