Marx identified another: owners of scarce things like land, coal, a patent, a licence to practise law, and so on. Such scarce things can improve productivity above the general productivity level–the same product can be produced in less labour time or with fewer means of production. That in turn creates ‘surplus profits’–what Smith and Ricardo might have thought of as ‘rent’–for capitalists, or landlords and proprietors, who can exploit these advantageous production conditions. Marx thus outlined a theory of ‘monopoly’ gain.
