Diogenes Laertius Vitae philosophorum 3.1.52

[52] Of these the former is a proposition, the latter a conception. Now where he has a firm grasp Plato expounds his own view and refutes the false one, but, if the subject is obscure, he suspends judgement. His own views are expounded by four persons, Socrates, Timaeus, the Athenian Stranger, ¹ the Eleatic Stranger. ² These strangers are not, as some hold, Plato and Parmenides, but imaginary characters without names, ³ for, even when Socrates and Timaeus are the speakers, it is Plato’s doctrines that are laid down. To illustrate the refutation of false opinions, he introduces Thrasymachus, Callicles, Polus, Gorgias, Protagoras, or again Hippias, Euthydemus and the like.