Ludwig von Mises strips economics down to its foundation: man acts. He removes three persistent misunderstandings—that economics describes a fictional "economic man," that it assumes pure selfishness, and that it requires perfect rationality—and replaces them with one proposition: human beings consciously strive to substitute more satisfactory conditions for less satisfactory ones. From there, he explains why economic knowledge is derived from reason rather than laboratory experiment, why there are no constant relationships in economics the way there are atomic weights in chemistry, and why statistics belong to economic history, not economic theory. Along the way, he corrects a fallacy that has persisted since Aristotle: that exchange requires equality between the items traded. It doesn't. Exchange occurs precisely because both parties value what they receive more than what they give—a point Marx missed, Bergson missed, and most textbooks still miss. The article closes with a warning: you cannot predict the future by studying the past, and the economist who confuses history with theory will be wrong every time.