The amount of work required to produce its use value under normal production conditions in a society and with the average social degree of skill and labor intensity. This amount of work is “socially necessary work”.
Use value and exchange value
All commodities have a use value, they are useful in a certain social context to a certain extent and in a precise way. However, they can be exchanged among themselves, then they have something in common with all of them that allows them to be considered as holding equivalent value, not for individuals subjectively, but for the society that produces them.
It is clear that it is precisely the abstraction of their use values that characterizes the exchange relationship between commodities. (…) As use values, commodities are, above all, different in terms of quality; as exchange values they can only differ by their quantity, and therefore they do not contain a single atom of use value.
Now, if we separate the use value, from the body of the goods, only one property will be subtracted from them: that of being products of work. However, the product of work has also been transformed in our hands. If we abstract its use value, we also abstract the components and corporeal forms that make it a use value. That product is no longer a table or house or thread or anything else useful. All its tangible properties are gone. Nor is it any longer the product of the work of the cabinetmaker or the bricklayer or the spinner or any other particular productive work. With the usefulness of the products of work, the usefulness of the work represented in them vanishes and, therefore, so do the various concrete forms of such work; they cease to be distinguished, being reduced in their entirety to undifferentiated human work, to abstract human work.
These things only make us aware that in their production human labor force was used, human labor was accumulated. As crystallizations of that social substance common to them, they are values.
In the very relationship of exchange between commodities, their exchange value was revealed to us as something entirely independent of their use values. If then the use value of the products of work is effectively abstracted, their value is obtained, as it has just been determined. That common thing that is manifested in the exchange relationship or in the exchange value of the goods is, therefore, their value. The development of this research will once again lead us to the exchange value as a mode of expression or form of necessary manifestation of value, which must however be considered independently of that form.
A use value or a good, therefore, only has value because it is objectified or materialized by abstract human work. How, then, can we measure the magnitude of its value? By the amount of “value-generating substance” – by the amount of work – contained in that use value. The amount of work itself is measured by its duration, and the work time, in turn, recognizes its measurement pattern in certain time fractions, such as hour, day, etc.
It might seem that if the value of a commodity is determined by the amount of labor spent in its production, the more lazy or clumsy a man is, the more valuable his commodity is, because he needs so much more time to make it. However, the work that generates the substance of values is undifferentiated human labor, the expenditure of the same human labor force. The whole of society’s labor force, represented in the values of the world of commodities, is here made up of one and the same human labor force, even though it is composed of innumerable individual labor forces. Each of these individual labor forces is the same human labor force as the others, insofar as it possesses the character of an average social labor force and operates as such an average social labor force, that is, insofar as, in the production of a commodity, it uses only the average necessary labor time, or socially necessary labor time. Socially necessary labor time is that required to produce any use value, under the normal production conditions in force in a society and with the average social degree of skill and labor intensity. After the adoption of the steam loom in England, for example, it took about half the labor required to convert a certain amount of yarn into fabric. In order to make this conversion, the English hand weaver needed to spend exactly the same amount of time as before, but the product of his individual hour of work represented only half an hour of social work, and its value therefore decreased to half of what it had before.
It is only the amount of socially necessary work, then, or the time of work socially necessary for the production of a use value, that determines its magnitude of value.
Karl Marx. Capital, 1857