This distinction was beyond Smith: he did not differentiate the twofold character of value-creating labour. Marx once claimed to have discovered the ultimate source of Smith's strange dogma--that the aggregate of produced values can be completely resolved into _v + s_--in his fundamentally erroneous theory of value.[79] Failure to differentiate between the two aspects of commodity-producing labour as concrete and useful labour on the one hand, and abstract and socially necessary labour on the other, indeed forms one of the most important characteristics of the theory of value as conceived not only by Smith but by all members of the classical school. Disregarding all social consequences, classical economics recognised that human labour alone is the factor which creates value, and it worked out this theory to that degree of clarity which we meet in Ricardo's formulation. There is a fundamental distinction, however, between Marx's theory of value and Ricardo's, a distinction which has been misunderstood not only by bourgeois economists but also in most cases by the popularisers of Marx's doctrine: Ricardo, conceiving as he did, of bourgeois economy in terms of natural law, believed also that the creation of value, too, is a natural property of human labour, of the specific and concrete labour of the individual human being. This view is even more blatantly revealed in the writings of Adam Smith who for instance declares what he calls the 'propensity to exchange' to be a quality peculiar to human nature, having looked for it in vain in animals, particularly in dogs. And although he doubted the existence of the propensity to exchange in animals, Smith attributed to animal as well as human labour the faculty of creating value, especially when he occasionally relapses into the Physiocrat doctrine: 'No equal capital puts into motion a greater quantity of productive labour than that of the farmer. Not only his labouring servants, but his labouring cattle, are productive labourers....'[80] 'The labourers and labouring cattle, therefore, employed in agriculture, not only occasion, like the workmen in manufactures, the reproduction of a value equal to their own consumption, or to the capital which employs them, together with its owner's profits, but of a much greater value. Over and above the capital of the farmer and all its profits, they regularly occasion the reproduction of the rent of the landlord.'[81] Smith's belief that the creation of value is a direct physiological property of labour, a manifestation of the animal organism in man, finds its most vivid expression here. Just as the spider produces its web from its own body, so labouring man produces value--labouring man pure and simple, every man who produces useful objects--because labouring man is by birth a producer of commodities; in the same way human society is founded by nature on the exchange of commodities, and a commodity economy is the normal form of human economy.