if not-being has no part in the proposition, then all things must be true; but if not-being has a part, then false opinion and false speech are possible, for to think or to say what is not—is falsehood, which thus arises in the region of thought and in speech.
Plato’s Stranger establishes the ontological condition for the possibility of falsehood: it requires that not-being genuinely participate in being, a participation the Sophist systematically denies in order to evade refutation.
, Sophist, -360thesis