The Probable Comes from the True
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Socrates — τὸ εἰκὸς διʼ ὁμοιότητα τοῦ ἀληθοῦς… ὁ ἀλήθειαν εἰδὼς κάλλιστα ἐπίσταται εὑρίσκειν.
Here the trap that has been building for pages finally closes on Tisias, and Socrates springs it by turning and speaking to him directly. The whole of Tisias's art rests on 'the probable,' to eikos. But what is the probable? Socrates reminds him of what 'we were saying long ago, before you even came along': that 'this probable arises in the many through likeness to the truth,' di' homoiotēta tou alēthous. The plausible is not a free-standing thing; it is whatever resembles the true closely enough to be believed. And likeness was the very subject of the earlier discussion of deception, which established the decisive point Socrates now recalls: 'everywhere it is the person who knows the truth who knows best how to find' the likenesses, kallista epistatai heuriskein. Put the two together and Tisias's whole position collapses. If the probable is likeness-to-truth, and if the truth-knower is best at finding such likenesses, then the master of the probable — the very thing Tisias claims to teach without any need for truth — must be the person who knows the truth. Tisias told his pupils to bid the truth farewell; but the truth turns out to be the one indispensable qualification for doing what Tisias promises. His art, which claimed to make truth unnecessary, secretly requires it more than any other.
Now Socrates turns and addresses Tisias directly, springing the trap. Long before you came along, Tisias, we were already saying that this 'probable' of yours arises in the minds of the many through its likeness to the truth. And we showed, just now, that in every case it is the person who knows the truth who is best at finding these likenesses. So if you have something else to teach about the art of speeches, we will listen — but if not, we will trust what we have shown.
This is one of Plato's most elegant refutations, and its beauty is that it defeats the opponent not by opposing his goal but by showing he cannot reach it without the very thing he discarded. Tisias wanted mastery of the plausible. Fine, says Socrates — and then demonstrates that mastery of the plausible is impossible without mastery of the true, because the plausible just is resemblance to the true. The argument has two joints, both established earlier in the dialogue and now snapped together. The first joint: the probable is defined as likeness to the truth. What makes an account believable to a jury is that it resembles how things really go; a story is plausible precisely insofar as it looks like the truth. Sever plausibility from all relation to truth and it becomes mere arbitrary assertion, believable to no one. The second joint, from the discussion of deception at 262: to work reliably with likenesses — to construct a resemblance that will hold up, and to see through an opponent's resemblance that will not — you must know the original, the truth, from which the likeness is drawn, because only the knower of the truth can tell a real likeness from a treacherous one and can lead a mind step by plausible step without stumbling into a gap. Combine these and the conclusion is inescapable: the master of plausibility must be a master of truth. Tisias's 'bid farewell to the truth' is thus not merely immoral but incoherent — self-defeating on its own terms. He is like a forger who despises real banknotes: the better he knows the genuine article, the better his forgeries, and a forger who genuinely 'bade farewell' to real money could not forge at all. And notice the deeper implication for the whole quarrel between philosophy and rhetoric. Plato is not saying, as a moralist might, merely that one ought to tell the truth. He is making the stronger, stranger claim that truth is the condition of rhetorical competence itself, even for the liar — that the knowledge the sophist scorns is exactly the knowledge his own trade secretly depends on. The philosopher, the knower of truth, turns out to be the best possible rhetorician, better even at the sophist's own game of plausibility than the sophist is, because he possesses the original of which plausibility is only the copy. This is the ultimate vindication of the dialogue's thesis. Real rhetoric is not the enemy of philosophy but its offspring; and the philosopher, precisely because he knows the truth, could out-persuade every sophist in Athens — which is exactly why it matters so much what he chooses to persuade people of, and why the art must be governed by the love of the good.
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