Only so much of the usefulness of an article as is the result of human labor becomes the object of mutual exchange, and consequently of remuneration.

The remuneration varies much, no doubt, in proportion to the intensity of the labor, of the skill which it requires, of its being _a propos_ to the demand of the day, of the need which exists for it, of the momentary absence of competition, etc.

But it is not the less true in principle, that the assistance received from natural laws, which belongs to all, counts for nothing in the price.
We do not pay for the air we breathe, although so useful to us, that we could not live two minutes without it.

We do not pay for it, because Nature furnishes it without the intervention of man's labor.

But if we wish to separate one of the gases which compose it, for instance, to fill a balloon, we must take some trouble and labor; or if another takes it for us, we must give him an equivalent in something which will have cost us the trouble of production.