"Cogito, ergo sum", "I think, therefore I am", said Descartes but how does one know that one thinks? Could "my" thoughts not be part of an illusion of someone/something else? Can "I" actually know that "I" exist?

I apologise if this question has already been answered (didn't find anything here or Google) or makes no sense and if my English is poor.

  • Your thoughts do not belong to "I", they in fact are the function, product of something else - (your) consciousness. Reflective consciousness (i.e. consciousness supposing consciousness) sees the thought being thought as an object, and it produces an illusory second object "who must be thinking it", the Ego (I).
    – ttnphns
    Commented Jan 21, 2018 at 21:17

2 Answers 2


Relevant SE link: "I think, therefore I am" - How does "I" establish "I" before "I" can "think"?

how does one know that one thinks?

Because something that doesn't exist can't think. Therefore you must exist (or rather I must exist, as I can't say anything about you beyond what my senses tell me is the case). If I didn't exist, then I couldn't be thinking. It is an absolute incoherent statement to say that I can think and not exist at the same time. We can't even begin to try to give a rational argument to justify such a position.

What form I exist isn't really knowable with certainty. Am I part of a dream? Maybe. Or perhaps I am a brain in a vat or in a supercomputer simulation of our universe, and my essence consists of nothing more than binary coding. Entirely possible.

What Descartes wanted to do with The Cogito was to start with a foundational philosophy rooted in the certainty he saw in self-doubt. Using it as an edifice to establish the rest of his philosophy. A lot of it was supported by the Christian God not being an "evil deceiver" tricking him into believing he was experiencing what he was experiencing through his perceptual apparatus.


Brueckner, Tony, "Skepticism and Content Externalism", The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/win2016/entries/skepticism-content-externalism/.


Suppose Descartes' Evil Demon exists. Why can't I and my thoughts be merely dreamt or fantasied by the Evil Demon ? I can imagine, now, a person, X, who in my imagination is thinking all sorts of things. Only, of course there is no X and X is not thinking anything. X is purely a creation of my imagination. Why can't I and my thoughts be equally unreal products of the Evil Demon's dreams or imagination ?

Do not suppose that I endorse the existence of Descartes' Evil Demon (malin génie). I am only probing logical possibilities, or what I take to be such.

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