Marx thus bases his argument on two principles which he borrows from current economic writers, without, however, observing the limitations under which those writers taught them, and introducing besides important modifications of his own. The one principle is that value comes from labour, or as economists stated their law, that the natural value of commodities is determined by the cost of their production. The second is only a special application of the first; that the natural wages of labour are determined by the cost of its production, and that the cost of the production of labour is the cost of the labourer's subsistence. The fault he finds with the present system is accordingly this, that while labour creates all value it is paid only by its stated living, no matter how much value it creates; and he then goes over the phenomena of modern industrial life to show how each arrangement is invented so as to extract more and more value out of the labourer by prolonging his hours of work or enhancing its speed without giving him any advantage whatever from the increase of value so obtained. We shall get a fair view of Marx's argument, therefore, if we follow it through the successive heads: 1st, Value; 2nd, Wages; 3rd, Normal day of labour; 4th, Machinery; 5th, Piecework; 6th, Relative over-population. 1st. _Value._ Marx holds that all capital--all industrial advances except wages--is absolutely unproductive of value, and therefore not entitled to the acknowledgment known as interest. The original value of all such capital--the purchase price of the materials, together with a certain allowance made for tear and wear of machinery--is carried forward into the value of the product, and preserved in it, and even that could not be done except by labour. The old value is preserved by labour, and all new value is conferred by it, and therefore interest is a consideration entirely out of the question. It is obvious to object that labour by itself is as unproductive as capital by itself, but Marx would reply that while labour and capital are equally indispensable to produce new commodities, it is labour alone that produces new value, for value is only so much labour preserved, it is merely a register of so many hours of work. His whole argument thus turns upon his doctrine of the nature of value, and that doctrine must therefore be closely attended to.