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