“. . . for those retired, with too much time and no world, a world must be found, and not necessarily one that is heavily populated. One can join a group or work alone; the essential . . . is that the work be difficult, concentrated, and that definite progress can be measured . . . the purpose . . . is. . . to maintain a carefully directed intensity. . . . Here the question is one of time, and to what all that remaining time should be devoted. [pp. 45-46]”