According to this hypothesis,[147] the organic process accompanied by pleasure is to be conceived as a “restoration of equilibrium” after “disturbance”: so that the absence of appreciable pleasure in the case of certain normal activities is explained by the absence of antecedent disturbance. This view is obviously applicable to certain classes of pleasures which, though by no means rare are incidental in a normal life:--the pleasure of relief after physical pain, or after the strain of great anxiety, and the pleasure of repose after unusual exertions, intellectual or muscular. But when we attempt to apply it to sensational pleasures generally, the indefiniteness of the notion of “equilibrium,” as applied to the processes of a living organism, becomes manifest. For our physical life consists of a series of changes, for the most part periodically recurrent with slight modification after short intervals: and it is difficult to see why we should attach the idea of “disturbance” or “restoration of equilibrium” to any one among these normal processes rather than any other:--_e.g._ it is difficult to see why the condition of having expended energy should be regarded as a departure from equilibrium any more than the condition of having just taken in nutriment. In fact, to render the hypothesis we are considering at all applicable to normal pleasures of sense, we have to pass from the physiological to the psychological point of view, and take note of the psychical state of _desire_, as a consciously _unrestful_ condition, of which the essence is a felt impulse to pass out of this state towards the attainment of the desired object. Our hypothesis, then, may take this unrestful consciousness as a sign of what, from a physiological point of view, is “disturbance of equilibrium,” and similarly, the satisfaction of desire may be taken to be, physiologically, a restoration of equilibrium. On this assumption, the theory becomes undeniably applicable to those gratifications of sensual appetite which form the most prominent element of the pleasures of sense, as popularly conceived. Now we have already noted that by a wide-spread confusion of thought, desire has often been regarded as a species of pain. Accordingly, the theory that we are considering was originally prompted by the ethical motive of depreciating the vulgarly overvalued pleasures of satisfied bodily appetite, by laying stress on their inseparable connexion with antecedent pain. The depreciation, however, fails so far as the appetite which is a necessary antecedent condition of the pleasure is--though an unrestful state--not appreciably painful.[148]