The realities to which we actually refer appearances 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.