ONCE upon a time---long ago now it was possible to refer anyone who wanted to find out what economic theory had to say about society and its problems to Marshall’s classic Principles of Economics. There he could find the structure of economic theory spelled out in a language comprehensible to the average businessman. (Even when I went up to Cambridge in 1945, the myth still prevailed that “it is all mercifully in Marshall.”) That situation has been revolutionized, as a result of the introduction of geometrical and mathematical methods of theoretical investigation in the 1930’s, and of the post-war emphasis on the measurement of economic relationships. Economic theory is now written in set theory, mathematical models, and regression results, with the consequence that it seems to be too abstruse and remote from reality, as well as too technical, to be of any interest or use to the nonspecialist concerned with understanding and improving the world in which he lives. Yet economics remains essentially a social science, concerned to further our understanding of society, and the great advance of economic science that has occurred in the past thirty-odd years has made it more and not less capable of illuminating social questions. This, at least, is what I hope to suggest- necessarily in very broad and sketchy terms—in this article.