For instance, if Descartes reasoned that knowing 1+3 with certainty was impossible, for an evil demon or other countless possibilities may manipulate his mind into believing it was indeed 5, shouldn't it be equally true that an evil demon forced his mind to believe that cogito ergo sum was also true, when perhaps, it is false?

  • Good observation, couldn't agree more. But old Rene pulled a fast one when he claimed that the only thing he could not doubt was the 'cogito'. Read that part again, break it down carefully so that you can respond to the comments you'll receive!
    – user37981
    Commented Oct 12, 2020 at 20:49
  • There can be NO DOUBT that the dreamer is one who must exist. Why? To claim non-existent things can dream would make no sense. Furthermore, science people & math people would say if x is non existent them any property I give x will be deemed false. All unicorns are white is deemed false because unicorns are nonexistent. Thus unicorns can't possible dream. If I am a person who can dream then I must be an existing thing. Well we have existing things that are not alive such as stop signs. I need to prove I am alive now. But dead things can't dream either. Inanimate things can't dream either.
    – Logikal
    Commented Oct 12, 2020 at 21:05
  • 5
    Descartes reasoned that one can not be deceived about own existence because said existence is a precondition for getting deceived. But yes, this is a common criticism of cogito. Peirce wrote, for example:"Here is a man who utterly disbelieves and almost denies the dicta of memory. He notices an idea, and then he thinks he exists. The ego of which he thinks is nothing but a holder together of ideas. But if memory lies there may be only one idea. If that one idea suggests a holder-together of ideas, how it can do so is a mystery".
    – Conifold
    Commented Oct 12, 2020 at 21:17
  • 3
    Does this answer your question? Could 'cogito ergo sum' possibly be false?
    – Kevin
    Commented Oct 12, 2020 at 21:31
  • 1
    @Nelson Alexander Spinoza, who was the stand out Descartes interpreter of his time, (see the 'Cogito Metaphysica'), vigorously denied that it is physically and intellectually possible to deny anything known with certainty. This is no minor claim. It calls into question any validity for Descartes skeptical position.
    – user37981
    Commented Oct 13, 2020 at 17:52

2 Answers 2


Descartes doesn't actually reason that nothing is knowable. In Meditation I he merely practises what is generally called methodological doubt. In constructing the foundations of knowledge he will not accept any belief if beliefs of that kind could be false. So he will not accept sense-based beliefs because the senses can deceive us. He will not believe that he is awake since when he is asleep he sometimes thinks he is awake, and when he is awake he thinks he is awake: so how does he know which state, awake or asleep, he is in? As for what he believes to be the truths of mathematics, how can he tell whether he knows these are truths and not merely matters which he psychologically cannot doubt, courtesy of the deceptions of the evil demon?

In spite of his language, Descartes does not in fact take the cogito as infallible knowledge even though he is certain of its truth as he is relatedly of the nature of the 'I' which exists - that it (he) is essentially a thing that thinks. He has been certain before - and wrong before. A deceiving God could have made him falsely certain - there is this bare, metaphysical posibility (Med III: tenuis et, ut ita loquar, Metaphysica dubitandi ratio est). So he sets out to discover whether there is or could be a deceiving God. 'For if I do not know this, it seems I can never be [justifiably] quite certain about anything else.'

He takes the cogito to be a clear and distinct idea, and he lays down a principle, a general rule (regula generali), in Meditation III that he can rely on any other ideas which are as clear and distinct as the cogito. 'I now seem to be able to lay it down as a general rule that whatever I perceive ... clearly and distinctly is true'. On this basis he 'proves' by what are evidently taken to be clear and distinct ideas the existence of a God who is no deceiver (Ex quibus satis patet illum fallacem esse non posse) - and God would be a deceiver if he did not guarantee the truth of clear and distinct ideas. The whole process is circular and is well-known as the 'Cartesian circle'. He uses clear and distinct ideas to prove the reliability of clear and distinct ideas.

So to underscore the point: Descartes does not 'exempt' the cogito, since the truth of the cogito has to be vindicated by demonstrating the existence of a non-deceiving God who underwrites the reliability of our clear and distinct ideas, of which the cogito is one (he thinks). That the whole process is circular, as described, is another matter. Since Descartes' general rule, 'that whatever I perceive ... clearly and distinctly is true', is also justified only by reference to God (Discourse, Part IV: it is a rule the truth of which is 'assured only for the reasons that God is or exists'), the circularity is reinforced.


Descartes, Meditations on First Philosophy, tr. J. Cottingham, Cambridge: CUP, 1996: 12-23 & 102-6.

Descartes, Discourse on the Method, tr. J. Cottingham et al., The Philosophical ritings of Descartes, I, Cambridge: CUP, 1985: 130.

Bernard Williams, Descartes: the Project of Pure Enquiry, London: Routledge, 2008: 93, 175-189.

  • Yup. Descartes' evil demon could not fool Descartes into believing he exists if Descartes doesn't exist in the first place, because the evil demon has limits on its powers, it cannot do everything. However, God, not having any limits on his power, could make Descartes believe he exists when he doesn't.
    – Gabriel
    Commented Apr 23 at 18:02

The main point is that the Cogito is for each one of us to decide whether it is conclusive of not.

As I see it, only the subject knows that he knows. Knowledge is nothing but subjective experience: I know pain only when I am subjectively experiencing pain. Pain is nothing but whatever I am experiencing whenever I am in pain.

And then of course, it becomes trivial that Descartes knows he is thinking at the moment he is thinking and that therefore there is this thinking thing which is experiencing itself and therefore exists as such and knows it exists as such.

If you don't accept that you know you are subjectively experiencing pain whenever you are subjectively experiencing pain, so be it. It is for you to decide.

And if you define knowledge for example as what is written in scientific books, then you won't be convinced.

  • How does whether only the subject knows or the entire population answer the question of how Descartes knows his mind isn't being manipulated?
    – Cell
    Commented Oct 13, 2020 at 12:03
  • @Cell Deception implies you don't know. Either you know or you don't. If you do, manipulation is logically impossible. Goes for knowing and knowing that you know. To claim otherwise is not only saying that knowledge of anything is impossible, but also that the concept of knowledge is nonsensical. The idea that we can know yet be deceived is inconsistent but you can choose to believe it. We can be manipulated into believing things exists outside our mind but what we subjectively experience is as real as could be. Commented Oct 13, 2020 at 16:40
  • Deception means to be convinced into believing or knowing something that is not true and it involves manipulation. Not knowing is not the same as knowing something not true. I may not know the true shape of the spheroid planet Earth in as much as I don't know the exact geometrical parameters to draw a precise model, but flat Earthers know incorrectly that the Earth is flat and disk shaped. Different things.
    – Cell
    Commented Oct 13, 2020 at 17:36
  • @Cell Sorry, I don't get it. Commented Oct 14, 2020 at 9:25

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