(1) All the appearances of different stars in a given place, or,
(2) All the appearances of a given star in different places.
But when I speak of "appearances," I do so only for brevity: I do not
mean anything that must "appear" to somebody, but only that happening,
whatever it may be, which is connected, at the place in question, with a
given physical object--according to the old orthodox theory, it would be
a transverse vibration in the aether. Like the different appearances
of the table to a number of simultaneous observers, the different
particulars that belong to one physical object are to be collected
together by continuity and inherent laws of correlation, not by their
supposed causal connection with an unknown assumed existent called a
piece of matter, which would be a mere unnecessary metaphysical thing in
itself. A piece of matter, according to the definition that I propose,
is, as a first approximation,* the collection of all those correlated
particulars which would normally be regarded as its appearances or
effects in different places. Some further elaborations are desirable,
but we can ignore them for the present. I shall return to them at the
end of this lecture.
*The exact definition of a piece of matter as a construction
will be given later.
According to the view that I am suggesting, a physical object or piece
of matter is the collection of all those correlated particulars which
would be regarded by common sense as its effects or appearances in
different places. On the other hand, all the happenings in a given place
represent what common sense would regard as the appearances of a number
of different objects as viewed from that place. All the happenings in
one place may be regarded as the view of the world from that place. I
shall call the view of the world from a given place a "perspective." A
photograph represents a perspective. On the other hand, if photographs
of the stars were taken in all points throughout space, and in all such
photographs a certain star, say Sirius, were picked out whenever it
appeared, all the different appearances of Sirius, taken together,
would represent Sirius. For the understanding of the difference between
psychology and physics it is vital to understand these two ways of
classifying particulars, namely:
(1) According to the place where they occur;
(2) According to the system of correlated particulars in different
places to which they belong, such system being defined as a physical
object.
Given a system of particulars which is a physical object, I shall
define that one of the system which is in a given place (if any) as the
"appearance of that object in that place."
When the appearance of an object in a given place changes, it is found
that one or other of two things occurs. The two possibilities may be
illustrated by an example. You are in a room with a man, whom you see:
you may cease to see him either by shutting your eyes or by his going
out of the room. In the first case, his appearance to other people
remains unchanged; in the second, his appearance changes from all
places. In the first case, you say that it is not he who has changed,
but your eyes; in the second, you say that he has changed. Generalizing,
we distinguish--
(1) Cases in which only certain appearances of the object change, while
others, and especially appearances from places very near to the object,
do not change;
(2) Cases where all, or almost all, the appearances of the object
undergo a connected change.
In the first case, the change is attributed to the medium between the
object and the place; in the second, it is attributed to the object
itself.*
* The application of this distinction to motion raises
complications due to relativity, but we may ignore these for
our present purposes.
It is the frequency of the latter kind of change, and the comparatively
simple nature of the laws governing the simultaneous alterations
of appearances in such cases, that have made it possible to treat a
physical object as one thing, and to overlook the fact that it is a
system of particulars. When a number of people at a theatre watch an
actor, the changes in their several perspectives are so similar and so
closely correlated that all are popularly regarded as identical with
each other and with the changes of the actor himself. So long as all
the changes in the appearances of a body are thus correlated there is no
pressing prima facie need to break up the system of appearances, or to
realize that the body in question is not really one thing but a set of
correlated particulars. It is especially and primarily such changes that
physics deals with, i.e. it deals primarily with processes in which
the unity of a physical object need not be broken up because all its
appearances change simultaneously according to the same law--or, if not
all, at any rate all from places sufficiently near to the object, with
in creasing accuracy as we approach the object.
The changes in appearances of an object which are due to changes in the
intervening medium will not affect, or will affect only very slightly,
the appearances from places close to the object. If the appearances
from sufficiently neighbouring places are either wholly un changed,
or changed to a diminishing extent which has zero for its limit, it
is usually found that the changes can be accounted for by changes in
objects which are between the object in question and the places from
which its appearance has changed appreciably. Thus physics is able
to reduce the laws of most changes with which it deals to changes in
physical objects, and to state most of its fundamental laws in terms 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 distinguish
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 appearances resulting from
moving x nearer and nearer to the object will approach to a limiting
set, and this limiting set will be that system of appearances which the
object would present if the laws of perspective alone were operative
and the medium exercised no distorting effect. This limiting set of
appearances may be defined, for purposes of physics, as the piece of
matter concerned.