ABSTRACT
VERY early in the dialogue, at 145E, Socrates asks the young Theaetetus for a definition of knowledge. Like various other persons questioned by Socrates, Theaetetus at first fails to grasp the nature of general definition, but he soon comes to under stand what Socrates wants, and, at 151E, he puts forward the view that knowledge is perception. Socrates' reaction to this is to point out that such a definition is tied both to the relativism of Protagoras and to the all-flowing philosophy of the Heracliteans. He proceeds to develop a number of consequences of the flowing philosophy, but does so in such a way that Theaetetus is not clear as to whether this philosophy (and thus his own definition) is being attacked or not 157C Even after the passage i57Eff. in which the flowing philosophy turns out to provide no criterion by which waking may be distinguished from dreams or sanity from insanity, Theaetetus is apparently still in doubt, since he raises no objection when Socrates says 160D: Therefore you were quite right in saying that knowledge is nothing else than perception, and there is complete identity between the doctrine of Homer and Heraclitus and all their followers-that all things are in motion, like streams-the doctrine of the great philo sopher Protagoras that man is the measure of all things-and the doctrine of Theaetetus that, since these things are true, perception is knowledge. But after the speech 161Cff. in which Socrates suggests that Protagoras' measure might as well be a pig or a dog-faced
baboon, Theaetetus' faith in his definition is severely shaken; he says 162C, 'when we were discussing the meaning of the doctrine that whatever appears to each one really is to him, I thought it was good; but now it has suddenly changed to the opposite.' It is at this point, when Theaetetus has begun to wonder whether his definition will hold after all, that the pas sage with which I am concerned in this chapter begins.
