Anonymous.
An axiom from economics popular in the 1960s, the words have no known source, though have been dated to the 1840s, when they were used in saloons where snacks were offered to customers. Ascribed to an Italian immigrant outside Grand Central Station, New York, in Alistair Cooke's America (epilogue, 1973), the expression appears in Robert A. Heinlein's The Moon is a Harsh Mistress, ch. 11 (1966), but has become most closely associated with economist Milton Friedman, who made it the title of a book in 1975.