as hallucination. As I have
remarked in setting out, illusion and hallucination shade one into the
other much too gradually for us to draw any sharp line of demarcation
between them. And here we see that hallucination differs from illusion
only in the proportion in which the causes are present. When the
internal imaginative impulse reaches a certain strength, it becomes
self-sufficient, or independent of any external impression.

This intimate relation between the extreme form of active illusion and
hallucination may be seen, too, by examining the physical conditions of
each. As I have already remarked, active illusion has for its
physiological basis a state of sub-excitation, or an exceptional
condition of irritability in the structures engaged in the act of
interpretative imagination. The greater the degree of this irritability,
the less will be the force of external stimulation needed to produce the
effect of excitation, and the more energetic will be the degree of this
excitation. Moreover, it is plain that this increase in the strength of
the excitation will involve an extension of the area of excitation till,
by-and-by, the peripheral regions of the nervous system may be involved
just as in the case of external stimulation. This accounts for the
gradual displacement of the impression of the moment by the mental
image. It follows that when the irritability reaches a certain degree,
the amount of external stimulus needed may become a vanishing quantity,
or the state of sub-excitation may of itself develop into one of full
activity.

_Hallucinations._

I do not propose to go very fully into the description and explanation
of hallucinations here, since they fall to a large extent under the
category of distinctly pathological phenomena. Yet our study of
illusions would not be complete without a glance at this part of the
subject.

Hallucination, by which I mean the projection of a mental image outwards
when there is no external agency answering to it, assumes one of two
fairly distinct forms: it may present itself either as a semblance of an
external impression with the minimum amount of interpretation, or as a
counterfeit of a completely developed percept. Thus, a visual
hallucination may assume the aspect of a sensation of light or colour
which we vaguely refer to a certain region of the external world, or of
a vision of some recognizable object. All of us frequently have
incomplete visual and auditory halluci