Now what gives the increase in these cases is something which, though it generally requires labor to utilize it, is yet distinct and separable from labor—the active power of nature; the principle of growth, of reproduction, which everywhere characterizes all the forms of that mysterious thing or condition which we call life. And it seems to me that it is this which is the cause of interest, or the increase of capital over and above that due to labor. There are, so to speak, in the movements which make up the everlasting flux of nature, certain vital currents, which will, if we use them, aid us, with a force independent of our own efforts, in turning matter into the forms we desire—that is to say, into wealth.
While many things might be mentioned which, like money, or planes, or planks, or engines, or clothing, have no innate power of increase, yet other things are included in the terms wealth and capital which, like wine, will of themselves increase in quality up to a certain point; or, like bees or cattle, will of themselves increase in quantity; and certain other things, such as seeds, which, though the conditions which enable them to increase may not be maintained without labor, yet will, when these conditions are maintained, yield an increase, or give a return over and above that which is to be attributed to labor.
Now the interchangeability of wealth necessarily involves an average between all the species of wealth of any special advantage which accrues from the possession of any particular species, for no one would keep capital in one form when it could be changed into a more advantageous form. No one, for instance, would grind wheat into flour and keep it on hand for the convenience of those who desire from time to time to exchange wheat or its equivalent for flour, unless he could by such exchange secure an increase equal to that which, all things considered, he could secure by planting his wheat. No
