What resists doubt?
On the axis: Relation to certainty · the one who doubts
Pushing doubt to its limit, dream, illusion, a possible deceiver, does anything resist, or must we admit that no certainty holds?
This problem is brought out by the thought experiment The dream and the evil demon:
You have surely dreamed a scene so vivid that, in the moment, nothing set it apart from reality. But then, what assures you that at this very instant you are not dreaming? Descartes presses the test further: imagine an all-powerful, deceiving mind, an evil demon, bending all its cunning to make you take the false for true, down to the sky, the earth, and your own body. As long as this hypothesis is not ruled out, almost all your beliefs could be false without your being able to know it.
Descartes, Meditations on First Philosophy (1641)
Faced with this doubt, what remains to you? Does anything withstand the test, or must we admit that no definitive certainty is within our reach?
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Justification
Against , Descartes turns doubt against itself: he drives it to the point of , supposing an evil genius who deceives him about everything, to see whether anything withstands it. Yet the very act of doubting presupposes a subject who doubts: I can call everything into doubt except that I am thinking, and therefore that I am. The cogito is no dogma laid down in advance but a certainty won through doubt itself, an indubitable foothold from which the whole of knowledge can be rebuilt. The conclusion is thus firmly : full certainty is attainable, and skepticism refutes itself when pushed to its limit.
I think, therefore I am.Discourse on Method, IV
Meditations on First Philosophy, I and II · Discourse on Method, IV
Justification
The testimony of Aristocles, gathered from , reduces all wisdom to three questions: how are things by nature, what attitude should we take toward them, and what follows from this for us. The answer to the first is what makes Pyrrho original: things are indeterminate, unstable and indiscriminable, so that neither our sensations nor our opinions tell the true or the false. From this verdict about the object, and not from an examination of our faculties, he concludes that we should place no trust in them and remain . More radical than fallibilism, which revises its beliefs, he refuses even to assert that nothing is knowable, and is content to say that things are "no more this than that".
Things are no more this than that, nor both at once, nor neither.Aristocles, cited by Eusebius, Preparation for the Gospel, XIV, 18
Aristocles (after Timon), cited by Eusebius, Preparation for the Gospel, XIV, 18 · Diogenes Laertius, Lives of the Philosophers, IX, 61-108