Mill is often thought of as an eclectic and unsystematic writer, whose views on freedom contradict his views on moral right and wrong, whose views on causation contradict his views on syllogistic inference and so on. This text argues that Mill both saw his views as part of a systematic defence of empiricist epistemology and utilitarian ethics, and was to a large extent successful in offering a coherent and connected defence of this system. Mill aimed to show that we could possess a knowledge of individual and social human nature equal to our knowledge of the material world; the point of showing this was to erect on the science of human nature a utilitarian ethics in which freedom and self-realization for as many people as possible could be achieved. From the philosophy of mathematics to the defence of individual liberty, Mill attacked the prevailing intuitive theories and put a subtle empiricism in their place. Since the first edition of this study in 1970, many writers have contributed a more systematic understanding of Mill’s programme for philosophy, ethics and social science, and the author’s preface to this second edition briefly assesses the way Mill appeared in this later climate of opinion.