rms of
matter. It is only in those cases in which the unity of the system of
appearances constituting a piece of matter has to be broken up, that the
statement of what is happening cannot be made exclusively in terms of
matter. The whole of psychology, we shall find, is included among such
cases; hence their importance for our purposes.

We can now begin to understand one of the fundamental differences
between physics and psychology. Physics treats as a unit the whole
system of appearances of a piece of matter, whereas psychology is
interested in certain of these appearances themselves. Confining
ourselves for the moment to the psychology of perceptions, we observe
that perceptions are certain of the appearances of physical objects.
From the point of view that we have been hitherto adopting, we
might define them as the appearances of objects at places from which
sense-organs and the suitable parts of the nervous system form part
of the intervening medium. Just as a photographic plate receives a
different impression of a cluster of stars when a telescope is part of
the intervening medium, so a brain receives a different impression
when an eye and an optic nerve are part of the intervening medium.
An impression due to this sort of intervening medium is called a
perception, and is interesting to psychology on its own account, not
merely as one of the set of correlated particulars which is the physical
object of which (as we say) we are having a perception.

We spoke earlier of two ways of classifying particulars. One way
collects together the appearances commonly regarded as a given object
from different places; this is, broadly speaking, the way of physics,
leading to the construction of physical objects as sets of such
appearances. The other way collects together the appearances of
different objects from a given place, the result being what we call a
perspective. In the particular case where the place concerned is a
human brain, the perspective belonging to the place consists of all the
perceptions of a certain man at a given time. Thus classification by
perspectives is relevant to psychology, and is essential in defining
what we mean by one mind.

I do not wish to suggest that the way in which I have been defining
perceptions is the only possible way, or even the best way. It is the
way that arose naturally out of our present topic. But when we approach
psychology from a more introspective standpoint, we have to dist