Philosophical scepticism doesn't ask whether you're wrong about specific things. It asks a more unsettling question: could you be systematically deceived about everything, in a way no amount of ordinary checking could ever catch? René Descartes pushed this idea to its extreme with his thought experiment of an "evil demon" — a hypothetical deceiver powerful enough to fake every sensation, every memory, your entire experience of an external world, indistinguishably from reality. If that were true, you'd have no way to tell from the inside. Descartes couldn't fully refute the possibility. He did find one thing the demon couldn't touch.
The one certainty that survives total doubt
Descartes' famous response, "cogito, ergo sum" — I think, therefore I am — identifies a single fact that remains true even under the most extreme sceptical scenario imaginable. Even if every one of your beliefs about the external world were false, even if your senses, memories, and reasoning were all being manipulated by a deceiver, the very act of doubting or thinking requires something doing the doubting. You can be wrong about literally everything else, Descartes argued, but you cannot coherently doubt that you are, in that moment, a thinking thing having some experience — the doubt itself proves a doubter exists. This became Descartes' single foundational certainty, the one point his systematic doubt couldn't dissolve, from which he then attempted to rebuild broader knowledge.
Later responses: shifting the burden, not defeating the demon
Descartes' own attempt to rebuild reliable knowledge from that single starting point, largely by arguing for the existence of a non-deceiving God, has been widely regarded by later philosophers as one of the weaker parts of his argument. Since then, philosophers have developed alternative responses to radical scepticism that don't rely on refuting the sceptical scenario outright — arguing instead that ordinary knowledge doesn't require the impossible standard of absolute certainty scepticism demands, that some beliefs can be justified as reasonable default assumptions unless positively challenged, or that the sceptical hypothesis, precisely because it's unfalsifiable by definition, doesn't actually threaten everyday knowledge claims the way it first appears to. None of these responses claims to logically disprove the possibility of radical deception; they instead argue the demand for that level of certainty was misguided to begin with.
What we're still unsure about
That radical sceptical scenarios like Descartes' evil demon (or its modern equivalent, the "brain in a vat") cannot be logically disproven from the inside is a widely accepted point among epistemologists — it's not something philosophy has settled by refutation. What remains genuinely contested is which of the various proposed responses, if any, actually gives ordinary knowledge claims a solid enough foundation despite that unrefuted possibility, and epistemologists continue to disagree sharply over whether scepticism has been adequately answered, productively sidestepped, or whether it points to a genuine, permanent limit on what certainty human knowledge can ever claim.
This sits inside Scepticism & Responses to Scepticism, one of seven topics in Epistemology, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.