Book III shows that the natural progress of opulence is to direct capital, first to agriculture, then to manufactures, and lastly to foreign commerce, but that this order has been inverted by the policy of modern European states.
Book IV deals with two different systems of political economy: (1) the system of commerce, and (2) the system of agriculture, but the space given to the former, even in the first edition, is eight times as great as that given to the latter. The first chapter shows the absurdity of the principle of the commercial or mercantile system, that wealth is dependent on the balance of trade; the next five discuss in detail and show the futility of the various mean and malignant expedients by which the mercantilists endeavoured to secure their absurd object, namely, general protectionist duties, prohibitions and heavy duties directed against the importation of goods from particular countries with which the balance is supposed to be disadvantageous, drawbacks, bounties, and treaties of commerce. The seventh chapter, which is a long one, deals with colonies. According to the forecast at the end of chapter i this subject comes here because colonies were established in order to encourage exportation by means of peculiar privileges and monopolies. But in the chapter itself there is no sign of this. The history and progress of colonies is discussed for its own sake, and it is not alleged that important colonies have been founded with the object suggested in chapter i .
In the last chapter of the Book, the physiocratic system is described, and judgement is pronounced against it as well as the commercial system. The proper system is that of natural liberty, which discharges the sovereign from “the duty of superintending the industry of private people and of directing it towards the employments most suitable to the interest of the society.”