nticipation 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. 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. Thus, when a patient takes any small
objects, as pebbles, for gold and silver, under the influence of the
dominant idea of being a millionaire, it is obvious that external
suggestion has very little to do with the self-deception. The confusions
into which the patient often falls with respect to the persons before
him show the same state of mind; for in many cases there is no
discoverable individual resemblance between the person actually present
and the person for whom he is taken.

It is evident that when illusion reaches this stage, it is scarcely
distinguishable from what is specially known