A comparison of value with weight will aid in arriving at a clearer understanding of the nature of the former. The weight of a body is the force which draws it to the earth. The value of an object is a certain power of attraction which it has for human beings. The only way we have of measuring this value or power of attraction in goods is to compare it with or to oppose it to some other value or power of attraction. This is done when a person chooses between two goods, accepting the one and rejecting the other. The one which he keeps has more value than the other. We have a similar experience with weight. We learn the amount of the weight of a body by comparing the attraction of the earth for the body with the similar attraction which the earth has for some other body. The comparison is made by means of a balance. In order that we may express the amount of weight in an object it is necessary that we have a unit weight, such as the pound. A pound is a certain definite quantity of something which has weight. Although a pound is the same all over the world, the weight of a pound differs in different latitudes. In a similar way, the dollar is a certain amount of the good, gold, which has value. The value of the dollar is compared with other values, and in this way we are able to speak of the number of units of value which a good contains. And although the amount of gold which constitutes a dollar remains constant, the value of a dollar is continually varying.