| 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.
Only one marginal utility at one moment
Note carefully that on the one diagram are represented a large number of marginal utilities which never exist at one and the same moment. At any one moment there is a given number of units and there is but one marginal utility, and this is the same for each of the units. It is quite erroneous to say that when there are 30 units the utility of the tenth unit is 36; of the twentieth, 25; of the thirtieth, 19. It is equally incorrect to say that when there are 60 units the "total utility" is equal to the area between the right angle and the curve a-g, while the value is equal to the rectangle below and to the left of the point g. The curve from a-g but marks the height of marginal utilities that have no existence when the supply is 30. The "total utility," often spoken of in this connection, if it has any existence certainly cannot be calculated. The diagram must be understood as representing indicatively at any given moment but one marginal utility, the same for every unit of like goods. The other perpendicular lines are expressed in the conditional mood; they are what the marginal utility would be were the numbers of units different.
Changing feelings changes utility
5. Since goods possess utility only as they gratify wants, it follows that if wants change, the utility changes. Utility does not rest unchanging in the goods as something "intrinsic," but it depends on the relation of goods to men. This truth, unrecognized for many centuries, is now seen to be fundamental to the whole problem of value. The portions of a good added later do not appeal to the same man as the earlier portions. The man has been changed by what he has enjoyed. In changing his feelings, goods have also changed his wants. Hence, the added portions of the good are changed in respect to their utility or power to gratify a man's wants. Though physically and chemically, i.e., in every material way, they are exactly like the earlier portions, they cannot have the same want-gratifying power until he again changes, for they are not in the presence of the same feelings.
Wants are constantly shifting; different kinds of goods are compared in man's thought and arranged on a scale at every moment according to their felt utility. An increase in the amount of a good will drop the marginal utility of the added portions down the scale of usefulness for the next moment. When we rise in the morning, we want our breakfast; the breakfast eaten, another breakfast does not appeal to us. Our tasks done, we take a boat-ride or go golfing; then, appetite returning, we are tempted to our dinner. And thus from hour to hour wants are gratified, are altered and are shifted, until, wearied with the day's labor and pastimes, we go to rest. In a well-ordered life, in an advanced economic society, the means for gratifying our wants as they arise are provided in advance. The changing series of desires is met by a changing series of goods. Life has been defined as a constant adjustment of inner relations to outer conditions. Economic life is therefore like physical life, a constant adjustment; and this adjustment of goods but reflects the shifting and adjustment of feelings.
Choice is constantly shifting
6. The substitution of goods in men's thought is the shifting of the choice from a good that does not give the highest gratification economically possible at the time, to another good that does. The shifting that takes place on the scale of gratification makes it necessary for man to shift constantly his choice of goods. This again is the problem of "economy." Waste results when goods continue to be used to secure a lower degree of gratification, if they might be used to secure a higher. The change of choice may be because of a change in the man, or because of a change in the quality or the quantity of the goods; or because of a change in the ratio at which the goods can be secured.