SOCRATES: Let us carry the principle which has just been affirmed, that
nothing is self-existent, and then we shall see that white, black, and
every other colour, arises out of the eye meeting the appropriate motion,
and that what we call a colour is in each case neither the active nor the
passive element, but something which passes between them, and is peculiar
to each percipient; are you quite certain that the several colours appear
to a dog or to any animal whatever as they appear to you?

THEAETETUS: Far from it.

SOCRATES: Or that anything appears the same to you as to another man? Are
you so profoundly convinced of this? Rather would it not be true that it
never appears exactly the same to you, because you are never exactly the
same?

THEAETETUS: The latter.

SOCRATES: And if that with which I compare myself in size, or which I
apprehend by touch, were great or white or hot, it could not become
different by mere contact with another unless it actually changed; nor
again, if the comparing or apprehending subject were great or white or hot,
could this, when unchanged from within, become changed by any approximation
or affection of any other thing. The fact is that in our ordinary way of
speaking we allow ourselves to be driven into most ridiculous and wonderful
contradictions, as Protagoras and all who take his line of argument would
remark.

THEAETETUS: How? and of what sort do you mean?

SOCRATES: A little instance will sufficiently explain my meaning: Here