and I say, secondly, that it always leads to
a better understanding of a thing's significance to consider its
exaggerations and perversions its equivalents and substitutes and
nearest relatives elsewhere. Not that we may thereby swamp the thing
in the wholesale condemnation which we pass on its inferior congeners,
but rather that we may by contrast ascertain the more precisely in what
its merits consist, by learning at the same time to what particular
dangers of corruption it may also be exposed.

Insane conditions have this advantage, that they isolate special
factors of the mental life, and enable us to inspect them unmasked by
their more usual surroundings. They play the part in mental anatomy
which the scalpel and the microscope play in the anatomy of the body.
To understand a thing rightly we need to see it both out of its
environment and in it, and to have 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 g