as explained above. Thus, for example, we fall into the illusion of
hearing two voices when our shout is echoed back, just because the
second auditory impression irresistibly calls up the image of a second
shouter. On the other hand, a man experiences the illusion of seeing
spectres of familiar objects just after exciting his imagination over a
ghost-story, because the mind is strongly predisposed to frame this kind
of percept. The first class of illusions arises from without, the
sense-impression being the starting-point, and the process of
preperception being controlled by this. The second class arises rather
from within, from an independent or spontaneous activity of the
imagination. In the one case the mind is comparatively passive; in the
other it is active, energetically reacting on the impression, and
impatiently anticipating the result of the normal process of
preperception. Hence I shall, for brevity's sake, commonly speak of them
as Passive and Active Illusions.[16]

I may, perhaps, illustrate these two classes of illusion by the simile
of an interpreter poring over an old manuscript. The first would be due
to some peculiarity in the document misleading his judgment, the second
to some caprice or preconceived notion in the interpreter's mind.

It is not difficult to define conjecturally the physiological conditions
of these two large classes of illusion. On the physical side, an
illusion of sense, like a just perception, is the result of a fusion of
the nervous process answering to a sensation with a nervous process
answering to a mental image. In the case of passive illusions, this
fusion may be said to take place in consequence of some point of
connection between the two. The existence of such a connection appears
to be involved in the very fact of suggestion, and may be said to be
the organic result of frequent conjunctions of the two parts of the