as 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 te