I am going in this lecture on ‘Philosophy and Practice’ first to say something about philosophy and then something about practice, in order to show you how they bear on one another. But I must start by paying a tribute to the President of the Society for Applied Philosophy, Professor Sir A. J. Ayer, who has kindly agreed to take the chair at this lecture. I can honestly say that he is more responsible than anybody else for putting me on the right track in moral philosophy. He did this by convincing me, when young, that the ways people were doing it at that time had no future. In the famous chapter on ethics in his marvellously readable and exciting book,Language, Truth and Logic, Ayer was thought to be trying to show that moral philosophy itself, and perhaps even ordinary first-order moral thinking, was a waste of time. From later work of his, and from his occasional pronouncements about moral and political questions, it is evident that the second of these slanders was false. But even on the theoretical side the lessons I learnt from his book were positive as well as negative. That is not to say that the negative lessons were unimportant. Some people have still not absorbed them, and continue to waste our time. But here are two positive points which you will find in Ayer's book, and which for me were crucia.