It must be emphasized, because the contrary has so often been asserted, that Marx was not trying to reduce everything to economic terms. He was rathe… - Paul Sweezy

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 prices proportional to labor times will be established only if the forces of competitive supply and demand are allowed to work themselves out freely. The competitive supply-and-demand theory of price determination is hence not only not inconsistent with the labor theory; rather it forms an integral, if sometimes unrecognized, part of the labor theory.

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.

In possessing use value a commodity is in no way peculiar. Objects of human consumption in every age and in every form of society likewise possess use value. Use value is an expression of a certain relation between the consumer and the object consumed. Political economy, on the other hand, is a social science of the relations between people.