"It took from a hundred to a hundred and fifty or two hundred years for the astronomy of Kepler to become the astronomy of Newton and , and for the mechanics of Galileo to become the mechanics of d'Alembert and Lagrange. On the other hand, less than a century has elapsed between the publication of Adam Smith’s work and the contributions of Cournot, Gossen, Jevons, and myself."

“I say that things are useful whenever they can be put to any use at all; whenever they are seen to be capable of satisfying a want. In this connection, there is no need to consider the subtle shades of meaning classified in ordinary language under terms ranging from the necessary to the useful, from the useful to the agreeable, from the agreeable to the superfluous. For present purposes, necessary, useful, agreeable and superfluous simply mean more or less useful. Furthermore, we need not concern ourselves with the morality or immorality of any desire which a useful thing answers or serves to satisfy. From other points of view the question of whether a drug is wanted by a doctor to cure a patient, or by a murderer to kill his family is a very serious matter, but from our point of view, it is totally irrelevant. So far as we are concerned, the drug is useful in both cases, and may even be more so in the latter case than in the former.”