Cantillon, writing forty years before The Wealth of Nations, employed the term “entrepreneur” to describe the worthy gentleman who bought goods “at a fixed price” and sold them “at an uncertain price.” This conception clearly had some kinship with that of Adam Smith, since it fastened on a crucial feature of that “propensity to truck, barter and exchange.” The undertaker is essentially the ideal entrepreneur, and the men of the 15th century could have found no better title for him than “adventurer.” The explanation, in default of which the early economists postulated the operation of a natural law, has been provided by Marshall’s Principle of Substitution, and this principle, though like most economic discoveries both simple and obvious, is the keystone of whatever modern theory of enterprise may be said to exist. The preoccupation of the undertaker is essentially with quantitative measurement, operating through the medium of money-price.