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.