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 h