Marx’s teaching on the “form of value” (i.e., the social form taken by the product of labor) is the result of a particular social form of labor itself and represents Marx’s new and unique contribution to the theory of labor value… Marx drew a precise distinction between the material-technical process of production and its social form, between labor as an aggregation of technical functions (concrete labor) and labor viewed in terms of its social form in commodity-capitalist society (abstract labor or universal labor). The specificity of commodity economy consists of the fact that the material-technical process of production is not organized by society and is undertaken by separate commodity producers. Concrete labor is simultaneously the private labor of separate individuals. The private labor of the individual commodity producer is connected with the labor of all other commodity producers and becomes social labor only insofar as its product is equated with all other commodities in the market. As we have seen, this market equalization of all commodities, expressed through the valuation of them all in terms of one and the same commodity, gold (or money), simultaneously signifies the equalization of all the concrete types of labor expended in different spheres of the economy. This means that the private labor of a separate individual assumes the character of social labor not in the process of production itself but in the act of exchange, which represents an abstraction from the concrete specificities of particular things and particular types of labor. The equalization of all types of labor through market equalization of all the products of labor as values — this is what Marx means by the concept of abstract labor. And since the equalization of labor results from the social form of commodity economy, in which there is no direct social organization and equalization of labor, it follows that abstract labor is a social and historical concept. Abstract labor does not express a physiological equality of the various types of labor, but rather the social equalization of various types of labor that occurs in the specific form of market equalization of the products of labor as values.
Isaak Rubin, Fundamental Features of Marx’s Theory of Value and How It Differs from Ricardo’s Theory, 1924, in Responses to Marx’s Capital: From Rudolf Hilferding to Isaak Illich Rubin, ed. by Richard Day and Daniel Gaido, Chicago, IL: Haymarket Books, 2017, p. 561.