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.