There is at least one clear point on which philosophers influential in the evolution of economic ideas agree, even when they are otherwise radically at odds with each other: they all believe in the power of ideas to transform human conduct and institutions. An explicit agreement on this point unites economic philosophers as far apart on values and principles as D. Hume and W. Paley in the eighteenth century, J.S. Mill and K. Marx in the nineteenth or, more lately, J. Robinson and F.A. Hayek. But what are these ideas, and how do they interfere with human action in ordinary life?

A good starting–point to answer these questions is Hume's assertion that ‘though men be much governed by interest, yet even interest itself, and all human affairs, are entirely governed by opinion’ (E,51). There is no interest without ‘opinion of interest’. By opinion of interest, Hume means that the very self–interest which supposedly motivates social agents into action is in fact a certain perception of interest, that is, an ordering of preferences grounded on expectations and beliefs. The pre–eminent example of opinion shaping the perception of interest for Hume is the fact that a great number of agents are prepared to give up their individual, self–centered pursuit of advantages when the benefit of their own family is at stake. Even the most staunch cases of selfishness usually stop short of family knots.