As far as I know Decartes tried to prove that he existed from the fact that he was thinking. And he thought this was only proof. My question is following:

-- why is the argument brought up above, stronger, than argument like this for example, I can see sky, therefore I exist? If someone says because this is illusion and maybe you are in a dream. Then why can't it happen that in dream, I "think" that I think, but in reality I don't.

Not sure if I properly formulated my argument but I hope you get the idea what I am trying to say. Why is Decartes argument about thinking more powerful that if he used other things, like I feel pain, therefore I exist.

  • 2
    It is not more powerful. If you feel pain, even if the pain is an illusion, something is performing the act of feeling nonetheless and must exist, and that something is you.
    – armand
    Commented Feb 8, 2021 at 17:04
  • @armand So why is that argument from decartes described in the literature, and not the one about pain from OP for example, if they have same strength?
    – user50263
    Commented Feb 8, 2021 at 17:13
  • 3
    Descartes later clarified that cogito ergo sum was not meant as an argument, but rather a direct existence claim that he "could not bring himself" to doubt. The extra "power", if any, comes from self-referential nature of it: even if you are doubting that you are thinking you are still thinking to do the doubting. This won't work with the blue sky.
    – Conifold
    Commented Feb 8, 2021 at 18:47
  • 4
    Does this answer your question? Could 'cogito ergo sum' possibly be false?
    – Conifold
    Commented Feb 8, 2021 at 18:48

4 Answers 4


Welcome, don. Thanks for a nice, reflective question.

The cogito has much less importance for Descartes than is commonly supposed. Its significance is twofold: (1) it is (merely) the first truth which Descartes has come upon which (as he supposes) 'is necessarily true whenever it is put forward by me or conceived in my mind' (Med. II: Cottingham, II: 17). (2) Descartes realises that there is very little he can do with the cogito as an isolated truth. What he takes from it is that it is a 'clear and distinct idea' and that 'I now seem able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true' (Med. III: Cottingham, II: 24). Med. III is packed with other ideas that Descartes incorporates into his argument on the ground that they are equally clear and distinct - e.g. 'there must be at least as much in the efficient and total cause as is in the effect', 'the objective being of an idea cannot be produced by potential being', &c.(Cottingham: 28. 32).

The cogito is also not self-standing. Descartes needs to make certain that clear and distinct ideas really are necessarily true. This leads him into what many take to be the 'Cartesian circle'. He proves that clear and distinct ideas are necessarily true by using clear and distinct ideas in order to prove the existence of a perfect God who is no deceiver and guarantees the truth of clear and distinct ideas. (This is a contested area but whatever the case about the Cartesian circle, Descartes very evidently does believe that he needs to secure the necessary truth of clear and dsitinct ideas. So even the cogito, whatever Descartes epistemological relief at its discovery, needs reinforcement.

That aside, and at the heart of your question, Descartes has no difficulty in accepting, 'I feel pain, therefore I exist'. In fact he accommodates just such a case in Med.III: 'But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, is wiling, is unwilling, and also imagines and has sensory perceptions [et sentiens]' (Cottingham: 19). Among sensory perceptions are pains.

As for, 'Then why can't it happen that in dream, I "think" that I think, but in reality I don't', I suggest that we already have the answer to this. What would replace your 'I "think"' except 'I imagine' - I don't think but imagine that I do. But Descartes has already included imagination in thought, therefore in dreams I imagine = think that I think: so I think.


The Philosophical Writings of Descartes, II, ed. J. Cottingham et al., Cambridge: Cambridge University Press, 1984/ 2008.


I don't think that Descartes:

tried to prove that he existed from the fact that he was thinking.

Rather, he tried to remove errors in his thinking by assuming a skeptical stance toward anything that he thought one might reasonably doubt. Then discovered for himself (although he likely read it beforehand) that he could not reasonably doubt his own existence, because the mere experience of doubting seemed to confirm his own existence.

