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