The economist studies the disposal of scarce means. He is interested in the way different degrees of scarcity of different goods give rise to different ratios of valuation between them, and he is interested in the way in which changes in conditions of scarcity, whether coming from changes in ends or changes in means—from the demand side or the supply side—affect these ratios. Economics is a science which studies human behavior as a relationship between ends and scarce means which have alternative uses.

An Essay on the nature and significance of Economic Science (1932), Chapter I: The Subject Matter of Economics