ccupation of the insane, and
is rightly made use of by Shakespeare as a mark of incipient mental
aberration in Hamlet; and yet this very same occupation is quite natural
to children, and to imaginative adults when they choose to throw the
reins on the neck of their phantasy. Our luminous circle of rational
perception is surrounded by a misty penumbra of illusion. Common sense
itself may be said to admit this, since the greatest stickler for the
enlightenment of our age will be found in practice to accuse most of his
acquaintance at some time or another of falling into illusion.

If illusion thus has its roots in ordinary mental life, the study of it
would seem to belong to the physiology as much as to the pathology of
mind. We may even go further, and say that in the analysis and
explanation of illusion the psychologist may be expected to do more than
the physician. If, on the one hand, the latter has the great privilege
of observing the phenomena in their highest intensity, on the other
hand, the former has the advantage of being familiar with the normal
intellectual process which all illusion simulates or caricatures. To
this it must be added that the physician is naturally disposed to look
at illusion mainly, if not exclusively, on its practical side, that is,
as a concomitant and symptom of cerebral disease, which it is needful to
be able to recognize. The psychologist has a different interest in the
subject, being specially concerned to understand the mental antecedents
of illusion and its relation to accurate perception and belief. It is
pretty evident, indeed, that the phenomena of illusion form a region
common to the psychologist and the mental pathologist, and that the
complete elucidation of the subject will need the co-operation of the
two classes of investigator.

In the present volume an attempt will be made to work out the
psychological side of the subject; that is to say, illusions will be
viewed in their relation to the process of just and accurate perception.
In the carrying out of this plan our principal attention will be given
to the manifestations of the illusory impulse in normal life. At the
same time, though no special acquaintance with the pathology of the
subject will be laid claim to, frequent references will be made to the
illusions of the insane. Indeed, it will be found that the two groups of
phenomena--the illusions of the normal and of the abnormal
condition--are so similar, and pass into