Finally, the human capacity to perform work, which Marx called “labor power,” must not be confused with the power of any nonhuman agency, whether natural or man made. Human labor, whether directly exercised or stored in such products as tools, machinery, or domesticated animals, represents the sole resource of humanity in confronting nature. Thus for humans in society, labor power is a special category, separate and inexchangeable with any other, simply because it is human. Only one who is the master of the labor of others will confuse labor power with any other agency for performing a task, because to him, steam, horse, water, or human muscle which turns his mill are viewed as equivalents, as “factors of production.” For individuals who allocate their own labor (or a community which does the same), the difference between using labor power as against any other power is a difference upon which the entire “economy” turns. And from the point of view of the species as a whole, this difference is also crucial, since every individual is the proprietor of a portion of the total labor power of the community, the society, and the species.

It is important to take note of the historical character of this phenomenon. While the purchase and sale of labor power has existed from antiquity,* a substantial class of wage-workers did not begin to form in Europe until the fourteenth century, and did not become numerically significant until the rise of industrial capitalism (that is, the production of commodities on a capitalist basis, as against mercantile capitalism, which merely exchanged the surplus products of prior forms of production) in the eighteenth century. It has been the numerically dominant form for little more than a century, and this in only a few countries. In the United States, perhaps four-fifths of the population was self-employed in the early part of the nineteenth century. By 1870 this had declined to about one-third and by 1940 to no more than one-fifth; by 1970 only about one-tenth of the population was self-employed. We are thus dealing with a social relation of extremely recent date. The rapidity with which it has won supremacy in a number of countries emphasizes the extraordinary power of the tendency of capitalist economies to convert all other forms of labor into hired labor.

But if the capitalist builds upon this distinctive quality and potential of human labor power, it is also this quality, by its very indeterminacy, which places before him his greatest challenge and problem. The coin of labor has its obverse side: in purchasing labor power that can do much, he is at the same time purchasing an undefined quality and quantity. What he buys is infinite in potential, but in its realization it is limited by the subjective state of the workers, by their previous history, by the general social conditions under which they work as well as the particular conditions of the enterprise, and by the technical setting of their labor. The work actually performed will be affected by these and many other factors, including the organization of the process and the forms of supervision over it, if any.