THIS book is in the form of a debate about a thesis. The original essay which gave rise to the discussion was intended as a fairly systematic, although extremely summary, exposition of a point of view which had been implied in a short popular booklet which I had recently published under the title of The Scientific Attitude. The essay was submitted to the editors of Nature, who invited a number of authorities to comment upon it. Other authors were moved to contribute to the discussion, which became too voluminous for the correspondence columns of a weekly journal. The whole debate, both that portion which has not yet appeared in print as well as the original public discussion, appears to constitute a valuable contribution to a subject the profound importance of which is becoming ever more generally recognized. In collecting it together, and recording it in a form more permanent than a private correspondence, every attempt has been made to edit it in such a way that it does not lose the essential character of a discussion, that of being an interchange of views. The age-long endeavour to find an intellectual basis for ethics is an enterprise of such importance, and of such difficulty, that any explorer of that country must always be glad to hear the voiCes of his fellowtravellers. "This," Wittgenstein once said to me, "is a terrible business-just terrible! You can at best stammer when you talk of it." This book is communal, perhaps even co-operative, stammering.