The aim of this monograph is to study a strand of ethical enquiry which has been present throughout the history of ethical thought, but which has in fairly recent times been given an independent development and has thereby illuminated the whole field of ethical questions. Axiology or Value-Theory began as a tailpiece to Ethics, but it arguably ought to end as the tail which wags the dog, which by illuminating the ends of practice alone makes the prescription of norms for practice itself a practicable undertaking. This monograph will study the thought of those who have done most to detach value-theory as a discipline from the practical enquiries in which it had its origin, and on which it in its turn can be used to throw light.