Is becoming thinkable? Plato poses this question in the dialogue entitled The Sophist (or: "On Being"), which follows on from Theaetetus, and which features, in addition to Socrates and the young Athenian Theaetetus, a foreigner who, although born in Elea (in "Magna Graecia", southern Italy), is said to be able to take a step back from Parmenides' teachings. Indeed, to think about becoming, we have to admit a passage, a transition between being and non-being. In the school of Parmenides, this is unthinkable: what is is posited to exist, what is not does not exist, and of what does not exist there is nothing to say. But what is a sophist? It's someone who boasts of being able to talk about anything, and support any thesis, giving the impression that he knows (when perhaps he's making it up?). To invent is to make be what is not. To tell the sophist that what he's saying is false, we're going to have to talk about what he's saying that doesn't exist, so we're going to make it, in a way, exist; conversely, what really happened yesterday no longer exists, but we're not going to claim "that reality and becoming don't exist" (245 d). Thus: "The many-headed sophist forces us to grant, in spite of ourselves, that non-being exists in some way" (240 c).
Plato had loved Parmenides' teachings. Through the Stranger, Plato tells us that, to flush the sophist out of his lair, we'll have to "challenge the thesis of Parmenides, our father, and do him violence by proving that in a certain respect, Non-Being exists, and that, on the other hand, Being, for its part, in some way does not exist" (241 d). And the Stranger solicits the indulgence of his interlocutor: no, I'm not a parricide, but we're going to re-examine Parmenides' thesis, and "force non-being, under certain conditions, to be, and being, in turn, under certain modalities, not to be"(ibid.).
