It is very curious that Smith should have failed to make the best possible use of this theory. Its full significance was lost upon him. The theory of division of labour alone was sufficient to dispose of the whole Physiocratic system. Nevertheless, in the last chapter of Book IV we find him still valiantly struggling to disprove the conclusions of the Physiocrats, by the aid of arguments not always very convincing. Forgetting his principle of division of labour, he even adopts a part of their thesis and finds himself entangled by the invalid distinctions which they had drawn between productive and unproductive workers. He simply gives another definition and describes as unproductive all works which “perish in the very instant of their performance, and seldom leave any trace or value behind them for which an equal quantity of service could afterwards be procured.”[142] All these services, which comprise the labours of domestic servants, of administrators and magistrates, of soldiers and priests, of counsellors, doctors, artists, authors, musicians, etc., Say classed together as “immaterial products.” By restricting the term “productive” to material objects only, Smith gave rise to a very useless controversy on the nature of productive and unproductive works—a controversy that was first taken up by Say and revived by Mill, but which to-day seems to be decided against Smith, thanks to a more exact interpretation of his own doctrines. It is, indeed, quite clear that all these services constitute a part of the annual revenue of the nation, and that “production” in a general sense would be diminished if some persons did not exclusively devote themselves to the performance of such tasks. After criticising the Physiocratic distinction drawn between the wage-earning classes and the productive, Smith immediately admits that the labour of artisans and traders is not as productive as that of farmers and agricultural labourers, for the latter not only return the capital employed by them together with profits, but they also furnish the proprietor with rent.[143] Whence this hesitation on the part of Smith? Where did he come by the idea of the special and superior productivity of agriculture? An attempt to account for it may prove interesting, and it will help us to give Smith his true place in a history of economic doctrines.