The turning of the tables, whereby Plato (Theaetetus, 171 a) opposes the relativism of Protagoras . Protagoras holds the doctrine that whatever seems true for a person is true for them; hence he must accept that those who believe that the doctrine of Protagoras is false are right: its falsity is true for them. It became a standard form of argument, used by both Epicurus and Sextus Empiricus . See also Ishmael effect.
Philosophy dictionary. Academic. 2011.