All social theorists and philosophers who seek to explain human action have a 'model of man', a metaphysical view of human nature. Some make man a plastic creature of nature and nurture, some present him as the autonomous creator of his social world, some offer a compromise. Each view needs its own theory of scientific knowledge calling for philosophic appraisal and the compromise sets harder puzzles than either. Passive accounts of man, for example, have a robust notion of causal explanation but cannot either find or dispense with a self to apply them to. Active accounts rightly stress an autonomous self, but lack a proper concept of explanation. Martin Hollis takes these tensions and contrasts from the thought of sociologists, economists, and psychologists. He then develops a model of his own - one which seeks to connect personal and social identity through an ambitious theory of rational action and a priori knowledge, proposing a sense in which men can act freely and still be a subject for scientific explanation.