judgement I call a "judgement of perception." The proposition "knowledge is perception" must be
interpreted as meaning "knowledge is judgements of perception." It is only in this form that it is
grammatically capable of being correct.
To return to likeness and unlikeness, it is quite possible, when I perceive two colours
simultaneously, for their likeness or unlikeness to be part of the datum, and to be asserted in a
judgement of perception. Plato's argument that we have no sense-organ for perceiving likeness
and unlikeness ignores the cortex, and assumes that all senseorgans must be at the surface of the
body.
The argument for including likeness and unlikeness as possible perceptive data is as follows. Let
us assume that we see two shades of colour A and B, and that we judge "A is like B." Let us
assume further, as Plato does, that such a judgement is in general correct, and, in particular, is
correct in the case we are considering. There is, then, a relation of likeness between A and B, and
not merely a judgement on our part asserting likeness. If there were only our judgement, it would
be an arbitrary judgement, incapable of truth or falsehood. Since it obviously is capable of truth or
falsehood, the likeness can subsist between A and B, and cannot be merely something "mental."
The judgement "A is like B" is true (if it is true) in virtue of a "fact," just as much as the
judgement "A is red" or "A is round." The mind is no more involved in the perception of likeness
than in the perception of colour.
I come now to existence, on which Plato lays great stress. We have, he says, as regards sound and
colour, a thought which includes both at once, namely that they exist. Existence belongs to
everything, and is among the things that the mind apprehends by itself; without reaching
existence, it is impossible to reach truth.
The argument against Plato here is quite different from that in the case of likeness and unlikeness.
The argument here is that all that Plato says about existence is bad grammar, or rather bad syntax.
This point is important, not only in connection with Plato, but also with other matters such as the
ontological argument for the existence of the Deity.
Suppose you say to a child "lions exist, but unicorns don't," you can prove your point so far as
lions are concerned by taking him to the Zoo and saying "look, that's a lion." You will not, unless
you are a