And here we recur once more to gratuitous and onerous utility, the former being substituted for the latter by virtue of Exchange. Is it not very clear, that if men were deprived of the power of Exchange, and were obliged to produce ice under the equator, and sugar at the poles, they must spend much pains in doing what heat and cold do gratuitously, and that for them an immense proportion of the Forces of nature would remain inoperative? Thanks to Exchange, these forces are rendered useful to us wherever we encounter them. Corn land is sown with wheat—in wine-growing countries the land is planted with vines—there are fishermen on the coasts, and wood-cutters among the mountains. In one place a wheel which does the work of ten men is set in motion by water—in another, by wind. Nature becomes a slave, whom we have neither to feed, nor to clothe, nor to pay—who costs nothing either to our purse or our conscience.[26] The same amount of human efforts, that is to say, the same services, the same value, realizes a constantly increasing amount of utility. For each given result a certain portion only of human exertion is absorbed; the remainder, by means of the intervention of natural Forces, is rendered disposable, and it sets to work to overcome new obstacles, to minister to new desires, to realize new utilities. The effects of Exchange upon our intellectual Faculties are so great, that we can scarcely even imagine their extent. “Knowledge,” says M. de Tracy, “is the most precious of all our acquisition, since it directs and governs the employment of our forces, and renders them more prolific, in proportion as it is sounder and more extensive. No man can himself observe [p107] everything, and it is much easier to learn than to invent. But when several men communicate with each other, what is observed by one is soon known to the rest; and if there be among them but one person of superior ingenuity, precious discoveries speedily become the property of all. In such circumstances, knowledge is much more rapidly increased than it could be in a state of isolation, without taking into account the power of preserving it, and consequently of accumulating it from one generation to another.”