By the close of WN Book III, Smith has completed his system and his empirical (historical) case for his system's persuasiveness. However, there were alternative systems available, and in WN Book IV, “Of Systems of political Oeconomy,” Smith addresses his competition.

The competitors he identifies are the “agricultural systems” (WN, 663) of the Physiocrats and the “mercantile System” (WN, 429). Smith makes the case that both of these are less persuasive than his model, but in one very important respect, he sees these two systems very differently. Although he disagrees with the Physiocrats, he admires and respects them as noble philosophers. In contrast, he sees the contemporary advocates of the mercantile system as selfish rogues who have captured and reshaped a philosophical enterprise in order to advocate policies that line their own pockets under the guise of philosophy.

This misuse of philosophy galls Smith doubly. First, having prevailed in the Parliament, the mercantile interests have led Britain down a path that he fears is very self-destructive. This concerns Smith, the policy person, deeply. Second, by presenting themselves as bearing the mantle of a philosophy concerned only for the well-being of the state, the mercantile interests have invaded and sullied Smith's own beloved realm: philosophy.

There are nine chapters in the final version of WN Book IV. Eight cover the mercantile system. Only one is dedicated to the agricultural system.