Let us consider now some propositions which follow from the determination of exchange value by labor-time.
As a use-value, every commodity owes its usefulness to itself. Wheat, e. g., serves as an article of food. A machine saves labor to a certain extent. This function of a commodity by virtue of which it serves only as use-value, as an article of consumption, may be called its service, the service which it renders as use-value. But as an exchange value, a commodity is always regarded as a result; the question in this case is not as to the service which it renders, but as to the service10 which it has been rendered in its production. Thus, the exchange value of a machine is determined not by the quantity of labor-time which it saves, but by the quantity of labor-time which has been expended on its own production and which is, therefore, required to produce a new machine of the same kind.
If, therefore, the quantity of labor-time required for the production of commodities remained constant, their exchange value would remain the same. But the ease and the difficulty of production are constantly changing. If the productivity of labor increases, the same use-value will be produced in less time. If the productivity of labor declines, more time will be required for the production of the same use-value. Thus, the labor-time contained in a commodity or its exchange-value is a variable quantity, increasing or diminishing in an inverse ratio to the rise and fall of the productivity of labor. The productive power of labor which is applied in the manufacturing industry on a predetermined scale depends in the agricultural and extractive industries also on natural conditions which are beyond human control. The same labor will yield a greater or less output of various metals, according to their more or less close occurrence in the earth’s crust. The same labor may be embodied in two bushels of wheat in a favorable season, and only in one in an unfavorable season. In this case, scarcity or abundance, as natural conditions, seem to determine the exchange value of commodities, because they determine the productivity of certain kinds of labor which depend upon natural conditions.