Theorien über den Mehrwert
In "Theories of Surplus Value", Marx says in classical economics the "surplus" referred to "an excess of gross income over cost", which implied that the value of goods sold was greater than the value of the costs involved in producing or supplying them. That was how you could "make money". The surplus represented a net addition to the stock of wealth. A central theoretical question was then to explain the kinds of influences on the size of the surplus, or how the surplus originated, since that had important consequences for the funds available for re-investment, tax levies, the wealth of nations, and (especially) economic growth.