We do not pay for the air we breathe, although it is so _useful_ to us, that, without it, we could not live two minutes. We do not pay for it, nevertheless; because nature furnishes it to us without the aid of human labour. But if, for example, we should desire to separate one of the gases of which it is composed, to make an experiment, we must make an exertion; or if we wish another to make that exertion for us, we must sacrifice for that other an equivalent amount of exertion, although we may have embodied it in another product. Whence we see that pains, efforts, and exertions are the real subjects of exchange. It is not, indeed, the oxygen gas that I pay for, since it is at my disposal everywhere, but the labour necessary to disengage it, labour which has been saved me, and which must be recompensed. Will it be said that there is something else to be paid for, materials, apparatus, etc.? Still, in paying for these, I pay for labour. The price of the coal employed, for example, represents the labour necessary to extract it from the mine and to transport it to the place where it is to be used. We do not pay for the light of the sim, because it is a gift of nature. But we pay for gas, tallow, oil, wax, because there is here human labour to be remunerated; and it will be remarked that, in this case, the remuneration is proportioned, not to the utility produced, but to the labour employed, so much so that it may happen that one of these kinds of artificial light, though more intense, costs us less, and for this reason, that the same amount of human labour affords us more of it. Were the porter who carries water to my house to be paid in proportion to the _absolute utility_ of water, my whole fortune would be insufficient to remunerate him. But I pay him in proportion to the exertion he makes. If he charges more, others will do the work, or, if necessary, I will do it myself. Water, in truth, is not the subject of our bargain, but the labour of carrying it. This view of the matter is so important, and the conclusions which I am about to deduce from it throw so much light on the question of the freedom of international exchanges, that I deem it necessary to elucidate it by other examples. The alimentary substance contained in potatoes is not very costly, because we can obtain a large amount of it with comparatively little labour. We pay more for wheat, because the production of it costs a greater amount of human labour. It is evident that if nature did for the one what it does for the other, the price of both would tend to equality. It is impossible that the producer of wheat should permanently gain much more than the producer of potatoes. The law of competition would prevent it.