At first glance all the basic concepts of Political Economy (value, money, capital, profit, rent, wages, etc.) have a material character. Marx showed that under each of them is hidden a definite social production relation which in the commodity economy is realized only through things and gives things a determined, objectively-social character, a "determination of form" (more precisely: a social form), as Marx often put it. Analyzing any economic category, we must first of all point to the social production relation expressed by it. Only if the material category is an expression of a precisely given, determined production relation, does it enter the framework of our analysis. If this material category is not related to a given production relation among people, we pull it out of the framework of our analysis and set it aside. We classify economic phenomena into groups and build concepts on the basis of the identity of the production relations which the phenomena express, and not on the basis of the coincidence of their material expressions. For example, the theory of value deals with exchange between autonomous commodity producers, with their interaction in the labor process through the products of their labor. The fluctuation of the value of products on the market interests economists not for itself, but as it is related to the distribution of labor in society, to the production relations among independent commodity producers. For example, if land (which is not the product of exchange) appears in exchange, then production relations in this case do not connect commodity producers with commodity producers, but with a landowner; if the price fluctuations of plots of land have a different influence on the course and distribution of the production process from the price fluctuations of the products of labor, then we are dealing with a different social relation, a different production relation, behind the same material form of exchange and value. This social relation is subject to special analysis, namely in the context of the theory of rent. Thus land, which has price, i.e., a money expression of value (as a material category), does not have "value" in the sense mentioned above, i.e., in the act of exchange the price of land does not express the functional social relation which relates the value of the products of labor with the working activity of independent commodity producers. This led Marx to the following formulation, which has often been misinterpreted: "Objects that in themselves are not commodities, such as conscience, honor, &c., are capable of being offered for sale by their holders, and of thus acquiring, through their price, the form of commodities. Hence an object may have a price without having value. The price in that case is imaginary, like certain quantities in mathematics. On the other hand, the imaginary price-form may sometimes conceal either a direct or an indirect real value relation; for instance, the price of uncultivated land, which is without value, because no human labor has been incorporated in it" (C., I, p. 102). These words of Marx, which have often puzzled and even provoked the mockery of critics,  express a profound idea about the possible divergence between the social form of working relations and the material form which corresponds to it. The material form has its own logic and can include other phenomena in addition to the production relations which it expresses in a given economic formation. For example, in addition to the exchange of products of labor among independent commodity producers (the basic fact of the commodity economy), the material form of exchange includes the exchange of plots of land, the exchange of goods which cannot be multiplied by labor, exchange in a socialist society, etc. From the standpoint of the material forms of economic phenomena, the sale of cotton and the sale of a painting by Raphael or a plot of land do not in any way differ from each other. But from the standpoint of their social nature, their connection with production relations, and their impact on the working activity of society, the two phenomena are of a different order and have to be analyzed separately.
Marx frequently emphasized that one and the same phenomenon appears in a different light depending on its social form. Means of production, for example, are not capital in the workshop of a craftsman who works with them, though the same things become capital when they express and help to realize a production relation between wage laborers and their employer-capitalist. Even in the hands of a capitalist, means of production are capital only within the limits of the production relation between the capitalist and the wage laborers. In the hands of a money-capitalist, the means of production play a different social role. "Means of production are capital if, from the worker's standpoint, they function as his non-property, i.e., as someone else's property. In this form, they function only as opposed to the labor. The existence of these conditions in the form of an opposition to labor transforms their owner into a capitalist, and the means of production which belong to him, into capital. But in the hands of money-capitalist A, capital lacks this quality of opposition which transforms his money into capital, and thus the ownership of money into the ownership of capital. The real determination of form (Formbestimmtheit) through which money or commodities are transformed into capital has disappeared in this case. Money capitalist A is not in any way related to a worker, but only to another capitalist, B" (Theorien uber den Mehrwert, Vol. III, pp. 530-531, emphasis by Marx). Determination of social forms, which depends on the character of production relations, is the basis for the formation and classification of economic concepts.
Political Economy deals with determined material categories if they are connected with social production relations. Inversely, the basic production relations of the commodity economy are realized and expressed only in material form, and they are analyzed by economic theory precisely in this material form. The specific character of economic theory as a science which deals with the commodity capitalist economy lies precisely in the fact that it deals with production relations which acquire material forms. Of course the cause of this reification of production relations lies in the spontaneous character of the commodity economy. Precisely because commodity production, the subject of economic theory, is characterized by spontaneity, Political Economy as the science of the commodity economy, deals with material categories. The logical specificity of theoretical-economic knowledge must be derived precisely from that material character of economic categories, and not directly from the spontaneity of the national economy. The revolution in Political Economy which Marx carried out consists in his having considered social production relations behind material categories. This is the genuine subject of political economy as a social science. With this new "sociological" approach, economic phenomena appeared in a new light, in a different perspective. The same laws which had been established by the Classical Economists were given a completely different character and meaning in Marx's system 
 "Real phenomena, such as the value of land, are presented as 'imaginary' and 'irrational,' while imaginary concepts, such as the mysterious 'exchange value,' which does not appear in exchange, are identified as the only reality" (Tugan-Baranovskii, Teoreticheskie osnovy marksizma, Theoretical Bases of Marxism, 4th Ed., 1918, p. 118). The passage by Marx which was cited above means that, even though the purchase and sale of land does not directly express relations between commodity producers through the products of their labor, it is nevertheless related to these relations and can be explained in terms of them. In other words, the theory of rent is derived from the theory of value. Riekes incorrectly interpreted this passage in the sense that the protection of landed property requires expenditures, i.e., labor, which is expressed in the price of land (Riekes, Hugo, Wert und Tauschwert, Berlin: L. Simion, [n.d.] p. 27).
 Ignorance of this essential distinction between Marx's theory of value and the theory of the Classical Economists accounts for the weakness of Rosenberg's book (Isaiah Rosenberg, Ricardo und Marx als Werttheoreriker; eine Kritische Studie, Wien: Kommissionsverlag von I. Brand, 1904).