r own case, the one group is our body (or
our brain), while the other is our mind, in so far as it consists of
perceptions. In the case of the photographic plate, the first group is
the plate as dealt with by physics, the second the aspect of the heavens
which it photographs. (For the sake of schematic simplicity, I am
ignoring various complications connected with time, which require some
tedious but perfectly feasible elaborations.) Thus what may be called
subjectivity in the point of view is not a distinctive peculiarity of
mind: it is present just as much in the photographic plate. And the
photographic plate has its biography as well as its "matter." But
this biography is an affair of physics, and has none of the peculiar
characteristics by which "mental" phenomena are distinguished, with the
sole exception of subjectivity.

Adhering, for the moment, to the standpoint of physics, we may define a
"perception" of an object as the appearance of the object from a place
where there is a brain (or, in lower animals, some suitable nervous
structure), with sense-organs and nerves forming part of the intervening
medium. Such appearances of objects are distinguished from appearances
in other places by certain peculiarities, namely:

(1) They give rise to mnemic phenomena;

(2) They are themselves affected by mnemic phenomena.

That is to say, they may be remembered and associated or influence our
habits, or give rise to images, etc., and they are themselves different
from what they would have been if our past experience had been
different--for example, the effect of a spoken sentence upon the hearer
depends upon whether the hearer knows the language or not, which is
a question of past experience. It is these two characteristics, both
connected with mnemic phenomena, that distinguish perceptions from the
appearances of objects in places where there is no living being.

Theoretically, though often not practically, we can, in our perception
of an object, separate the part which is due to past experience from the
part which proceeds without mnemic influences out of the character of
the object. We may define as "sensation" that part which proceeds in
this way, while the remainder, which is a mnemic phenomenon, will have
to be added to the sensation to make up what is called the "perception."
According to this definition, the sensation is a theoretical core in
the actual experience; the actual experience is the perception. It
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