But physiology demolishes the corner-stone of this edifice when it shows that pain, so far from being inseparable from existence, has merely become, through transmitted experience, nearly inseparable from the progressive cessation of existence. While action and reaction are equal in inorganic nature, the principle of life modifies the operation of this universal law of force by bringing in _nutrition_, which, were it complete, would antagonize reaction. In such a case, pleasure would be continuous, pain null; action constant, reaction hypothetical. As, however, nutrition in fact never wholly and at once replaces the elements altered by vital action, both physicians and metaphysicians have observed that pleasure is the fore-runner of pain, and has the latter as its certain sequel.[14-1] Physiologically and practically, the definition of pleasure is, _maximum action with minimum waste_. This latter generalization is the explanation of the esthetic emotions. The modern theory of art rests not on a psychological but a physiological, and this in turn on a physical basis. Helmholtz’s theory of musical harmony depends on the experimental fact that a continued impression gives a pleasant, a discontinuous an unpleasant sensation. The mechanics of muscular structure prove that what are called graceful motions are those which are the mechanical resultant of the force of the muscle,--those which it can perform at least waste. The pleasure we take in curves, especially “the line of beauty,” is because our eyes can follow them with a minimum action of its muscles of attachment. The popular figure called the Grecian figure or the walls of Troy, is pleasant because each straight line is shorter, and at right angles to the preceding one, thus giving the greatest possible change of action to the muscles of the eye. Such a mechanical view of physiology presents other suggestions. The laws of vibratory motion lead to the inference that action in accordance with those laws gives maximum intensity and minimum waste. Hence the pleasure the mind takes in harmonies of sound, of color and of odors. The correct physiological conception of the most perfect physical life is that which will continue the longest in use, not that which can display the greatest muscular force. The ideal is one of extension, not of intension.