What is “capital”? To Karl Marx, it was a social, political, and legal category—the means of control of the means of production by the dominant class. Capital could be money, it could be machines; it could be fixed and it could be variable. But the essence of capital was neither physical nor financial. It was the power that capital gave to capitalists, namely the authority to make decisions and to extract surplus from the worker.
Early in the last century, neoclassical economics dumped this social and political analysis for a mechanical one. Capital was reframed as a physical item, which paired with labor to produce output. This notion of capital permitted mathematical expression of the “production function,” so that wages and profits could be linked to the respective “marginal products” of each factor. The new vision thus raised the uses of machinery over the social role of its owners and legitimated profit as the just return to an indispensable contribution.