The study of physical functions shows in a dim way that in bodily activities there is an ideal mean. Both over-use and under-use may produce disorder. Stimulation of any organ up to a certain point causes development. Over-stimulation may produce changes that lead to atrophy. Under-stimula-tion or lack of stimulation may have the same effect.

In the realm of the non-physical the same conditions are met with, perhaps with more distinctness. The extrovert who pursues a course of unchecked and increasing extroversion will find himself in opposition to a force which comes from the unconscious and which may suddenly cut him off, by some disaster, from his line of action. The introvert who withdraws more and more from the world will find an opposite doctrine coming from the unconscious. Thus it is necessary to conceive of some ideal mean, of some middle path or line lying between introversion and extroversion, where a balance is struck between thought, feeling, and action. To those on one side of the line one set of proverbs and teachings will apply. To those on the other side of the line another set of proverbs and teachings will apply. Thus it is possible to see how it comes about that some of the great teachers of human conduct have spoken in a high form of paradox, and how human wisdom, preserved in proverbs, rests on the same basis. Por it is almost always possible to find one proverb to contradict another proverb.

To most people the conception of continuity, of steady progress, visualized as a straight line, underlies the idea of truth. In science this conception is tacitly affirmed. But in the history of the world this consistent and steady progress along a line is not found in the record of nations. History is sharply divided from any pattern of scientific truth. It does not present itself as a building rising slowly and persistently, but on the contrary it shows a continual spectacle of action and reaction, of evolution and involution, of extroversion and introversion. Overlying the whole in terms of a much vaster perspective than those given by ordinary historical methods, progress can perhaps be detected. But to the ordinary observer what is instantly apparent is that where life is concerned, the linear progress affirmed by science does not exist. Ebb and flow is everywhere visible. A nation blossoms forth into some great extroversion like that of the Elizabethan age; the tide turns and introversion sets in. If the national experience be contracted to fit into the circle of individual experience a complete correspondence is found, because it is simply human experience magnified. A man rises, blossoms out into some great individual extroversion, and then suddenly the tide turns, the star sets, the sap dries up, the branch withers, the fires die out— there are countless metaphors for this typical event —and try as he will he can do nothing more. He may still retain health and vigour, and probably will attribute his failures to bad luck. He may outlive his phase of special extroversion, as Napoleon did, and spend his time in looking for the cause in external events. But the true cause lies elsewhere, for something within himself has ceased to be as it once was. It is on this familiar experience, upon which the world's pessimism has been lavished, that the vanity of life finds support as a doctrine.

But truth does not necessarily lie on the surface. A man who spurs himself on to attain some high pinnacle of fame and comes down with a crash need not be taken as a last word on life, but serves rather as an introduction to the study of life. If you look for the explanations in external circumstances alone, you will find them. It will be possible to trace exactly where his judgment was at fault and how the threads of disaster converged. But if you shift the inquiry to another plane and seek for the causes behind the error of judgment, you will find yourself studying the unconscious mind, and that inner psychological situation which is comparable to a house divided against itself.