Ricardo's genius was limited by his adherence to the crude and narrow labour theory of value, and by his "ignorance of the terse language of the differential calculus". This principle of the identity of value-ratios with labour-ratios rested on conditions which defined the nature of the dominant tendencies in an exchange- society. The essential importance of this labour-principle was that it could be employed to determine the value of labour-power itself. In the case of land or capital clearly there were serious practical objections to taking them as a basis: difficulties which would have exceeded any of those which are charged against the labour-theory. Labour, conceived objectively as the output of human energy, was the measure and the essence of Ricardo’s "difficulty or facility of production". If the labour or activity required to replace what is used up is less than the labour embodied in the total product, a surplus emerges.