In short, Marx has not sufficiently recognised the fact that the development of the new social forces brought with it a new set of functions: that of initiating and directing industrial enterprise. These functions are not comprehended in the narrow definition of labour, but they are, nevertheless, most essential to progress; and the men that performed them have a most complete historical reason for their existence and a share in the results of industry. We need not add that such an argument does not justify all they did as the heads of the new industry. There is ample evidence that they were often rough, hard, cruel, and unscrupulous in the prosecution of their industrial enterprises. Nor does it prejudice the question whether the like direction of industry must and should continue in the future. There can be no doubt that in his theory of surplus value obtained from unpaid labour, Marx, as agitator and controversialist, has fallen into serious contradiction with himself as scientific historian and philosopher. The theory that labour is the source of value was widely accepted among economists during his early life, and by its justice and nobleness it was well adapted to the comfortable optimism prevalent among so many of the classical school. The economists, however, did not follow the principle to its obvious conclusion: that if labour is the source of wealth, the labourer should enjoy it all. It was otherwise with the socialists, who were not slow to perceive the bearing of the theory on the existing economic order. In his controversial treatise against Proudhon, Marx gives a list of writers (beginning with the political economy of Hopkins,[8] published in 1822, only five years after the appearance of Ricardo’s great work), by whom the principle was applied to revolutionary purposes. Its simplicity and seeming effectiveness must have made it most attractive. As posited by the classic economy, and applied by the socialists, Marx accepted the principle. It was an unanswerable _argumentum ad hominem_ when addressed to an economist of the Ricardian school; but it should have broken down when confronted with historical fact. Nevertheless it was made, and continued to be, the foundation-stone of the system of Marx, and is really its weakest point. His doctrine of surplus value is the vitiating factor in his history of the capitalistic system. The most obvious excuse for him is that he borrowed it from the classic economists.