44 NATURE OF CAPITAL AND INCOME [Caar. II

satisfactions from those goods are held. But the two are
none the less distinct. It is with utility or desirability

that we are concerned in this chapter.
§ 4

The desirability of any particular goods may relate to
the whole or to any part of the group of goods. The desirability
 of the entire group is called the total desirability;
the desirability of one unit more Or less of the group is
called the marginal desirability. In economic science we
have to do more with marginal than with total desirability,
and it is important that the concept of marginal desirability
 should be thoroughly understood.
That marginal desirability is the desirability of one unit,
more or less, may be illustrated as follows: If a person
airs, their marginal desirability is the differthe
 desirability of having ten
f having nine chairs; that is,
it is the desirability sacrificed by having one chair less.
Or, what is almost the same thing, the marginal desirability
of the group of ten chairs is the desirability of one chair
more, — the difference in desirability between eleven chairs
and ten. Whether the marginal desirability is taken as
referring to one unit more or to one unit less is usually of
so little importance as not to require separate designations
to distinguish them, and in case the commodity is one which
admits of indefinite subdivision, as flour, wheat, coal, etc.,
the two coalesce as the size of the increment is reduced indefinitely.!
 This fact is usually expressed by saying that
the marginal desirability of the chairs is the desirability of
“the tenth’ chair. But though this mode of statement is
correct, it is not intended to convey the idea that any particular
 chair is the “tenth” chair.
The group of goods the marginal desirability of which is
under consideration may be any specified group of goods
1 For a mathematical treatment see Appendix to Chap. III, § 1.

possesses ten ch
ence, in his mind, between
chairs and the desirability o