se sense-data will reappear
when I open my eyes, replace my arm, and begin again to rap with my
knuckles. The question we have to consider in this chapter is: What
is the nature of this real table, which persists independently of my
perception of it?

To this question physical science gives an answer, somewhat incomplete
it is true, and in part still very hypothetical, but yet deserving of
respect so far as it goes. Physical science, more or less unconsciously,
has drifted into the view that all natural phenomena ought to be reduced
to motions. Light and heat and sound are all due to wave-motions, which
travel from the body emitting them to the person who sees light or feels
heat or hears sound. That which has the wave-motion is either aether or
'gross matter', but in either case is what the philosopher would call
matter. The only properties which science assigns to it are position in
space, and the power of motion according to the laws of motion. Science
does not deny that it _may_ have other properties; but if so, such other
properties are not useful to the man of science, and in no way assist
him in explaining the phenomena.

It is sometimes said that 'light _is_ a form of wave-motion', but this
is misleading, for the light which we immediately see, which we know
directly by means of our senses, is _not_ a form of wave-motion, but
something quite different--something which we all know if we are not
blind, though we cannot describe it so as to convey our knowledge to a
man who is blind. A wave-motion, on the contrary, could quite well be
described to a blind man, since he can acquire a knowledge of space by
the sense of touch; and he can experience a wave-motion by a sea voyage
almost as well as we can. But this, which a blind man can understand, is
not what we mean by _light_: we mean by _light_ just that which a blind
man can never understand, and which we can never describe to him.

Now this something, which all of us who are not blind know, is not,
according to science, really to be found in the outer world: it is
something caused by the action of certain waves upon the eyes and nerves
and brain of the person who sees the light. When it is said that light
_is_ waves, what is really meant is that waves are the physical cause of
our sensations of light. But light itself, the thing which seeing people
experience and blind people do not, is not supposed by science to form
any part of the world that is independent of