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 act of
imagination, the less the additional force of external stimulus required
to excite them to full activity.

Considering the first division, passive illusions, a little further, we
shall see that they may be broken up into two sub-classes, according to
the causes of the errors. In a general way we assume that the impression
always answers to some quality of the object which is perceived, and
varies with this; that, for example, our sensation of colour invariably
represents the quality of external colour which we attribute to the
object. Or, to express it physically, we assume that the external force
acting on the sense-organ invariably produces the same effect, and that
the effect always varies with the external cause. But this assumption,
though true in the main, is not perfectly correct. It supposes that the
organic conditions are constant, and that the organic process faithfully
reflects the external operation. Neither of these suppositions is