serve to explain them.  Thus, when I ask:  Why do I perceive that tree now as faint and blue and now as vivid and green? the answer to the question is found in the notion of distance and position in space; it is found, in other words, in a 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.

Those who are familiar with the literature of the subject know that it has long been customary to make regarding space certain other statements to which the plain man does not usually make serious objection when he is introduced to them.  Thus it is said:—­