This doctrine taken as it stands seems to cut the mind off from the external world very completely; and the most curious thing about it is that it seems to be built up on the assumption that it is not really true.  How can one know certainly that there is a world of material things, including human bodies with their sense-organs and nerves, if no mind has ever been able to inspect directly anything of the sort?  How can we tell that a sensation arises when a nervous impulse has been carried along a sensory nerve and has reached the brain, if every mind is shut up to the charmed circle of its own ideas?  The anatomist and the physiologist give us very detailed accounts of the sense-organs and of the brain; the physiologist even undertakes to measure the speed with which the impulse passes along a nerve; the psychologist accepts and uses the results of their labors.  But can all this be done in the absence of any first-hand knowledge of the things of which one is talking?  Remember that, if the psychologist is right, any external object, eye, ear, nerve, or brain, which we can perceive directly, is a mental complex, a something in the mind and not external at all.  How shall we prove that there are objects, ears, eyes, nerves, and brains,—­in short, all the requisite mechanism for the calling into existence of sensations,—­in an outer world which is not immediately perceived but is only inferred to exist?

I do not wish to be regarded as impugning the right of the psychologist to make the assumptions which he does, and to work as he does.  He has a right to assume, with the plain man, that there is an external world and that we know it.  But a very little reflection must make it manifest that he seems, at least, to be guilty of an inconsistency, and that he who wishes to think clearly should strive to see just where the trouble lies.

So much, at least, is evident:  the man who is inclined to doubt whether there is, after all, any real external world, appears to find in the psychologist’s distinction between ideas and things something like an excuse for his doubt.  To get to the bottom of the matter and to dissipate his doubt one has to go rather deeply into metaphysics.  I merely wish to show just here that the doubt is not a gratuitous one, but is really suggested to the thoughtful mind by a reflection upon our experience of things.  And, as we are all apt to think that the man of science is less given to busying himself with useless subtleties than is the philosopher, I shall, before closing this chapter, present some paragraphs upon the subject from the pen of a professor of mathematics and mechanics.