This was the insight in Decartes, "I think, therefore I am". It's more profound than most people give it credit for.
Decartes realized that almost everything else is open to error -- even the things you directly see and touch might be illusions, or the result of hallucination (or "fakes" in the manner of the simulated world of "The Matrix", etc.)
But Decartes also realized that no matter how much of the perceived world might be illusory or understood in error, one primal fact is inarguable -- if you're pondering your perceptions and thoughts and wondering about their accuracy, you provably EXIST, even if all else might be hallucination. Because you couldn't be questioning your perceptions if you didn't exist in the first place. Your *existence* is real, *has* to be real; it can not be illusory.
Thus, the fundamental observation: "I think, therefore I am [exist]".
All else may be open to question, but that is not.
And A is A. Always will be.
And that brings up the other kind of unquestionable truth -- "true by definition".
I'd prefer to say, in this instance, "true by identity." I think it's more than a quibble. A definition isn't an axiom.
There are several ways in which this can be seen as a weak claim.
For example, to an evolutionist pondering the fact that you are made up of cells whose ancestors used to be freemoving, one celled cowboys, before the communist tyranny of multicellularity came along and fenced in the ranges. "You" are just a bio-chemical/bio-electical conglomeration whose ponderings are meaningless to the cells you are made of, whose existence is far more tangibly demonstrable than your maudlin arrogance about "being" because you are "thinking"--which merely amounts to a silly attempt to pretend there is meaning in a few fleeting congeries of electical impulses between cells.
Or, consider the atoms that make up your cells--with their vast empty spaces occupied by entities whose existence at any given point or time is highly probabilistic and fleeting--and cells have the arrogance to suppose that they exist just because their happen to be an fleeting accidental conjugation of these entities in atoms whose existence is so evanescent as to be virtually non-existent.
Or, for one more, consider the inversion of the usual way of looking at Descartes argument: Your self-detected visceral existence, however pursuasive you might find it, is a far cry from high on the heirarchy of what we commonly take for objective evidence. A proof, or a refereed scientific article about an experiment I can duplicate, if I want, or, just an ordinary chair, which more than one human can stub his toe on, and then get together to share notes with other chair-victims about. All of these things take priority over your self-awareness, as objective evidence.
All of which brings me around to my question: I think humans are, more than any other creatures we know about, defined by their existence as part of a community. We observe that those of us cut off from effective participation in our community go bonkers, become unhealthy, and meet untimely ends at a very high rate. I therefore suggest that, while Descartes formulation might be a very good one for, say, a muledeer, it will be rather deficient for a human: If all you do is think about what you are, you will shortly cease to be, however, if you suppress your philosophical misgivings, and you continue to work at being, as I aver you are supposed to, you may very well become something worth thinking exists.
At the end of a paricularly unproductive day, Decartes wanders into a bar. The bartender notices his grimace.
Bartender: "Yer lookin' down there, Decartes. How about a beer?"
Decartes: "I think not."
And disappears in a puff of logic.
Obligatory Descartes before hor's D'oeuvre joke:
Descartes enters a tavern and orders a glass of wine. After he's finished the bartender says, "Would you like another?" Descartes replies, "I think not," and vanishes.