... one is thinking, and thus one can neither doubt that one is not thinking nor not existing (at least as long as the proposition is being considered). For this reason, it can be said to be very effective at securing knowledge of oneself: even if a sceptic were to claim that the malicious demon could influence the mind as well, Descartes could reply that even if he were to doubt that he exists, then he will still be existing, because the act of thinking requires existence. Moreover, as Descartes points out when he begins to contemplate the evil demon hypothesis, for the deceiver to be effective, there would have to be someone to deceive. The cogito can therefore withstand such criticisms: as a result, the knowledge that his mental (i.e., his non-corporeal self) is secure. There has been much debate as to the way Descartes formulates the cogito, and whether it ...

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