Peritrope

Peritrope, from the Greek word for a turning back on itself, is a form of self-refutation argument used since antiquity against relativist and skeptical claims about truth. Its earliest recorded use is Socrates' argument in Plato's dialogue Theaetetus against Protagoras's claim that truth is entirely relative to each individual, and the technique was later named and analyzed explicitly by the skeptic Sextus Empiricus around 200 AD in his work Against the Logicians. The argument works by turning a general claim back against itself: if someone asserts that all truth is merely subjective or that no truths exist at all, that very assertion is itself either subject to the same relativism, undermining any reason to accept it as more than the speaker's private opinion, or else it is itself an exception that contradicts the universal claim being made, so the claim collapses under its own weight the moment it is examined.