It is, of course, often difficult to say, in such cases as these, how far elements of actual sensation co-operate in the production of the illusions.  Thus, in the case just mentioned, the odour of the earth may have been the starting-point in the illusion.  In many cases, however, an imaginative mind appears to be capable of transforming a vivid expectation into a nascent stage of sensation.  Thus, a mother thinking of her sick child in an adjoining room, and keenly on the alert for its voice, will now and again fancy she really hears it when others hear nothing at all.

Transition to Hallucination.

It is plain that in these cases illusion approaches to hallucination.  Imagination, instead of waiting on sensation, usurps its place and imitates its appearance.  Such a “subjective” sensation produced by a powerful expectation might, perhaps, by a stretch of language, be regarded as an illusion, in the narrow sense, in so far as it depends on the suggestive force of a complete set of external circumstances; on the other hand, it is clearly an hallucination in so far as it is the production of the semblance of an external impression without any external agency corresponding to this.

In the class of illusory expectations just considered the immediately present environment still plays a part, though a much less direct part than that observable in the first large group of illusions.  We will now pass to a second mode of illusory expectation, where imagination is still more detached from the present surroundings.

A common instance of this kind of expectation is the so-called “intuition,” or presentiment; that something is going to happen, which expectation has no basis in fact.  It does not matter whether the expectation has arisen by way of another’s words or by way of personal inclinations.  A strong wish for a thing will, in an exalted state of mind, beget a vivid anticipation of it.  This subject will be touched on again under the Illusions of Belief.  Here I am concerned to point out that such presentiments are fertile sources of sense-illusion.  The history of Church miracles, visions, and the like amply illustrates the effect of a vivid anticipation in falsifying the perceptions of external things.

In persons of a lively imagination any recent occupation of the mind with a certain kind of mental image may suffice to beget something equivalent to a powerful mode of expectation.  For example, we are told by Dr. Tuke that on one occasion a lady, whose imagination had been dwelling on the subject of drinking fountains, “thought she saw in a road a newly erected fountain, and even distinguished an inscription upon it, namely, ‘If any man thirst, let him come unto Me, and drink.’  She afterwards found that what she had actually seen was only a few scattered stones."[56] In many cases there seems to be a temporary preternatural activity of the imagination in certain directions, of which no very obvious explanation is discoverable.