To the Physiocrats the economic world was a hierarchy of classes. The agriculturist in some mysterious way bore the “whole weary weight of this unintelligible world” upon his own shoulders, giving to the other classes a modicum of that sustenance which he had wrested from the soil. Hence the fundamental importance of the agricultural classes and the necessity for making the whole economic system subordinate to them. Adam Smith, on the other hand, attempted to get a view of production as a whole. He regarded it as the result of a series of joint undertakings engineered by the various sections of society and linked together by the tie of exchange. The progress of each section is bound up with that of every other. To none of these classes is entrusted the task of keeping all the others alive; all are equally indispensable. The artisan who spares the labourer the task of building his house or of making his shoes contributes to the accumulation of agricultural products just as much as the ploughman who frees the artisan from turning the furrow or sowing the seed. The progress of national wealth cannot be measured in terms of a single net product; it must be estimated by the increase in the whole mass of commodities placed at the disposal of consumers. One very evident practical conclusion follows; namely, that taxation should fall, not upon one class, as the Physiocrats wished, but upon all classes alike. As against the _impôt unique_, Smith advocates multiple taxation which shall strike every source of revenue equally, labour and capital as well as land; and the fundamental rule which he lays down is as follows: “The subjects of every State ought to contribute towards the support of the Government, as nearly as possible, in proportion to their respective abilities; that is, in proportion to the revenue which they respectively enjoy under the protection of the State.”[140] This is his famous maxim of equality so frequently quoted in every financial discussion.[141]