impute to an increment of labor
whatever amount of product its presence adds to that which was created
before.

_Summary of Essential Facts._--The facts that are to be remembered
then are: first, that the capital remains fixed in amount, though the
forms of it change as the number of units of labor increases;
secondly, that that which we call the product of a unit of labor is
what that unit, coming into the field without any capital, can add to
the product of the labor and capital that were there before; and
thirdly, that this specific product of labor grows smaller as the
amount of labor grows larger, rendering the product of the last unit
the smallest of all. When the tenth and last unit is working, each one
of the nine earlier units is, of itself, producing no more than does
the final one, though it formerly produced more because of the larger
quota of capital with which it was formerly supplied.

[Illustration]

_The Test of Final Productivity._--There are now at work ten units of
capital and ten of labor, and we cannot go through the process of
building up the working force from the beginning. How, then, do we
measure the true product of a single unit of labor? By withdrawing
that unit, letting the industry go on by the aid of all the capital
and one unit of labor the less. Whatever one of the ten units of labor
we take away we leave only nine working. If the forms of the capital
change so as to allow the nine units to use it advantageously, the
product will not be reduced to nine tenths of its former size, but it
will still be reduced; and the amount of the diminution measures the
amount of product that can be attributed to one unit of bare labor. Or
we may add a certain number of workmen to a social force already at
work, making no change in the amount of the capital,--though changing
its forms,--and see how much additional product we get. That also is a
test of final productivity. It gives the same measurement as does the
experiment of taking away the little detachment of men and seeing how
much the product shrinks. By either process we measure an amount that
is attributable altogether to bare labor and not to capital.

The whole area _BCD_ in the diagram is an amount of product that is
attributable to capital and not to labor. It represents the total
surplus produced by labor and capital over the amount that can be
traced to the labor alone. The product of all the capital and all the
labor minus ten times