In adult life for casual and random activity is substituted activity directed by some end or purpose which determines the responses called into play. Professional and business, domestic and social enterprises and obligations take up most of the adult's energy. The contrast between the play of the child and the work of the adult is that in the case of the former actions are done for their own sake; and in the latter for some end. The child, we say, plays "for the fun of the thing," the adult works for pay, for professional success, for power, reputation, etc.
But even in the adult the desire for play powerfully persists. Not all the grown-up's energy is absorbed in his work, and even some types of work, like that of the poet or painter, or the building-up of a great business organization, may be intrinsically delightful and self-sufficient activity. Under the conditions of modern industry, however, especially of machine production, much--in many cases, most--of the activity by which an individual earns his living, utilizes only some of his native tendencies to act, while the working day does not, under normal conditions, absorb all his energy. Whatever vitality is not, therefore, absorbed in necessary work goes into forms of purely gratuitous activity. Which form "play" shall take in the adult depends on the degree to which certain impulses are in him stronger than others, either by native endowment or cultivation, and which impulses have not been sufficiently utilized in him during the day's work. A man musically gifted will find his recreation in some performance on a musical instrument, let us say; on the other hand, if his work is music, those impulses, strong though they be, that make him a musician, will have been sufficiently exhausted in the day's work to make some other activity a more satisfactory recreation.
The relations between play and work can be better understood by a consideration of the physiological importance of variety in activity. A certain regular recurrence of response may be pleasant, as in rowing or canoeing, or in listening to the rhythms of poetry or music, but a prolonged repetition of precisely the same stimulus or the same set of stimuli may make responses dissatisfying to the degree of pain. Ideal activity, biologically, would be one where every impulse was just sufficiently frequently called upon to make response easy, fluent, and satisfactory.