In this paper I explore a famous part of Plato’s Theaetetus where Socrates develops various models of the mind (picturing it first as a wax tablet and then as an aviary full of specimen birds). These are to solve some puzzles about how it is possible to make a mistake. On my interpretation, defended here, the discussion of mistakes is no digression, but is part of the refutation of Theaetetus’s thesis that knowledge is “true doxa”. It reveals that false doxa (...) is possible only if there is a certain stock of abstract knowledge, conceptual knowledge, that is not awareness of the particular individual that is being described. The individual must be identified under some description, or seen as something of a certain kind. Error can only occur if the description applied misdescribes the situation, but then if it is to be applied falsely it must first have been known from somewhere else. So knowledge cannot be reduced to the application of descriptions to particulars, but is to be found in the prior possession of abstract descriptions that can be deployed in identifying particular individuals on the ground. (shrink)