It should further be pointed out that this doctrine of Rent, though putting so strict a limit on the product that can be assigned to Labour, interferes with no view that the most ardent Socialist or Radical may entertain with regard to the moral rights of the labourer. If any one contends that the men who labour on the land, and who pay away part of the produce as rent to other persons, ought by rights to retain the whole produce for themselves, he is perfectly at liberty to do so, for anything that has been urged here. For the real meaning of such a contention is, not that the labourers do not already keep everything that is produced by their labour, but that they ought to own their land instead of hiring it, and so keep everything that is produced by the land as well. [Sidenote ◆1 The doctrine of Rent is the fundamental example of the reasoning by which to each agent in production a definite portion of the product is attributed.] This doctrine of Rent, then, which I have tried to make absolutely clear, involves no special pleading on behalf either of landlord or tenant, of rich or poor. It can be used with equal effect by Tory, Radical, or Socialist, and it would be as true of a Socialistic State as it is of any other. I have insisted on it here for one reason only. ◆¹ It illustrates, and is the fundamental example of, the following great principle—that in all cases where Human Exertion is applied to Land which yields only enough wealth to maintain the man exerting himself, practical logic compels us to attribute the entire product to his exertion, and to take the assumption that his exertion produces this much as our starting-point. But in all other cases—that is to say in all cases where the same exertion results in an increased product, we attribute the increase—we attribute the added product—not to Human Exertion, which is present equally in both cases, but to some cause which is present in the second case, and was not present in the first: that is to say, to some superior quality in the soil. And now let us put this in a more general form. When two or more causes produce a given amount of wealth, and when the same causes with some other cause added to them produce a greater amount, the excess of the last amount over the first is produced by the added cause; or conversely, the added cause produces precisely that proportion of the total by which the total would be diminished if the added cause were withdrawn. It is on this principle that the whole reasoning in the present book is based; and having seen how it enables us to discriminate between the amounts of wealth produced respectively by Human Exertion and Land, let us go on to see how it will enable us likewise to discriminate what is produced by Capital. CHAPTER III _Of the Products of Machinery or Fixed Capital, as distinguished from the Products of Human Exertion._