The relative irksomeness of two occupations may
affect the relative wages which will rule in the two cases; so,
certainly, will the differences in the cost of education and training
which they require. But these are matters which concern the
_apportionment_ of labor between different employments. There is no
good reason to suppose that the general wage-level would be reduced,
merely because work as a whole became less irksome, or involved a
smaller physical or mental strain. The supply of people is not
determined by the same kind of influences as is the supply of a
commodity. Parents do not produce children for the sake of the wages
which the children will receive when they go out to work; or, if this
happens, we rightly regard it as a horrible anomaly. In so far as
parents are affected by economic conditions it is by their own
economic conditions; the question is rather one of how many children
they can afford to have, than of a balancing of the cost to them
against the incomes which their children may subsequently acquire. But
other considerations enter in; and, in fact, it is doubtful how the
aggregate supply of labor will react to changes in prosperity.
Finally, the supply of land involves neither effort nor sacrifice;
and, among our money costs, we have to account for the item
of the rent of land. To dispose of this difficulty by arguing that
rent does not enter into marginal costs (in any sense which is not
equally true of wages and profits) is to lose contact with
reality. Thus the attempt to explain money costs in terms of the costs
of producing the ultimate agents of production leads us into a
quagmire of unreality and dubious hypothesis. For a systematic theory,
which will rest on firm foundations, we must interpret money costs in
very different terms.

The real costs which the price of a commodity measures are not
absolute, but comparative. Marginal money costs reduce themselves in
the last analysis to the payments which must be made to secure the use
of the requisite agents of productions. These payments _tend_ to equal
the payments which the same agents could have commanded in alternative
employments. The payments which they could have commanded in
alternative employments, tend in their turn to equal the derived
marginal utilities of their services in those employments. It is thus
the loss of _Utility_ which arises from the fact that these agents of
production are not available for alternative employments