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 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.