You ask:

why is the argument brought up above, stronger, than argument like this for example, I can see sky, therefore I exist?

Despite the phrase "cogito, ergo sum" (I think, therefore I am), Descartes did not see his awareness that he couldn't doubt his own existence as a syllogism, as a formal deductive argument based upon premises. Rather, his inability to seriously doubt his own existence was something of a psychological fact. I can doubt the existence of "the sky", and from a logical point of view it may be true that "I" must exist if it is true that "I doubt the existence of the sky". However, in doubting the existence of the sky, I may not be immediately aware of my own existence. My attention may be distracted by the appearance of the sky, and my knowledge that appearances may be illusions. However, when I turn my attention to the attempt to doubt my own existence, my mind immediately says "but wait! how can that be?"

So, from a purely logical deductive point of view, "I doubt the existence of X, therefore I am" may be a valid argument, from a psychological point of view, it is not as effective as the attempt to doubt my own existence.

At the risk of being redundant, I will answer your question

Why is Descartes argument about thinking more powerful that if he used other things, like I feel pain, therefore I exist.

If I am thinking about my pain, I may not be thinking about whether my own existence is something that I can reasonably doubt. My attention is on the pain, but not necessarily on the question "what may I reasonably have doubts about?"

  • To whomever downvoted my answer, could you explain your reasoning please? Commented Feb 10, 2021 at 1:34

The problem here lies in the concept of existence, sum, not the cogito.

There are multiple concepts of existence, but this one fits perfectly on this context: an object exists for a subject if the subject can interact with the object.

The statement seems quite evident. A number exists in my mind (the subject), because I can interact with the idea of such number (the object) in my mind. A rock exists for me (the physical subject) if I can physically interact with it (the object), for example, by seeing it our touching it. Jupiter is said to have 79 moons. Therefore, the 80th moon does not exist for science (even if it's somewhere in the sky), because scientists (or I) can't interact with it.

So, I (the subject) can apply such test to know if I (as an object) exist. Ergo, given that I (subject) can interact with me (object), for example, if I (subject) think of me (object), I can conclude that I (object) exist (for me-subject).

That's precisely the approach of cogito ergo sum: thinking is the proof of an interaction between the subject(the observer) and the object(the observed), no matter if the observer is the same observer. I've heard at least three interpretations of cogito ergo sum in such sense:

  1. If I can think of me, I do exist (same as Aristotle explaining Plato's, knowledge of knowledge: ...to be conscious that we are perceiving or thinking is to be conscious that we exist...).
  2. If can interact with me (by thinking), I exist. In general, I can interact with myself in multiple ways, not only by thinking, but for example by touching my own nose.
  3. If I can think I think, I do exist (Prof. JC.R., Toulouse University).

Corolarium: as you suggested, seeing the sky (object) is not a proof of my existence, but moreover of the existence of the sky for me (the subject). Nevertheless, if the sense is an interaction between a subject and an object which are the same, and the sky is just a contingent on the predicate, the observation takes the form if I (subject) perceive that I (object) perceive (the sky, just a contingent auxiliar), I do exist, which is equivalent to (3).


Descartes's famous platitude "I think therefore I am" should be changed to "I doubt therefore I am" imho. "think" is a very general term and more or less implies "some distinctive consciousness or rough reasoning" for most everyday language usage. Descartes's real emphasis was on "self-doubt" (doubt is always about self's previous perception or conception) which is much more intellectually critical and serious. Advanced animals can no doubt see the sky or even enjoy some styles of music, but seems none will doubt itself about it, as we never see a healthy dog rejects eating simply because it's meditating or doubting about something it just perceived or conceived of moment ago...

Self-Doubt/Critique is the sign of the beginning of rational thinking, and clearly Descartes valued this unique human aspect so much that he considered this is the true criterion of human existence, also this is why he's considered a rationalist. We all see people say a lot only to support their own view at the end of the day, but we seldom see one says a lot in order to conclude he or she may be wrong...

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