The marginal utility
4. Marginal utility is the gratification afforded by the added portion of the good. The marginal dose, increment, or portion is that which may be logically considered as coming last in the case of any good or group of goods divisible into small parts. In considering the strict theory of the case, in order to get at the principle involved, the doses may be spoken of as infinitesimally small. The marginal utility expresses the importance that men attach to one unit of this kind of goods under the particular circumstances at the moment existing, and not under certain conceivable conditions which do not in fact exist or need to be taken into account by the persons affected. The marginal unit of a homogeneous supply cannot be considered to have a greater utility than any other unit at the moment, and therefore the product of the marginal utility by the number of units, gives the total measure of importance of the supply then and there, and this is the value.
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.
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.
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.