Supply and demand will be in equilibrium only when the price of every commodity is proportional to the labor time required to produce it. Conversely … - Paul Sweezy

Under simple commodity production, to which we have so far largely confined our attention, each producer owns and works with his own means of production; under capitalism ownership of the means of production is vested in one set of individuals while the work is performed by another. Both means of production and labor power, moreover, are commodities; that is to say, both are objects of exchange and hence bearers of exchange value. It follows that not only the relations among owners but also the relations between owners and non-owners have the character of exchange relations. The former is characteristic of commodity production in general, the latter of capitalism only.

Discussions of methodology in economics, as in other fields, are likely to be tiresome and unrewarding. Nevertheless, to avoid the problem altogether is to risk serious misunderstanding.

The expansion of value, which is the objective basis or main-spring of the circulation M-C-M, becomes his subjective aim, and it is only in so far as the appropriation of ever more and more wealth in the abstract becomes the sole motive of his operations that he functions as a capitalist, that is, as capital personified and endowed with consciousness and a will. Use values must therefore never be looked upon as the real aim of the capitalist; neither must the profit on any single transaction. The restless never-ending process of profit-making alone is what he aims at.