After ownership had been introduced, and since all things were not of the same nature, and did not yield the same service to human necessities, and no one had that abundance which he desired for his needs, it soon became customary among men to exchange commodities. But it very often happened that things of unlike nature or use had to be transferred, and hence, that one party or the other might not suffer in this sort of exchange, it was necessary for human convention to assign things a quantity, according to which they could be compared and balanced with one another. This also was the case with actions, which men were unwilling to render for others' advantage for nothing. This quantity usually goes by the name of value.
Value is divided into common value and value par excellence.
The former is seen in things, and actions or services, which enter into trade, in so far as they bring men some use and pleasure. The latter is seen in money, in so far as it is understood virtually to contain the price of all things and services, and to furnish them a common standard. Of common value the foundation as such is that aptitude of the thing or the service, by which it can contribute something directly or indirectly to the necessities of human life, and to make it more comfortable or agreeable. Hence we usually call things that serve no use at all things of no value. Yet there are some things most useful for human life, upon which things no definite value is understood to have been set, either because they do not admit of ownership, and necessarily so, or because they are unsuited for exchange, and hence withdrawn from trade, or because in trade they are never considered otherwise than as an addition to something else.