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Most studies of the Theaetetus concentrate on Plato’s examination of Protagoras’s ‘Man is the Measure’ doctrine— and rightly so. The bulk of the dialogue is after all devoted to an exhaustive critique of this doctrine and its consequences, and in order to understand Plato’s views it is surely crucial to determine what position he sets up in contrast to his own. Commentators differ, however, when it comes to the finer points of Protagoras’s position— particularly concerning the validity of Plato’s infamous self-refutation argument against the Measure Doctrine at 171A6-C7—and its relation to Heraclitean flux. After some preliminaries on the overall structure of the Theaetetus, I single out in this paper two interpretations of the Measure Doctrine: Myles Bumyeat’s relativist reading and Gail Fine’s more recent infallibilist reading.

These interpretations require close investigation and comparison to determine which better fits Plato’s argument in the dialogue. Nonetheless, one of my aims here is to suggest that Plato allows both these readings of the Measure Doctrine: Protagoras’s position is inherently ambiguous, and by distinguishing between theoretical and practical objections to his doctrine, I argue that the Theaetetus provides a fitting response to both relativism and infallibilism. Furthermore, once Plato’s critiques of the Measure Doctrine and Heraclitean flux are read together, the consequences of Bumyeat’s and Fine’s interpretations turn out to be compatible. Plato’s strategy in this first part of the dialogue is to argue that Theaetetus’s empiricist theory of knowledge ultimately fails because it makes the shared agreements that are the touchstone of philosophical analysis either pointless or impossible. Before inquiring into the conditions for knowledge, therefore, he must first determine the conditions that make rational inquiry and discussion meaningful. Focusing on this latter demand assigns a purpose to the Theaetetus that has not yet been fully appreciated: a concern with the practice of dialectic. It also, I believe, yields some distinctly Platonic conclusions.


Tushar Irani presented “Theory and Practice in Plato’s Theaetetus: The Question of Knowledge and the Primacy of Dialectic” to the Society at its meeting with the Pacific Division in Pasadena in 2004.

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