The economic value of a man’s work is determined, on a free market, by a single principle: by the voluntary consent of those who are willing to trade him their work or products in return. This is the moral meaning of the law of supply and demand; it represents the total rejection of two vicious doctrines: the tribal premise and altruism. It represents the recognition of the fact that man is not the property nor the servant of the tribe, that a man works in order to support his own life—as, by his nature, he must—that he has to be guided by his own rational self-interest, and if he wants to trade with others, he cannot expect sacrificial victims, i.e., he cannot expect to receive values without trading commensurate values in return. The sole criterion of what is commensurate, in this context, is the free, voluntary, uncoerced judgment of the traders.

- Ayn Rand. Capitalism: The Unknown Ideal (1966).