Notwithstanding his recantation, Smith was never quite rid of Physiocratic influence. Writing of the Physiocratic system, he described it as perhaps “the nearest approximation to the truth that has yet been published.”[144] So indelible was the impression which the Physiocrats left upon him that both they and their doctrines, even when the latter are directly opposed to his own, are always spoken of with the greatest respect. The most important evidence of their power over him is the thesis just mentioned which he attempted to defend, namely, that between agriculture and other industries lies an essential distinction, because in industry and commerce the forces of nature are never brought into play, whereas in agriculture they always collaborate with man. “No equal quantity of productive labour employed in manufactures can ever occasion so great a reproduction. In them nature does nothing; man does all; and the reproduction must always be in proportion to the strength of the agents that occasion it.”[145] We almost think we are dreaming when we read such things in the work of a great economist. Water, wind, electricity, and steam, are they not natural forces, and do they not co-operate with man in his task of production? Considerations such as these were allowed to pass quite unheeded, and Smith persisted in his error because he believed that this new doctrine furnished him with an explanation of rent, that strange enigma which had puzzled English economists for so long. How was it that while other branches of production gave a return only sufficient to remunerate the capital and labour employed, agriculture, in addition to these two revenues, yielded a supplementary income known as rent? It was because “in agriculture nature labours along with man: and though her labour costs no expence, its produce has its value as well as that of the most expensive workman.” Thus “rent may be considered as the produce of those powers of nature, the use of which the landlord lends to the farmer.”[146] Had Smith arrived at a true theory of rent this recourse to the natural powers of the soil to furnish an explanation of the proprietor’s revenue would have been quite unnecessary, and in all probability he would not have so easily accepted the idea of the special productivity of the soil. But this false conception of nature has persisted in economic theory, and in it Smith thought he saw an additional reason for adhering to those errors which the Physiocrats had first induced him to commit.[147] Apart from his personal attachment to the Physiocrats we must also remember that Smith more than shared their predilection for agriculture.