tence? But now we make
the further discovery, that neither white or whiteness, nor any sense
or sensation, can be predicated of anything, for they are in a perpetual
flux. And therefore we must modify the doctrine of Theaetetus and
Protagoras, by asserting further that knowledge is and is not sensation;
and of everything we must say equally, that this is and is not, or
becomes or becomes not. And still the word 'this' is not quite correct,
for language fails in the attempt to express their meaning.

At the close of the discussion, Theodorus claims to be released from the
argument, according to his agreement. But Theaetetus insists that they
shall proceed to consider the doctrine of rest. This is declined by
Socrates, who has too much reverence for the great Parmenides lightly
to attack him. (We shall find that he returns to the doctrine of rest
in the Sophist; but at present he does not wish to be diverted from
his main purpose, which is, to deliver Theaetetus of his conception of
knowledge.) He proceeds to interrogate him further. When he says that
'knowledge is in perception,' with what does he perceive? The first
answer is, that he perceives sights with the eye, and sounds with the
ear. This leads Socrates to make the reflection that nice distinctions
of words are sometimes pedantic, but sometimes necessary; and he
proposes in this case to substitute the word 'through' for 'with.' For
the senses are not like the Trojan warriors in the horse, but have
a common centre of perception, in which they all meet. This common
principle is able to compare them with one another, and must therefore
be distinct from them (compare Republic). And as there are facts of
sense which are perceived through the organs of the body, there are also
mathematical and other abstractions, such as sameness and difference,
likeness and unlikeness, which the soul perceives by herself. Being is
the most universal of these abstractions. The good and the beautiful are
abstractions of another kind, which exist in relation and which above
all others the mind perceives in herself, comparing within her past,
present, and future. For example; we know a thing to be hard or soft by
the touch, of which the perception is given at birth to men and animals.
But the essence of hardness or softness, or the fact that this hardness
is, and is the opposite of softness, is slowly learned by reflection and
experience. Mere perception does not reach being, and therefore