1. Plato’s evidence, here and in Theaetetus, that Protagoras held the view that contradiction is impossible, is probably reliable. In fact, however, Protagoras held the view that truth is relative (see p. 336); his version of the impossibility of contradiction, then, is the impossibility of gainsaying anyone’s impressions or opinions, which are necessarily private and peculiar to that person.

2. It is not at all clear whom Plato has in mind. In Theaetetus, perhaps erroneously, he assimilates Protagorean relativism to the theory of Heraclitus of Ephesus (fl. c. 500) and his followers, on the grounds that they believed everything to be in continual flux. So he could have Heraclitus in mind. On the other hand, he could be interpreting Parmenides’ denial that you can say ‘is not’ about anything to entail the denial of contradiction.