Reification is the transformation of a social relation (a relation between people) into a thing or into a relation between things (`thing' referring here to a non-person material object). In some cases, reification is a kind of mental error, the mistake of theorizing a social relation as a thing or thing-relation. However, in the process of economic commodification, there is a sense in which reification is actual. This is because commodity economies (i.e., market economies) are unique in that in them, coordination between producers is not brought about by any wilful agent (such as the lord of the manor, or the central planning committee), but by the reaction of each producer to the movement of commodities – things – in the market. Thus the production relation between producers in a commodity economy is not a direct, people-to-people relation; it is, instead, achieved through the intermediacy of things. Therefore, during commodification (the transition to a commodity economy) people's production relations, which formerly were to other people, become, at least in their immediate aspect, relations to things. This is an actual and not merely subjective reification.
Connected with the actual reification of production relations in the commodity economy, Marx identified a subjective reification in the theories of bourgeois economists, which consisted in their attributing properties to commodities which he thought could be better theorised as being properties of the production relations. For example, many bourgeois economists consider that capital (by which they mean means of production) has the attribute of yielding, or providing, a profit. But Marx held that the yielding of profit is not an attribute of means of production (proof: means of production do not yield profit when they are used in non-commodity economic modes such as feudalism or socialism); rather, profit is an attribute of a certain type of production relation, namely the wage worker to employer relation.
Marx's theoretical preference is reflected in his terminology. He did not define capital as means of production, or as money used to buy means of production and hire labour (those are bourgeois definitions); he defined it as the relation.
In Marxist theory, if one is being rigorous about this distinction, one speaks of means of production, for example, not as being capital, but as being bearers of the capital relation. In a similar vein, it may be said that when they are used in the capitalist mode of production, they `acquire the social form of capital', or they `acquire the capital-form, or, in a way that sounds quite ordinary, that `they take the form of capital'.
Bourgeois economic theory's reification of social forms gives those forms an appearance of universality or permanence which is functional for bourgeois hegemony (and annoyng to Marxists). The deceptive logic is that if the forms are inherent in the material objects, and those material objects have a trans-historical existence, then the forms have a trans-historical existence. The false link in that logic, in the Marxian view, is that the forms are not inherent in the objects; they are the result of production relations which are historically mutable.
The article, as of Spring 2014, is heavily based on the first five chapters of Isaak Illych Rubin's Essays on Marx's Theory of Value, third edition, Moscow and Leningrad, 1928, as translated by Milos Samardzija and Freddy Perlman and published by Black & Red, Detroit, USA, 1972. The text is available from the Marxist Internet Archive 
- Rubin, Essays, p 23.
- `Capital is a social form which is acquired by means of reproduction when they are used by wage labour.' (Marx, Theories of Surplus Value, p 383 in the German edition – quoted in Rubin, Essays, p 38.
- For more examples of the kind of language used, see Rubin, Essays, pp 39 and 47.