The general proof of this thesis is scattered throughout the whole book. But there was one point especially upon which Smith was very anxious to show complete accord between public and private interest. This was in connection with the investment of capital. In his opinion capital spontaneously seeks, and as spontaneously finds, the most favourable field for investment—most favourable, that is to say, to the interest of society in general. This proof at first sight seems to apply only to one special fact, but it really has a more general import. We know the great stress which Smith laid upon capital. Division of labour depends upon it, and so does the abundance or scarcity of produce. It determines the quantity of work and fixes the limit of population. To show that the investment of capital conforms to the general interest is to show that all production is organised in the manner most favourable to national prosperity. Smith distinguishes between four methods of investing capital: in agriculture, in industry, in the wholesale and in the retail trades. Wholesale industry is further divided into three classes: domestic trade; foreign trade, furnishing the nation with foreign products; and the carrying trade which transports those goods from one country to another. Smith maintained that the order in which these various forms of activity were mentioned was also the order of their utility, agriculture being the most advantageous, industry the second best, etc. He also proposes two criteria for testing this hierarchy: (1) the quantity of productive labour put into operation by means of the capital employed by each; (2) the amount of exchange value annually added to the revenue by each of these employments. As we pass from agriculture to the other branches, the quantity of productive labour brought into operation and the amount of exchange value obtained gradually decreases, and with this decrease goes a diminishing utility for the country. Smith thought that a nation ought to employ its capital in the way he had suggested. It ought to give the preference to agriculture, and engage in the other branches only as the accumulation of capital permitted.