bour being in each of
the four cases the same, and the additional loaves resulting in three
cases only, it is obvious that the difference between the larger
products and the less is not due to the labour, but to certain
additional qualities present, in the three superior acres and not
present in the worst one. In other words, although in producing
loaves--or, as Mill describes it, "the effect"--the parts played by
labour and nature are indefinite and incommensurable so long as the
land, the labour, and the effect remain all three the same, the parts
become immediately measurable when the effect begins to vary, and one of
the causes, and only one of them, at the same time varies also.

This truth can be yet further elucidated by the very illustration which
Mill cities in disproof of it. It is absurd to ask, he says, whether the
number five or six does most, when they are multiplied together, to
produce "the effect" thirty. This is true so long as "the effect" thirty
is constant; but if on occasions the thirty is increased to forty, and
if whenever this happens the six has increased to eight, we know that
the extra ten which our multiplication yields us is not due to the five,
the number which remains unchanged, but to an extra two now present in
the number that was once six. Or again let us take as "the effect" the
speed of a motor-car which is raced over a mile of road. Unless two
conditions were present--the engine and some ground to run upon--the car
could not run at all; and if there were only one road and one car in
the world, it would be absurd to inquire how much of the speed was due
to the merits of the engine, and how much to the character of the road's
surface. But if, the car remaining unchanged, the surface of the road
was improved, and a speed was thereupon developed of thirty miles an
hour instead of twenty, we should, with regard to the increment, at once
be able to say that it was due to the surface of the road, and was not
due to the engine. Conversely, if the road were unchanged, but the car
had a new engine, and the speed under these conditions increased in the
same way, the increment would be evidently attributable to the engine
and not the road.

And the same observations apply to labour and directive ability,
whenever the operations of both are essential to a given product. If the
ability and the labour were always inevitably constant, and the product
as to quality and amount were similarly constan