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.

Thus, we sometimes find our minds dwelling on some absent friend, without being able to give any reason for this mental preoccupation.

And in this way arise strong temporary leanings to illusory perception.

It may be said, indeed, that all unwonted activity of the imagination, however it arises, has as its immediate result a temporary mode of expectation, definite or indefinite, which easily confuses our perceptions of external things.
In proportion as this pre-existing imaginative impulse becomes more powerful, the amount of actual impression necessary to transform the mental image into an illusory perception becomes less; and, what is more important, this transformation of the internal image involves a larger and larger displacement of the actual impression of the moment.

A man whose mind is at the time strongly possessed by one kind of image, will tend to project this outwards with hardly any regard to the actual external circumstances.
This state of things is most completely illustrated in many of the grosser illusions of the insane.