ingenious English economist has rightly maintained that money is merely a material instrument like a ship or a steam-engine, but not an expression of a social relation in the field of production and consequently not an economic category; and that it is, therefore, wrong to treat the subject in political economy, which really has nothing in common with technology.[15] The world of commodities implies the existence of a highly developed division of labor; this division is manifested directly in the great variety of use-values, which confront each other as particular commodities and which embody as many different kinds of labor. The division of labor embracing all the particular kinds of productive occupations, is the complete expression of social labor in its material aspect viewed as labor creating use-values. But from the standpoint of commodities and within the process of exchange, it exists only in its results, in the variety of the commodities themselves. The exchange of commodities constitutes the social metabolic process, i. e. the process in which the exchange of the special products of private individuals is the result of certain social relations of production into which the individuals enter in this interchange of matter. As they develop, the mutual relations of commodities crystalize into various aspects of the universal equivalent and thus the process of exchange becomes at the same time the process of the formation of money. The whole of this process which takes the form of a succession of processes, constitutes circulation. NOTES ON THE HISTORY OF THE THEORY OF COMMODITIES. The analysis of commodities according to their twofold aspect of use-value and exchange value by which the former is reduced to work or deliberate productive activity; and the latter, to labor time or homogeneous social labor, is the result of a century and a half of critical study by the classical school of political economy which dates from William Petty in England and Boisguillebert in France[16] and closes with Ricardo in the former country and Sismondi in the latter. Petty reduces use-value to labor, without deceiving himself as to the natural limitation of its creative power. As regards concrete labor, he sizes it up in the magnitude of its social aspect, as _the division of labor_.[17] This view of the source of material wealth does not remain more or less fruitless as in the case of his contemporary, Hobbes, but leads up to his _Political Arithmetic_, the first form in which Political Economy is differentiated as an independent science.