  At the beginning of Metaphysics IV 5, Aristotle says that PNC stands and falls with the doctrine of Protagoras, that each individual human being is the measure of all things. In his Theaetetus 151–183, Plato argues that Theaetetus, who holds that knowledge is nothing but perception, is committed to Protagoras’s view via an argument from conflicting appearances. If the wind appears cold to you but hot to me and knowledge is nothing but perception, then we must both be correct, as Protagoras says.