sensations. I must not confound
the sensations I receive from paper with those which I receive from a
piece of wood. Both identities and differences of sensations must be
known before I can say "this piece of paper." The same is true when I
go on to say that it "is white." This is only possible by classifying
it with other white objects, and differentiating it from objects of
other colours. But the senses themselves cannot perform these acts of
comparison and contrast. Each sensation is, so to speak, an isolated
dot. It cannot go beyond itself to compare itself with others. This
operation must be performed by my mind, which acts as a co-ordinating
central authority, receiving the isolated sensations, combining,
comparing, and contrasting them. This is particularly noticeable in
cases where we compare sensations of one sense with those of another.
Feeling a ball with my fingers, I say it feels round. Looking at it
with my eyes, I say it looks round. But the feel is quite a different
sensation from the look. Yet I use the same word, "round," to describe
both. And this shows that I have identified the two sensations. This
{181} cannot be done by the senses themselves. For my eyes cannot
feel, and my fingers cannot see. It must be the mind itself, standing
above the senses, which performs the identification. Thus the ideas of
identity and difference are not yielded to me by my senses. The
intellect itself introduces them into things. Yet they are involved in
all knowledge, for they are involved even in the simplest acts of
knowledge, such as the proposition, "This is white." Knowledge,
therefore, cannot consist simply of sense-impressions, as Protagoras
thought, for even the simplest propositions contain more than
sensation.

If knowledge is not the same as perception, neither is it, on the
other hand, the same as opinion. That knowledge is opinion is the
second false theory that Plato seeks to refute. Wrong opinion is
clearly not knowledge. But even right opinion cannot be called
knowledge. If I say, without any grounds for the statement, that there
will be a thunderstorm next Easter Sunday, it may chance that my
statement turns out to be correct. But it cannot be said that, in
making this blind guess, I had any knowledge, although, as it turned
out, I had right opinion. Right opinion may also be grounded, not on
mere guess-work, but on something which, though better, is still not
true understanding. We often feel intuit