CHAPTER XVIII Knowledge and Perception in PlatoM OST modern men take it for granted that empirical knowledge is dependent upon, or derived
from, perception. There is, however, in Plato and among philosophers of certain other schools, a
very different doctrine, to the effect that there is nothing worthy to be called "knowledge" to be
derived from the senses, and that the only real knowledge has to do with concepts. In this view, "2
- 2 = 4" is genuine knowledge, but such a statement as "snow is white" is so full of ambiguity and
uncertainty that it cannot find a place in the philosopher's corpus of truths.
This view is perhaps traceable to Parmenides, but in its explicit form the philosophic world owes
it to Plato. I propose, in this chapter, to deal only with Plato's criticism of the view that knowledge
is the same thing as perception, which occupies the first half of the Theaetetus.
This dialogue is concerned to find a definition of "knowledge," but ends without arriving at any
but a negative conclusion; several definitions are proposed and rejected, bur no definition that is
considered satisfactory is suggested.
The first of the suggested definitions, and the only one that I shall consider, is set forth by
Theaetetus in the words:
"It seems to me that one who knows something is perceiving the thing that he knows, and, so far
as I can see at present, knowledge is nothing but perception."
Socrates identifies this doctrine with that of Protagoras, that "man is the measure of all things,"
i.e. that any given thing "is to me such as it appears to me, and is to you such as it appears to you."
Socrates adds: "Perception, then, is always something that is, and, as being knowledge, it is
infallible."
A large part of the argument that follows is concerned with the