On the other hand, as the prosperity of a nation grows it necessarily attracts the precious metals because a multiplication of exchanges leads to a growing demand for money. These exportations and importations will depend, as Hume[196] had already shown, upon the relative cheapness or dearness of money. What is true of metallic money is also true of a special kind of money known as bank-notes. Smith has given us a vivid description of the functions of banks, and especially of the fortunes of the most famous bank of this period, the Bank of Amsterdam. This afforded him another opportunity of demonstrating how the quantity of notes offered spontaneously adapts itself to the quantity demanded. If banks issue more notes than the circulation warrants prices will rise. Buying from foreign countries will be resorted to and the notes will be returned to the banks to be exchanged for gold and silver—the only international money. The banks clearly have no interest in issuing too many notes, because it involves a greater metallic reserve as the result of the more frequent demands for payment which they will have to face. Of course, “every particular banking company has not always understood or attended to its own particular interest, and the circulation has frequently been over-stocked with paper money.”[197] But this does not affect the main principle, and we have one further proof of the spontaneous activity of the economic mechanism. We have now reviewed some of Smith’s principal themes, and we have seen how every phenomenon impresses him in the same fashion. Had space permitted we might have cited other examples all pointing to the same conclusion.[198] This conception of spontaneity and wise beneficence is by no means the product of mere _a priori_ thinking. It was no abstract theory that needed the backing of a rigid demonstration. It was a belief gradually borne in upon him in the course of his review of the economic field. This is characteristic of all his thought, and with every new vista we are reminded of it. The conclusion is hinted at again and again, and the impression left upon the reader’s mind is that no other conclusion could ever be possible. Smith thought of the economic order as an organism—the creation of a thousand human wills unconscious of the end whither they are tending, but all of them obedient to the impulse of one instinctive, powerful force. This force, the root of all economic activity, its constancy and uniformity triumphant over every artificial obstacle and giving unity to the whole system, what is it? We have already encountered it on more than one occasion. It is personal interest, or, as Smith prefers to call it, “the natural effort of every individual to better his own condition.”[199] Hidden deep in the heart of every individual lies this essential spring of human life and social progress.