of sensation answers to the important fact that, under
usual circumstances, the agency producing the sensation is applied at
this particular point of the organism, the knowledge of which point is
supposed by modern psychologists to have been very slowly learnt by the
individual and the race, through countless experiments with the moving
organ of touch, assisted by the eye.

Similarly, the reference of the impression, in the case of hearing and
sight, to an object in the environment, though, as we have seen, from
one point of view illusory, clearly answers to a fact of our habitual
experience; for in an immense preponderance of cases at least a visual
or auditory impression does arise through the action on the sense-organ
of a force (ether or air waves) proceeding from a distant object.

In some circumstances, however, even this element of practical truth
disappears, and the localization of the impression, both within and
without the organism, becomes altogether illusory. This result is
involved in the illusions, already spoken of, which arise from the
instinctive tendency to refer sensations to the ordinary kind of
stimulus. Thus, when a feeling resulting from a disturbance in the optic
nerve is interpreted as one of external light vaguely felt to be acting
on the eye, or one resulting from some action set up in the auditory
fibre as a sensation of external sound vaguely felt to be entering the
ear, we see that the error of localization is a consequence of the other
error already characterized.

As I have already observed, an excitation of a nerve at any other point
than the peripheral termination, occurs but rarely in normal life. One
familiar instance is the stimulation of the nerve running to the hand
and fingers, by a sharp blow on the elbow over which it passes. As
everybody knows, this gives rise to a sense of pain at the _extremities_
of the nerve. The most common illustration of such errors of
localization is found in subjective sensations, such as the impression
we sometimes have of something creeping over the skin, of a disagreeable
taste in the mouth, of luminous spots floating across the field of
vision, and so on. The exact physiological seat of these is often a
matter of conjecture only; yet it may safely be said that in many
instances the nervous excitation originates at some point considerably
short of its peripheral extremity: in which case there occurs the
illusion of referring the impressions to the