He brings it forward with special reference to agriculture, and aims it at the contention of a certain school of economists that nature in agriculture did more than in other industries. To urge this, says Mill, is nonsense, for the simple reason that though nature in agriculture does something, it is impossible to determine whether the something is relatively much or little. Let us, he says in effect, take the products of any farm, which we may for convenience' sake symbolise as so many loaves; and it is obviously absurd to inquire which produces most of them--the soil or the farm labourers. The soil without the labourers would produce no loaves at all. The labourers would produce no loaves if they had not the soil to work upon. 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.