Reasoning 'I existed last Tuesday because I recall thinking on that day' is unreliable, since one might merely be dreaming about that occasion.

Second, a present tense formulation is essential to the certainty of the cogito. It’s no good to reason that “I existed last Tuesday, since I recall that I was thinking on that day.” For all I know, I’m now merely dreaming about that occasion. Nor does it work to reason that “I’ll continue to exist, since I’m now thinking.” As the meditator remarks, “it could be that were I totally to cease from thinking, I should totally cease to exist” (Med. 2, AT 7:27, CSM 2:18). The privileged certainty of t