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 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 I do about it?” is the form the question tends to take. In the autobiography of that high‐souled woman, Mrs. Annie Besant, I read the following passage: “Plenty of people wish well to any good cause, but very few care to exert themselves to help it, and still fewer will risk anything in its support. ‘Some one ought to do it, but why should I?’ is the ever reëchoed phrase of weak‐kneed amiability. ‘Some one ought to do it, so why not I?’ is the cry of some earnest servant of man, eagerly forward springing to face some perilous duty. Between these two sentences lie whole centuries of moral evolution.” True enough! and between these two sentences lie also the different destinies of the ordinary sluggard and the psychopathic man. Thus, when a superior intellect and a psychopathic temperament coalesce—as in the endless permutations and