The nature of economic laws

"To make economics a science is to suppose that its conclusions, even if hypothetical, have the same certainty as, for instance, those of physics; to assume, in other words that the same constancy which is discoverable in the dropping of the stone and its fall to the ground, is also discoverable in the relation between the imposition of tariffs and the movement of prices. This latter relationship is not one between abstract concepts, but between men's acts, and human ingenuity is continually altering relations between them. In short, when the economist professes to give us universality, he forget that an economic law is not merely like a physical law, one which man can understand and use, it is also one that he can alter. For the laws intended to cover man's asked and all of those he, unlike the stone that is dropped, it is conscious, and this very consciousness involves freedom to act differently. A law of human action can therefore be at best no more than a statistical generalization about the past, and if that is all that it is, there is no a priori probability that it will have any applicability in the future; for men may change their minds about what they want, or they may be led, by the discovery of a new invention or the examination of statistics of the past, to act in ways that hitherto had not occurred to them or that had been regarded as impossible." -- T.M. Knox, "The Study of Economic Activity," Philosophy, 1936