Mill brings the argument forward with special reference to agriculture. Let us take, he says in effect, the products of any farm; and it is obviously absurd to inquire which produces most of it—the fields or the farm labourers. Now if all labour were equal, and if there were only one farm in the world, or if every acre of land, when the same labour was applied to it, yielded the same amount of produce, this would, no doubt, be true. The actual state of the case is, however, widely different. Acres vary very greatly in fertility; and if the produce of one—the least fertile—when cultivated by a given amount of labour, be symbolised by ten loaves, the produce of others, when cultivated by the same labour, will be symbolised by loaves to the number of twelve, fifteen, or twenty. Here, then, we have a constant quantity of labour, which produces ten loaves from each of the four acres in question; but when applied to the first, it produces ten loaves only; when applied to the three others, it produces two, or five, or ten loaves in addition. About the first ten loaves, in each case, it is not possible to argue. So far as they are concerned, the result is in each case the same; with regard to them we cannot {199} make any comparison; and we must admit that the parts played by land and labour in producing them are “_indefinite and incommensurable_,” precisely as Mill says they are. But the two, the five, or the ten extra loaves which result when labour is applied to the second, the third, and the fourth acre respectively, but do not result at all so long as it is applied only to the first, constitute phenomena of a different order altogether. The labour being in each of the four cases the same, and these additional loaves resulting in three cases only, these additional loaves are obviously not due to labour, but to certain additional qualities present in the last three acres and not present in the first. In other words, though in producing the loaves, or, as Mill puts it, “_the effect_,” the parts played respectively by land and labour are incommensurable so long as the land, the labour, and the effect remain the same, the parts become immediately mensurable as soon as the effect begins to vary, and one of the causes, and one of the causes only, varies also.