By a commodity we shall understand any object, substance, action or service, which can afford pleasure or ward off pain. - William Stanley Jevons

We shall never have a science of economics unless we learn to discern the operation of law even among the most perplexing complications and apparent interruptions.

In a certain sense all knowledge is inductive. We can only learn the laws and relations of things in nature by observing those things. But the knowledge gained from the senses is knowledge only of particular facts, and we require some process of reasoning by which we may collect out of the facts the laws obeyed by them. Experience gives us the materials of knowledge: induction digests those materials, and yields us general knowledge. When we possess such knowledge, in the form of general propositions and natural laws, we can usefully apply the reverse process of deduction to ascertain the exact information required at any moment. In its ultimate foundation, then, all knowledge is inductive—in the sense that it is derived by a certain inductive reasoning from the facts of experience.

The theory which follows is entirely based on a calculus of pleasure and pain; and the object of economics is to maximize happiness by purchasing pleasure, as it were, at the lowest cost of pain.