Besides their ability to satisfy human needs (directly as consumer articles or indirectly as means of production), means of activity have in common the fact that they are themselves products of activity. Marx introduced the concept of “abstract labor” in volume I of Das Kapital (1867) to bring all concrete types of activity to a general equivalent so that the quantity of labor can be calculated: “If then we leave out of consideration the use value of commodities, they have only one common property left, that of being products of labor” (Marx and Engels 1975-2004, vol. 35, p. 48).
Marx introduced the concept of “abstract labor” but did not explain what its substance is. Georg Simmel wrote in his The Philosophy of Money (1900) that unlike energy, which is essentially unchanging but can manifest itself in different ways (such as heat, electricity, mechanical motion), different types of labor do not have a general equivalent or substance that would allow comparisons between, for example, “muscular” and “mental” labor. At the same time, Simmel admitted that such an equivalent might exist:
“From the outset I admit that I do not simply rule out the possibility that in the future the mechanical equivalent for the psychic activity will be discovered. Of course, the importance of its content, its factually determined position in logical, ethical and aesthetic contexts is completely separate from all physical movements, roughly in the same manner as the meaning of a word is quite separate from its physiological-acoustic sound. Yet the energy that the organism must expend upon the thought of this content as a cerebral process is, in principle, just as calculable as that necessary for a muscular exertion. If this were to be achieved one day, then one could at least make the amount of energy necessary for a specific muscular exertion a unit of measurement on the basis of which the mental use of energy would be determined. Mental labor would then be dealt with on the same footing as manual labor, and its products would enter into a merely quantitative balancing of value with those of the latter” (Simmel 2004, p. 421).
As Simmel correctly assumed, the general equivalent that makes it possible to treat mental and manual labor on the same footing is not mechanical in nature. Nor is it energy or time. Rather, it is directed information: meaning is the sought-after substance of all human activity and its results.