Here is my chain of reasoning and criticism regarding Descartes’s idea.

https://plato.stanford.edu/entries/descartes-epistemology/#2 “In the same way, I began by taking everything that was doubtful and throwing it out, like sand” - Descartes

“All the mistakes made in the sciences happen, in my view, simply because at the beginning we make judgments too hastily, and accept as our first principles matters which are obscure and of which we do not have a clear and distinct notion.” - Descartes.

Descartes begins by doubting everything. This is the beginning of his argument. We can say that it is the first assumption or starting point of his reason, that he can doubt everything.

At this point I want to pinpoint it out, that since I or Descartes, whoever does the thinking, are allowed to doubt everything, we can also doubt “if doubt is thought”. Just because we are simply allowed to doubt everything. I am not saying that doubt is not thought, but pointing out that at this point in reasoning where we have no extra assumptions, I can say that doubt might or might not be thought.

Then Descartes says: https://en.wikipedia.org/wiki/Cogito_ergo_sum#Discourse_on_the_Method “Accordingly, seeing that our senses sometimes deceive us, I was willing to suppose that there existed nothing really such as they presented to us And finally, when I considered that the very same thoughts (presentations) which we experience when awake may also be experienced when we are asleep, while there is at that time not one of them true, I supposed that all the objects (presentations) that had ever entered into my mind when awake, had in them no more truth than the illusions of my dreams. But immediately upon this I observed that, whilst I thus wished to think that all was false, it was absolutely necessary that I, who thus thought, should be something; And as I observed that this truth, I think, therefore I am, was so certain and of such evidence that no ground of doubt, however extravagant, could be alleged by the Skeptics capable of shaking it.” - Descartes.

Here Descartes says that he is certain that he cannot doubt that he is thinking. This entails a second assumption or a second point in reasoning which is “ All doubt is definitely thought”. Again, I am not saying that the assumption is good or bad, but merely pointing it out. This assumption is after the first one we have established above.

Now, comes my argument. I am saying that I need not make the second assumption, and I can establish the statement “ I think, therefore I must be”, without that second assumption. Since my argument is minus one assumption, compared to Descartes’s, it is a stronger truth. Here is my original argument as well, although it might be hard to understand( In a way it is circular logic, meaning that I propose to oppose Descartes’s argument through contradiction, and this requires a discussion to understand): Could 'cogito ergo sum' possibly be false?

But, forget about that argument of mine for a moment, and think about this: Descartes in his first assumption says that he is allowed to doubt everything. Then infers that doubt must definitely be thought, without any doubt at all.

This seems to me a logical fallacy. It is a logical fallacy if you do not make the second assumption which I have mentioned. I am only trying to pinpoint that out(The second assumption), and say that I can establish a more definitive minimum inference, which would be “ I think, therefore I must be”, by assuming one less statement. For Descartes’s argument to work, I would need to make a contradictory second assumption, which would be “Doubt is definitely thought, and I cannot doubt that”. Could anyone please pinpoint where I am getting this wrong?

  • 1/define logically valid 2/ why do you want your inferences to be ''logically valid'' beforehand? Commented Sep 1, 2017 at 10:24
  • 1
    Source for claim Descartes says he is allowed to doubt everything? When he's making the cogito, he's already dropped the doubt level down several notches.
    – virmaior
    Commented Sep 1, 2017 at 11:17
  • I've flagged this as a duplicate as it now appears you will continue making this thread until someone agrees with you.
    – Braydon
    Commented Sep 1, 2017 at 14:09

2 Answers 2


You pose the following apparent contradiction and I gather that your question asks why it isn't considered to be a logical fallacy in Descartes' argument:

Descartes in his first assumption says that he is allowed to doubt everything. Then infers that doubt must definitely be thought, without any doubt at all.

I think the chink in your line of reasoning is the assumption that in the phrase "doubt everything", Descartes uses the word everything to mean literally everything, including doubts.

In the context you've supplied, Descartes is using an implicitly iterative approach to discarding whatever can be discarded on the basis that they are not necessarily true (in the sense of correspondence of those things with reality). Two of the iterations are noted, which:

  • discard sensory perception because "our senses sometimes deceive us"; and
  • discard thoughts being real because in dreams, "there is at that time not one of them true".

Note that Descartes distinguishes between thoughts and doubts, so the word thoughts is used in a somewhat more limited fashion than the arbitrary subject matter of thinking. That is, one can think thoughts and one can think doubts, which Descartes treats as quite separate categories. (This might be considered a fallacy in itself today.)

After several iterations, Descartes is left with untrusted thoughts (or doubts as your quote has it). But even though those thoughts were untrusted, their existence could not be denied (i.e. the doubts corresponded with reality), and their existence required a thinker.

Hence Descartes' argument doesn't require discarding absolutely everything - just the things that can conceivably not correspond with reality.

  • I have migrated to my first question, since this has been marked as duplicate.
    – user12196
    Commented Sep 2, 2017 at 5:40
  • And will answer all your points in 3-4 days.
    – user12196
    Commented Sep 2, 2017 at 5:40

Again this critic is not logically valid. The first issue is drawing your distinction between doubt and thought, when it is inaccurate. Doubts are by definition a type of thought. (They are a subset of thought.) You cannot get around the fact that doubts are thoughts without changing the definition of the word. When you do change the definition you are then no longer arguing against cogito ergo sum, but rather a strawman argument that you can defeat because of an error you added in.

Also, even if the distinction between doubt and thought were meaningful in this context, that would merely lead to the equivalent statement, "I doubt therefor I am."