A puzzle about false judgement is raised in the Theaetetus (187d-200c), but not successfully answered there. On the proposed account, the confusion that explicitly vitiates Theaetetus’ final attempt to define knowledge is already at work implicitly in this puzzle. Theaetetus shares popular assumptions about knowledge (epistēmē), but also accepts that there are cognitive constraints on judgement (doxa): the puzzle arises because he fails to distinguish the one cognitive condition from the other.