For me, G. E. L. Owen's ‘Plato on Not-Being’ radically improved the prospects for a confident overall view of its topic. Hitherto, passage after passage had generated reasonable disagreement over Plato's intentions, and the disputes were not subject to control by a satisfying picture of his large-scale strategy; so that the general impression, as one read the Sophist, was one of diffuseness and unclarity of purpose. By focusing discussion on the distinction between otherness and contrariety (257B1–C4), Owen showed how, at a stroke, a mass of confusing exegetical alternatives could be swept away, and the dialogue's treatment of not-being revealed as a sustained and tightly organised assault on a single error. In what follows, I take Owen's focusing of the issue for granted, and I accept many of his detailed conclusions. Where I diverge from Owen – in particular over the nature of the difficulty about falsehood that Plato tackles in the Sophist (§§5 and 6 below) –it is mainly to press further in the direction he indicated, in the interest of a conviction that the focus can and should be made even sharper.

2. By 256E5–6 the Eleatic Stranger (ES) can say ‘In the case of each of the forms, then, what is is multiple and what is not is indefinite in number.’ Yet it is only at 258B6–7 that Theaetetus is allowed to announce the availability, at last, of the application for ‘what is not’ that was needed in order to flush the sophist from his refuge.