e
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
nervous operation in our past history. In the case of active illusions,
however, which spring rather from the independent energy of a particular
mode of the imagination, this point of organic connection is not the
only or even the main thing. In many cases, as we shall see, there is
only a faint shade of resemblance between the present impression and the
mental image with which it is overlaid. The illusions dependent on
vivid, expectation thus answer much less to an objective conjunction of
past experiences than to a capricious subjective conjunction of mental
images. Here, then, the fusion of nervous processes must have another
cause. And it is not difficult to assign such a cause. The antecedent
activity of imagination doubtless involves as its organic result a
powerful temporary disposition in the nervous structures concerned to go
on acting. In other words, they remain in a state of sub-excitation,
which can be raised to full excitation by a slight additional force. The
more powerful this disposition in the centres involved in the ac