Karl Marx the analysis which the classical economists had conducted disclosed only half of the problem. Value, Marx emphasized, was not a mysterious intrinsic attribute of things: it was merely an expression of a social relation between men. To explain surplus-value, therefore, in terms of some property of an object was to relapse into what Marx termed the Fetishism of Commodities-a species of animism in which post-Ricardian "vulgar economy" became increasingly enmeshed. The essential difference between Marx and classical Political Economy lay, therefore, in the theory of surplus- value. It seems to be generally forgotten, when Marx is criticized for giving no adequate "proof" of his theory of value in Das Kapital, that he was not propounding a novel and un- familiar doctrine, but was adopting a principle which was part of the settled tradition of classical Political Economy and without which he considered any determinate statement to be impossible.