Political economy abstracts from the social fact of landownership, to present rent as a quality of the land. It abstracts from the social form of wage-labour, to present wages as the recompense for labour. It abstracts from the social form of capital, to present profit as a quality of the means of production. It abstracts from the social form of exchange, to present exchange as an expression of a rational, natural propensity to ‘truck, barter and exchange’.
However this is an illegitimate form of abstraction, for it is only in a particular form of society that land generates a rent, means of production a profit, and labour a wage. It is only in a particular form of society that the private labour of individuals is related through exchange. To treat these categories in abstraction from their social form is to deprive them of any content, to make them into purely formal categories that exist wherever there are land, labour, means of production or co-operation. Thus the categories of political economy are given an eternal status, and are even applied to societies within which neither wages, nor profits, nor rent, nor exchange, actually exist.