For over two thousand years the idea of natural law has played a prominent part in thought and in history. It was conceived as the ultimate measure of right and wrong, as the pattern of the good life or "life according to nature". It provided a potent incentive to reflection, the touchstone of existing institutions, the justification of conservatism as well as of revolution. But recourse to natural law was never entirely unchallenged. The notion was laden with ambiguity even in the days when it was considered self-evident. In the last century and a half it has been assailed from many sides as critically unsound and as historically pernicious. It was declared to be dead, never to rise again from its ashes. Yet natural law has survived and still calls for discussion. It is the purpose of this book to examine the reasons for that vitality and the claim of natural law to have served the cause of humanity well.
But how is natural law best approached and how should it be handled? This is a serious difficulty to the modern student. There is no doubt that, for a number of reasons, we have grown unfamiliar with the whole body of doctrine and with its terminology. We find ourselves confronted with a variety of definitions, and we can see no reason why we should make our start from one rather than from another. There is, however, one important restriction which must be laid down from the outset, to circumscribe the ground which this book proposes to cover. The notion of natural law which it discusses is a notion which refers to human behaviour, not to physical phenomena. Our concern is with ethics and politics, not with the natural sciences. The word nature is the cause of the equivocation. The failure to distinguish clearly between its different meanings was the source of all the ambiguities in the doctrine of natural law.1
Prima facie, there seem to be two possible lines of approach to our subject. I would call the one historical, the other philo-____________________