I was thinking of the sentence

"I think therefore I am",

which I had for a long time considered indisputable because it's self-evident.

Then I considered the hypothetical situation where my thoughts are illogical – and of course I can't be aware that they are – and so to a logical outsider I am not saying something that is actually indisputable and self-evident, but in fact may be total nonsense. Perhaps there is a hypothetical evil demon altering my ability to reason. However convincing it seems to me that I MUST exist since, in whatever since "I" or "exist" is, there is the ability to question whether I do, the conclusion can't actually be trusted without doubt since I cannot know if I am in fact reasoning logically.

Even what I consider to be trivial tautologies require this assumption of sanity, and therefore there is no true conclusion without assumption – an assumption about the nature of my own reasoning. The only absolute truth I have is unspeakable subjective experience, and any articulation (and I realize the apparent circular reasoning I've cornered myself into here...) is necessarily uncertain.

I realize that perhaps this doesn't even make sense, but was just curious if anyone had entertained this line of thought and come to some satisfying resolution.

  • 1
    No proof without assumptions. Commented Apr 27, 2017 at 6:58
  • For a "self-evident" truth, that we can "grasp" without proof or some sort of inference/argument (if any): maybe. Commented Apr 27, 2017 at 6:59
  • This does make sense, and "I think therefore I am" has been criticized exactly for making unstated assumptions, see Could 'cogito ergo sum' possibly be false? And if we think of "I think therefore I am" as inference it can not possibly be self-evident, "I think" and "I am" are perhaps self-evident separately (even this is disputed), but that one implies the other requires logical justification and "assumptions".
    – Conifold
    Commented Apr 27, 2017 at 20:46
  • Does 1+1=2 require assumption?
    – Marxos
    Commented Dec 10, 2021 at 3:22

2 Answers 2


Although the point of departure is a bit different, I think that Ludwig Wittgenstein's On Certainty proposes a resolution to the problem that you describe. Wittgenstein suggested, that there are some "assumptions" that even a radical skeptic must make, if her doubt is to make minimal sense. There must be some rational discourse already in place, in order for rational doubt to get off the ground.

  1. The idealist's question would be something like: "What right have I not to doubt the existence of my hands?" (And to that the answer can't be: I know that they exist.) But someone who asks such a question is overlooking the fact that a doubt about existence only works in a language-game. Hence, that we should first have to ask: what would such a doubt be like? (On Certainty)

So yes, some "assumptions" seem to be inevitable. But these assumptions seem to be inevitable even for a radical skeptic. And to that extent they seem to be less of a problem.


Well, first we have to distinguish "true conclusions" from "assumptions".

I think it should be clear I could have an erroneous stab at the truth and get it right regardless of whether my stab was based on correct assumptions, or even correct arguments.

Now, I'm not a fan of the "I think therefore I am", because it really undercuts the essence of the thought. The full quote is "Je doute donc je pense, je pense donc je suis".

Which is the more powerful: "I doubt therefore I am". It is doubt that is undoubtable, as it is impossible to doubt my own doubt, since that would be doubting itself.

So there is an assumption of the "I" in that one, which leaves as our only certain and true conclusion: "There are doubts, doubts are thoughts, there is thought".

Do be careful with the evil deceiver, he is also an "assumption", one which we have no particular reason to make, except to try to destroy logic and mathematics. Descartes found no other way to doubt logic.

That being said, if I'm not mistaken, your concern is with general epistemology, or "how do we know things?". Some dude (BERKELEY, I think) has a pretty compelling argument for this.
Thinking happens, as we've established.
Thinking happens in language.
For language to be a thing, there must be something with whom it was developed so that ideas could be shared, otherwise there wouldn't be language.

Hence there need to be at least 1 thing besides my thoughts to communicate through language those thoughts, which is (to him) enough to show that there is

  1. An external world b
  2. Other mind(s)
  3. Language

These three come together and cannot exist without one another.

Without an external world, there's no one to talk to, and no language
Without other minds, there's no need for language and hence thought.
Without language, there's no thought or hence doubt.

It might not be the most satisfactory response, but there is at least, ironically, certainty in doubt.

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