physics, but only part of the intellectual apparatus which we
have manufactured for manipulating physics. Their self-evidence, if this
is so, lies merely in the fact that they represent our decision as to
the use of words, not a property of physical objects.

Judgments of perception, such as "this buttercup is yellow," are in
a quite different position from judgments of logic, and their
self-evidence must have a different explanation. In order to arrive at
the nucleus of such a judgment, we will eliminate, as far as possible,
the use of words which take us beyond the present fact, such as
"buttercup" and "yellow." The simplest kind of judgment underlying the
perception that a buttercup is yellow would seem to be the perception of
similarity in two colours seen simultaneously. Suppose we are seeing
two buttercups, and we perceive that their colours are similar. This
similarity is a physical fact, not a matter of symbols or words; and it
certainly seems to be indubitable in a way that many judgments are not.

The first thing to observe, in regard to such judgments, is that as they
stand they are vague. The word "similar" is a vague word, since there
are degrees of similarity, and no one can say where similarity ends
and dissimilarity begins. It is unlikely that our two buttercups have
EXACTLY the same colour, and if we judged that they had we should have
passed altogether outside the region of self-evidence. To make our
proposition more precise, let us suppose that we are also seeing a
red rose at the same time. Then we may judge that the colours of the
buttercups are more similar to each other than to the colour of the
rose. This judgment seems more complicated, but has certainly gained
in precision. Even now, however, it falls short of complete precision,
since similarity is not prima facie measurable, and it would require
much discussion to decide what we mean by greater or less similarity. To
this process of the pursuit of precision there is strictly no limit.

The next thing to observe (although I do not personally doubt that most
of our judgments of perception are true) is that it is very difficult to
define any class of such judgments which can be known, by its intrinsic
quality, to be always exempt from error. Most of our judgments of
perception involve correlations, as when we judge that a certain noise
is that of a passing cart. Such judgments are all obviously liable to
error, since there is no correlation of