This paper argues for a possible-worlds interpretation of the arguments marshalled by Socrates against Protagoras in Plato’s Theaetetus. Specifically, it reads Protagoras’ position as implying a limited form of modal realism, and evaluates both the self-refutation sequence at 170a–71d and the Future Argument at 177c–9c on the basis of this reading. It emerges that Socrates’ project is only partly successful: while the three main arguments of the self-refutation sequence force Protagoras into ever more awkward and metaphysically top-heavy positions, and the (...) Future Argument is psychologically compelling, these arguments do not force a collapse of the relativist position. Protagoras can still defend himself, though the cost is high. (shrink)