This brings us to the Ricardian theory of value. We know that he considered that the value of everything was determined by the amount of labour necessary for its production.[334] Adam Smith had already declared that value was proportional to the amount of labour employed, but that this was the case only in primitive societies. “In civilised society, on the contrary, there is a still smaller number [of cases] in which it consists altogether in the wages of labour.” Labour was regarded by Smith as one of the factors determining value—though by no means the only one, land and capital being obviously the others. But Ricardo simplified matters, as abstract thinkers frequently do, by neglecting the last-named factors. This leaves us only labour. Land is dismissed because rent contributes nothing to the creation of value, but is itself entirely dependent upon value.[335] Corn is not dear because land yields rent, but land yields rent because corn is dear. “The clearly understanding this principle is, I am persuaded, of the utmost importance to the science of political economy.” As for capital, why should we make a special factor of it, seeing that it is only labour? Its connotation might be extended so as to include “the labour bestowed not on their immediate production only, but on all those implements or machines required to give effect to the particular labour to which they were applied.”[336] But Ricardo was not thoroughly satisfied with this identification of capital and labour, and, great capitalist that he was, it must have caused him much searching of heart. Furthermore, it was not very easy to apply the conception to such commodities as timber and wine, which increase in value as they advance in age. In a letter to McCulloch he admits the weakness of his theory. After all the study that he had given to the matter, he had to confess that the relative value of commodities appeared to be determined by two causes: (1) the relative quantity of labour necessary for its production; (2) the relative length of time required to bring the commodity to market. He seems to have had a presentiment of the operation of a new and distinct factor, to which Böhm-Bawerk was to ascribe such importance. The usual method of stating the Ricardian theory of value is to say that value is determined by cost of production. It is also the correct way, inasmuch as he stated it thus himself. It is, however, quite a different thing to say on the one hand that value is determined by labour and on the other that it depends upon the sum of wages and profits (supposing we omit rent).[337] On this point, as on several others, obscurity of thought alone saves Ricardo from the reproach of self-contradiction.