ider
that the additional utility she will derive from buying 6 pounds a
week rather then 5 pounds is worth as much as 8 cents. But she shows
at the same time that she thinks it worth 7 cents. For, when the price
is 7 cents, no one compels her to buy that sixth pound. She could
stop, if she chose, at five; and it may serve to make the point quite
plain if we suppose her actually to hesitate before she buys the
sixth. She has hitherto, let us say, been buying 5 pounds a week at 8
cents. To-day she enters the shop and finds the price is down to 7
cents. She asks for her customary 5 pounds; then she pauses, and a
minute later turns her order into six. What are the alternatives which
she has been weighing one against the other in that momentary pause?
Not the utility of the whole 6 pounds of sugar against the total price
of 42 cents. For she has already ordered the first 5 pounds; and the
decision to buy the sixth is taken independently and subsequently. She
has been sizing up the _increment_ of utility which a sixth pound
would yield, and she decides that this is worth the expenditure of a
further 7 cents. Again, when the price was 8 cents she need not have
bought as many as 5 pounds. She could have stopped at 4 had she
chosen, and the fact that she did buy 5 pounds shows that the
increment of utility derived from buying a fifth pound, when she might
be said already to have 4, was worth at least 8 cents in her judgment.

This trite illustration enables us to lay down two important laws
relating to utility. To state them shortly, it is convenient to employ
one or two technical terms, which, unlike every term employed
hitherto, are not very commonly used in their present sense in
everyday life. Their adoption is desirable not merely for the sake of
convenience, but because they help to stamp clearly on the mind a most
illuminating conception, that of the "margin," which supplies the clue
to many complicated problems. The last pound of sugar which the
housewife purchased, the fifth pound when the price was 8 cents, or
the sixth pound when the price was 7 cents, we call the "marginal"
pound of sugar. And the increment of utility which she derives from
buying this marginal pound we call the "marginal utility" of sugar to
her. We are thus able to state the fact that the more a person has of
anything the less urgently does he require a little more of it, in the
following formal terms:--

LAW V. The marginal utility of a commodit