reference to the
real world of touch things, for which visual experiences serve as
signs. Under certain circumstances, the mountain _ought_ to be robed
in its azure hue, and, under certain circumstances, it _ought not_.
The circumstances in each case are open to investigation.

Now, let us substitute for the real world of touch things, which
furnishes the explanation of given visual experiences, that philosophic
fiction, that pseudo-real nonentity, the Unknowable. Now I perceive a
tree as faint and blue, now as bright and green; will a reference to
the Unknowable explain why the experiences differed? Was the
Unknowable in the one instance farther off in an unknowable space, and
in the other nearer? This, even if it means anything, must remain
unknowable. And when the chemist puts together a volume of chlorine
gas and a volume of hydrogen gas to get two volumes of hydrochloric
acid gas, shall we explain the change which has taken place by a
reference to the Unknowable, or shall we turn to the doctrine of atoms
and their combinations?

The fact is that no man in his senses tries to account for any
individual fact by turning for an explanation to the Unknowable. It is
a life-preserver by which some set great store, but which no man dreams
of using when he really falls into the water.

If, then, we have any reason to believe that there is a real external
world at all, we have reason to believe that we know what it is. That
some know it imperfectly, that others know it better, and that we may
hope that some day it will be known still more perfectly, is surely no
good reason for concluding that we do not know it at all.

[1] "First Principles," Part I, Chapter IV, section 26.

CHAPTER VI

OF SPACE

23. WHAT ARE WE SUPPOSED TO KNOW ABOUT IT.--The plain man may admit
that he is not ready to hazard a definition of space, but he is
certainly not willing to admit that he is wholly ignorant of space and
of its attributes. He knows that it is something in which material
objects have position and in which they move about; he knows that it
has not merely length, like a line, nor length and breadth, like a
surface, but has the three dimensions of length, breadth, and depth; he
knows that, except in the one circumstance of its position, every part
of space is exactly like every other part, and that, although objects
may move about in space, it is incredible that the spaces themselves
should be shifted about.