Didn't Descartes assume the act of doubting, before "proving" that "I think therefore I exist". Its possible to "feel uncertain about feeling uncertain" i.e. doubt the act of doubting Meaning the "act of doubting" goes into the uncertain category by his rules

Furthermore he wrote "it is prudent never to trust those who have deceived us even once", making another assumption not to trust people who deceive.

What does this mean for his arugment?

If you are certain you feel uncertain about X, you are uncertain about X.

Does this mean it is just not possible to be certain since you will always have to make an assumption?

  • Cogito ergo sum, is just the best we have.
    – 8Mad0Manc8
    Jan 6 at 4:10
  • Descartes statement is I think therefore I am. Speaking personally, I think western philosophy has been damaged by taking Descartes to be a serious philosopher. I enjoyed coordinate geometry in school but that is quite separable from Cogito ergo sum!
    – Rushi
    Jan 6 at 7:36
  • 1
    Basically, the Cogito is not an argument but a sort of immediate intuition. Jan 6 at 8:36
  • i think you should cite where he says this, as he's usually quoted as "I think therefore I exist" or similar
    – user70707
    Jan 7 at 15:52

7 Answers 7


Descartes did not think so

he does “not deny that one must first know what thought, existence and certainty are, and that it is impossible that that which thinks should not exist” (AT, Vol. 8A, p. 8; CSM, Vol. 1, p. 196). In order to avoid an obviously vicious circle, Descartes must intend that the last claim be read as a conceptual truth. Hence, Descartes goes on to say, “But because these are very simple notions, and ones which provide us with no knowledge of anything that exists, I did not think they needed to be listed” (ibid.). If I am right, the truth of “if x thinks, x exists” is analytic for Descartes, which is why he does not think he needs an additional argument to infer his existence from his awareness of his thinking.



Doubting that you are doubting is just not possible because feeling uncertain about feeling uncertain means we are somewhat certain, which makes no sense; either I am certain or I am not.

That impossibility of doubting if we are uncertain is the base for "I think, therefore I am".


So, for Descartes, it's important to remember that truth means rational truth. (In contrast to something like the immediate truth we feel when we simply experience being.) For Descartes, truth must be explicable, in a sense that is very close to a mathematical one. This is what it means to him to do philosophy via his method.

I do agree with you that there is a fundamental tautology in his argument. A cynical translation might be: "If I do philosophy, then I can say that I exist." Just remember that it is easier for us to see this today, because of all that followed. I feel that it's important when reading Descartes, to consider the trajectory of philosophy in his time, the changes (for example) in how knowledge was determined, and the value that his sceptical method brought with it.


Please note that the title of your question is wrong. Descartes concluded cogito ergo sum, i.e. "I think hence I exist".

His argument relies on the fact, that doubting certain states of affairs is a kind of thinking. And if I think, then - at least - I must exist.

  • But this line of thought assumes an "I"
    – Fraser Pye
    Jan 6 at 11:54
  • @FraserPye "Cogito = I think" is the first person singular.
    – Jo Wehler
    Jan 6 at 12:02
  • 2
    Maybe "cogitatur, ergo est esse" then. Jan 6 at 14:09

It seems the main assumption of "I think therefore I am" is the "I". By the time anyone is philosophising in such a degree of sophistication the questioning entity is already enmeshed and integrated in a cultural network: Being & Time ¶ 25. An Approach to the Existential Question of the "Who" of Dasein:-

In clarifying Being-in-the-world we have shown that a bare subject without a world never 'is' proximally, nor is it ever given. And so in the end an isolated "I" without Others is just as far from being proximally given. If, however, 'the Others' already are there with us [mit da sind] in Being-in-the-world, and if this is ascertained phenomenally, even this should not mislead us into supposing that the ontological structure of what is thus 'given' is obvious, requiring no investigation.

Even though local interests are ascribed to the "I" the being of the "I" is a networked entity formed in a society, so by the time thought questions existence it is socialised thought.

Essentially, the doubting Descartes of "dubito, ergo cogito, ergo sum" cannot even be certain of his body. In doubting, there is thought, therefore there is existence of thought.

In the Heideggerian œuvre the "I" is a part of the being of Dasein that involves ownership (Jemeinigkeit) but it is not necessarily all of Dasein. However, the being of Dasein cannot be fully explained in thought and language because it is what produces thought in the first place. It is in the objective conception of the "I" formed retrospectively that the questionable assumption of the OP's question can arise.

The way Heidegger relates the "I" to Dasein (the primary mode of which is Being-in-the-world) is indicated in Being & Time ¶ 26. The Dasein-with of Others and Everyday Being-with:-

Thus in characterizing the encountering of Others, one is again still oriented by that Dasein which is in each case one's own. But even in this characterization does one not start by marking out and isolating the 'I' so that one must then seek some way of getting over to the Others from this isolated subject? To avoid this misunderstanding we must notice in what sense we are talking about 'the Others'. By 'Others' we do not mean everyone else but me—those over against whom the "I" stands out. They are rather those from whom, for the most part, one does not distinguish oneself—those among whom one is too. This Being-there-too [Auch-da­-sein] with them does not have the ontological character of a Being-present­-at-hand-along-'with' them within a world. This 'with' is something of the character of Dasein; the 'too' means a sameness of Being as circumspectively concernful Being-in-the-world. 'With' and 'too' are to be understood existentially, not categorially. By reason of this with-like [mithaften] Being-in-the-world, the world is always the one that I share with Others. The world of Dasein is a with-world [Mitwelt].

So when Descartes deduces his existence (as "ergo sum") that existential Dasein already extends to the with-world in which he learned to think (with others). He is not isolated in a solipsistic way to the "I"; he discovers a cultivated intelligence.


Yes, at the surface, he just reiterated ("therefore I exist") of what he had assumed ("I am").

But in shortening this as a phrase, did little justice to what I interpret he meant of the statement (what little justice was done is clearer if one reads the addendum below).

He meant "I am" -- I think, I feel, I am self-aware. Hold on to that last one, self-awareness. See, Descartes had gone through the journey of stripping away everything that he was taught or conditioned to believe, down to irreducible principles & facts. Explaining his foray in mathematics. He even goes, what if this whole world is an illusion, projected to me by a fiend? From this it's easy to beg the question, do I even exist? It turns out he can establish he exists, because he is exercising the questioning of his existence, which requires a consciousness, a self.


The phrase is actually usually translated to English as, "I think, therefore I am", which would've gotten to my point more elegantly, if you had used that translation.




What Descarte is describing is experience. That is, he could not deny that the experience of doubt was happening, because that is the one thing that he had direct access to. Given that the experience was happening at all, Descarte reasoned that there must be something doing the experiencing, that is, himself.

You could argue that you could doubt that the experience that you are having is doubt, but in that case you wouldn't be doubting that the experience you are having is doubt, because to doubt the experience of doubt is to experience doubt.

But even if you somehow convinced yourself that you could doubt without experiencing doubt, you would have to acknowledge that you were experiencing something, and that's really all you need to follow Descarte to the conclusion that you exist. There's a lot to argue against once you get past that point, but you really can't argue coherently against the first principle.

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