inguish
sensations and perceptions, if possible, from other mental occurrences,
if any. We have also to consider the psychological effects of
sensations, as opposed to their physical causes and correlates. These
problems are quite distinct from those with which we have been concerned
in the present lecture, and I shall not deal with them until a later
stage.

It is clear that psychology is concerned essentially with actual
particulars, not merely with systems of particulars. In this it differs
from physics, which, broadly speaking, is concerned with the cases
in which all the particulars which make up one physical object can be
treated as a single causal unit, or rather the particulars which are
sufficiently near to the object of which they are appearances can be so
treated. The laws which physics seeks can, broadly speaking, be stated
by treating such systems of particulars as causal units. The laws which
psychology seeks cannot be so stated, since the particulars themselves
are what interests the psychologist. This is one of the fundamental
differences between physics and psychology; and to make it clear has
been the main purpose of this lecture.

I will conclude with an attempt to give a more precise definition of
a piece of matter. The appearances of a piece of matter from different
places change partly according to intrinsic laws (the laws of
perspective, in the case of visual shape), partly according to the
nature of the intervening medium--fog, blue spectacles, telescopes,
microscopes, sense-organs, etc. As we approach nearer to the object, the
effect of the intervening medium grows less. In a generalized sense,
all the intrinsic laws of change of appearance may be called "laws
of perspective." Given any appearance of an object, we can construct
hypothetically a certain system of appearances to which the appearance
in question would belong if the laws of perspective alone were
concerned. If we construct this hypothetical system for each appearance
of the object in turn, the system corresponding to a given appearance
x will be independent of any distortion due to the medium beyond x, and
will only embody such distortion as is due to the medium between x and
the object. Thus, as the appearance by which our hypothetical system
is defined is moved nearer and nearer to the object, the hypothetical
system of appearances defined by its means embodies less and less of the
effect of the medium. The different sets of app