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.