The English word value is derived from the Latin, valere, which means to be strong, to have power. It is usual to distinguish between two different kinds of value, value in use and value in exchange. When value is spoken of in economics without any qualification, value in exchange is meant. A treatment of value in exchange belongs in a later chapter, but we shall anticipate that treatment by saying that value in exchange or simply value will be defined as power in exchange, or the power which ownership of a good gives to its owner of securing other goods in exchange for it. We shall here confine our attention to value in use. In a discussion of value in exchange two persons who are exchangers are to be considered. In a discussion of value in use only one person is considered. Two goods are balanced against each other, not in the minds of two persons, but in the mind of one person. That person chooses one good and rejects the other. He prefers the one to the other. In other words one good has a greater power to influence his choice than the other has. The power of a good to influence a person to prefer it to some other good is its value in use. The man in our illustration in § 27 balanced in his mind the gratifications which could be obtained from a fourth unit of good a, a third unit of good b, a second unit of good c, and a first unit of good d; and considered that the gratifications were of equal weight. At this margin of indifference of consumption the amount of a which would just balance in his mind a unit of b was worth to him a unit of b. It had the same power to influence him to purchase it that a unit of b had.