The difficulty arises when we attempt to interpret, by means of pleasure, the increase and development of life to which the course of evolution tends, and which is sometimes put forward as the end which the evolution-theory prescribes for conduct. And the difficulty also meets us when we seek to explain the conception of a maximum of pleasures as the end, by means of the conception of evolution. As long as we are content to look upon human nature as consisting of constant sources of activity and enjoyment, and having fixed susceptibilities for pleasure and pain, it is easy to adopt the increase of pleasure and diminution of pain as our aim. But the case is altered when we take into consideration the fact that man's actions and sensibilities are subject to indefinite modification. Pleasure, as we have seen, is a feeling of the subject dependent upon the objects, sensory and motor, present at any time to consciousness. These objects alone can be our end; but we may aim at certain of them rather than others, simply on account of their pleasurable accompaniment. It may happen, however, that an object or action at one time pleasurable becomes painful at another time, and that what is now painful ceases to be so and becomes pleasurable. In this case our course of action, if motived by pleasure, would have to be entirely changed, our practical ethics revised and reversed. And, although no sudden alteration such as this ever takes place, the theory of evolution shows that a gradual modification of the kind is going on. [Sidenote: (_b_) The conditions of pleasure and pain:] The conditions of pleasure and pain, physiological and psychological, are matter of dispute; and the dispute is complicated by the confusion of the physiological with the psychological problem. It will be evident, however,--if only we keep different things clear of each other,--that both kinds of explanation are possible, and that they are distinct from one another. The question of the nervous antecedents and concomitants of feeling is one thing, and quite distinct from the question which now arises of the mental antecedents or concomitants of feeling. And here the theories which have attempted a generalisation of the phenomena are, in the light of recent inquiry, mainly two: the theory that pleasure follows, or is the sense of, increase of life, and that which holds it to be the concomitant of unimpeded conscious functioning or of medium activities. [Sidenote: (α) Pleasure not definable as the sense of increased vitality;]