It may not be quite correct to say that all pleasurable sensations are immediately derived from or directly lead to others of a painful character. There are some belonging to the organic life, such as the satisfaction of the desire for food or for muscular exercise, in which mere repose or remission is enough to repay the nervous drafts. But how close these are to pain is visible from the acuteness of the pangs of hunger, or the painfulness of prolonged positions of restraint. Even exalted esthetic and intellectual pleasures, which seem to leave no sting behind and to require no goad for their advance, constantly make unfelt though heavy drafts on the nervous centres, the results of which, under the various forms of cerebral and spinal exhaustion and degeneration, physicians are obliged to take cognizance of only too often. So close is the analogy between the two antagonistic sensations, that the one becomes merged into the other without our being able to mark the dividing line between them. A moderate pain may by its active diffusion actually bring a surplus of pleasure, as Professor Bain has remarked; and that an acute sensation of pleasure may pass into one of pain is matter of common observation. Thus, Pain is born of Pleasure, Pleasure of Pain; and memory and association have a tendency to diminish the extremes of each by recalling to the mind the sources from which it sprang. Some physiologists, therefore, refuse to discriminate between such inseparable feelings, and prefer to regard them both as manifestations of the same, calling it the “Pleasure-Pain sensation.” In all these respects what is true of these sensations also holds good of their analogues in the realm of emotions, as joy and sorrow, mirth and sadness, hope and fear, and the like. Each of these exercises, and by exercising exhausts, some fibres of the nervous system, and thus tends to develop its contrary. The general law which, as far as it goes, explains these facts is that of the alternation or periodicity of the manifestations of force. This is the fundamental law of the Universe, beyond which no analysis has proceeded. Force, or Power, or Energy, wherever we discern it and under every one of its exhibitions, expresses itself in undulatory or rhythmical movements, pulsations of the primal potency, vibrations of the central harmony, of the universe. In the rise and fall of sensation and emotion the sequences of height and depth are as closely related as are the proportions of the crest and trough in the waves of the ocean, or those of the vibration of the strings of a musical instrument.