Now, if there were only one farm in the world, and one grade of labour, and if every acre of this farm, when the same labour was applied to it, would always yield the same amount of produce—­let us say one loaf—­Mill’s argument would be true.  The actual state of the case is, however, very different.  Acres vary very greatly in quality; and if we take four acres of varying degrees of fertility, to all of which is applied the same amount of labour, then, while from the worst of the acres this labour will elicit one loaf, it will elicit from the others, let us say, according to their degrees of fertility, two loaves, three loaves, and four loaves respectively.  Here the labour 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.