simplest possible
illustration. Consider a certain star, and suppose for the moment
that its size is negligible. That is to say, we will regard it as, for
practical purposes, a luminous point. Let us further suppose that it
exists only for a very brief time, say a second. Then, according to
physics, what happens is that a spherical wave of light travels outward
from the star through space, just as, when you drop a stone into a
stagnant pond, ripples travel outward from the place where the stone hit
the water. The wave of light travels with a certain very nearly constant
velocity, roughly 300,000 kilometres per second. This velocity may be
ascertained by sending a flash of light to a mirror, and observing
how long it takes before the reflected flash reaches you, just as the
velocity of sound may be ascertained by means of an echo.

What it is that happens when a wave of light reaches a given place we
cannot tell, except in the sole case when the place in question is a
brain connected with an eye which is turned in the right direction. In
this one very special case we know what happens: we have the sensation
called "seeing the star." In all other cases, though we know (more or
less hypothetically) some of the correlations and abstract properties
of the appearance of the star, we do not know the appearance itself. Now
you may, for the sake of illustration, compare the different appearances
of the star to the conjugation of a Greek verb, except that the number
of its parts is really infinite, and not only apparently so to the
despairing schoolboy. In vacuo, the parts are regular, and can be
derived from the (imaginary) root according to the laws of grammar,
i.e. of perspective. The star being situated in empty space, it may be
defined, for purposes of physics, as consisting of all those appearances
which it presents in vacuo, together with those which, according to
the laws of perspective, it would present elsewhere if its appearances
elsewhere were regular. This is merely the adaptation of the definition
of matter which I gave in an earlier lecture. The appearance of a star
at a certain place, if it is regular, does not require any cause or
explanation beyond the existence of the star. Every regular appearance
is an actual member of the system which is the star, and its causation
is entirely internal to that system. We may express this by saying that
a regular appearance is due to the star alone, and is actually part of
t