To cause this, it is only necessary that less [time and] human labor should be required to furnish it.
When the water-boat comes to supply my ship, were I to pay in proportion to the _absolute utility_ of the water, my whole fortune would not be sufficient.

But I pay only for the trouble taken.

If more is required, I can get another boat to furnish it, or finally go and get it myself.

The water itself is not the subject of the bargain, but the labor required to obtain the water.

This point of view is so important, and the consequences that I am going to draw from it so clear, as regards the freedom of international exchanges, that I will still elucidate my idea by a few more examples.
The alimentary substance contained in potatoes does not cost us very dear, because a great deal of it is attainable with little work.