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
