Overwhelmed by objections to the thesis that knowledge is perception, Theaetetus abandons it. Socrates asks him for another definition. He replies, it consists of judging (doxazein). Socrates plods him to be more specific. Theaetetus observes that there is false as well as true judgment, so “True judgment would have to be knowledge.”[52]
Socrates addresses the question of false judgment, which has remained a thorny problem in Greek philosophy. In Greek idiom the phrase legein ouden, literally: ‘say nothing’ means to speak falsely or lie. But how can one say nothing? Parmenides had argued that say or even think what-is-not.[53] Socrates suggests that false judgment is not judging what is not, but a kind of misjudgment (allodoxia) in which one confuses one thing for another. Plato will expand on this insight in the sequel to this dialogue, the Sophist.[54] He also supposes that thinking consists of carrying on a silent conversation with yourself, in which you make tacit judgments in your head. In effect, then, we entertain propositions mentally before we utter them as statements. This account of thinking as mental assertion will also appear in Plato’s Sophist.[55]
If this model of judging is correct, we can conceive of erroneous judgment as some kind of confusion of mental image A with mental image B. But if we have both images in mind, how can we confuse them? Socrates suggests a piece of wax in which images can be stamped (as in sealing wax). When we perceive something, the image gets stamped on our mind, and this constitutes our memory of an object. We might have memory images of two persons; we see one of them in the distance and hastily associate our image of that person with the wrong memory image; that would be false judgment. We could also imagine that some agents have wax that is either too soft and malleable to make reliable memory images, or too hard to form any image.
Theaetetus finds this account promising. But Socrates realizes that it will not do. At least, it will not account for, for instance, someone’s making an error in calculation when eleven is confused with twelve. In such a case, there would be no sense perception to confuse with a memory image. If we have mastered the science of arithmetic, we should know the numbers and not confuse them. It seems we need a more complex model of knowledge to deal with false judgment.
[52] Plato Theaetetus 187a-b.
[53] Parmenides fr. 2, fr. 6.1-2.
[54] Plato Sophist 236d-238c; 256d-259b.
[55] Plato Sophist 263a-264b, connecting mental speech with truth and falsity.