Nature does more, however, than supply materials; she also supplies powers. Of natural powers, some are practically unlimited, others limited in quantity, and much of the economy of society depends on the limited quantity in which some of the most important natural agents exist, and more particularly land. As soon as there is not so much of a natural agent to be had as would be used if it could be obtained for the asking, the ownership or use of it acquires an exchangeable value. Where there is more land wanted for cultivation than a place possesses of a certain quality and advantages of situation, land of that quality and situation may be sold for a price, or let for an annual rent. Labour employed on external nature in modes subservient to production is employed either directly, or indirectly, in previous or concomitant operations designed to facilitate, perhaps essential to the possibilities of, the actual production. One of the modes in which labour is employed indirectly requires particular notice, namely, when it is employed in producing subsistence to maintain the labourers while they are engaged in the production. This previous employment of labour is an indispensable condition to every productive operation. In order to raise any product there are needed labour, tools, and materials, and food to feed the labourers. But the tools and materials can be remunerated only from the product when obtained. The food, on the contrary, is intrinsically useful, and the labour expended in producing it, and recompensed by it, needs not to be remunerated over again from the produce of the subsequent labour which it has fed. The claim to remuneration founded on the possession of food is remuneration for abstinence, not for labour. If a person has a store of food, he has it in his power to consume it himself in idleness. If, instead, he gives it to productive labourers to support them during their work, he can claim a remuneration from the produce. He will, in fact, expect his advance of food to come back to him with an increase, called, in the language of business, a profit.