and the brain, and tells us that when certain nervous impulses have
traveled, let us say, from the eye or the ear to the brain, one has
sensations of sight or sound.

He describes for us in detail how, out of such sensations and the
memories of such sensations, we frame mental images of external things.
Between the mental image and the thing that it represents he
distinguishes sharply, and he informs us that the mind knows no more
about the external thing than is contained in such images. That a
thing is present can be known only by the fact that a message from the
thing is sent along the nerves, and what the thing is must be
determined from the character of the message. Given the image in the
absence of the thing,--that is to say, an hallucination,--the mind will
naturally suppose that the thing is present. This false supposition
cannot be corrected by a direct inspection of the thing, for such a
direct inspection of things is out of the question. The only way in
which the mind concerned can discover that the thing is absent is by
referring to its other experiences. This image is compared with other
images and is discovered to be in some way abnormal. We decide that it
is a false representative and has no corresponding reality behind it.

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