because the table exists continuously that all these 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