The value of goods, as has been indicated, is the measure of the dependence felt by men on a portion of the outer world, as the condition of gratifying their wants. From the very nature of wants, which reside in feelings, a dependence that is not felt, a relation between things and gratification that is not recognized, can have no influence on value. Now, it is at this margin of supply that dependence is felt. Men do not concern themselves about that which they have in superfluity--unless, indeed, the excess causes them some discomfort. It is well that they do not, for a wise direction of effort can only take place when men think mainly of their need of things that they want, and want most, and direct their efforts toward securing them. [Sidenote: From marginal utility to value] The diminishing utility of successive portions (doses or increments, as they are called) may be represented by a curve of utility. [Illustration: _Scale of Supply_] The diagram is constructed on the hypothesis that a tenth unit of a certain good would have a utility expressed as 36; a fifteenth unit of 30, etc., and that the value of the whole supply is estimated according to these marginal units. Of course if the conditions were that "all or none" was to be taken, the result would be different. Unit of Supply Marginal Utility Value of Whole Supply 10 36 360 15 30 450 20 25 500 30 19 570 40 15 600 50 10 500 60 5 300 This diagram is frequently used, and it is important to guard against some misunderstandings. The marginal unit of any given supply--for example, ten units--is not any particular unit, it is any one of the ten units. In the presence of nine units of the good the person or persons find all the various wants that are dependent on that good gratified to such a degree that the tenth unit has an importance expressed by 36. But as this last or marginal unit of supply may be used for any of the purposes, the importance of each and every unit likewise will be expressed by 36. Any one of the units, when once present is, in a logical sense, a marginal unit. When, however, it is a question of increasing the supply, some one unit may properly be looked upon as marginal. The dependence felt by men on the whole group is the product of the units by the marginal utility. As the number of units increases, the marginal utility decreases, until at length it may reach zero, and the total value would be nothing. A point of maximum value evidently will be found somewhere between the two extremes. [Sidenote: Only one marginal utility at one moment]