acquaintance with the whole range of its variations. The study
of hallucinations has in this way been for psychologists the key
to their comprehension of normal sensation, that of illusions has
been the key to the right comprehension of perception. Morbid
impulses and imperative conceptions, "fixed ideas," so called,
have thrown a flood of light on the psychology of the normal
will; and obsessions and delusions have performed the same
service for that of the normal faculty of belief.

Similarly, the nature of genius has been illuminated by the
attempts, of which I already made mention, to class it with
psychopathical phenomena. Borderland insanity, crankiness,
insane temperament, loss of mental balance, psychopathic
degeneration (to use a few of the many synonyms by which it has
been called), has certain peculiarities and liabilities which,
when combined with a superior quality of intellect in an
individual, make it more probable that he will make his mark and
affect his age, than if his temperament were less neurotic.
There is of course no special affinity between crankiness as such
and superior intellect,[7] for most psychopaths have feeble
intellects, and superior intellects more commonly have normal
nervous systems. But the psychopathic temperament, whatever be
the intellect with which it finds itself paired, often brings
with it ardor and excitability of character. The cranky person
has extraordinary emotional susceptibility. He
is liable to fixed ideas and obsessions. His conceptions tend to
pass immediately into belief and action; and when he gets a new
idea, he has no rest till he proclaims it, or in some way "works
it off." "What shall I think of it?" a common person says to
himself about a vexed question; but in a "cranky" mind "What must