Aristotle, Customs, Chapter 7, Book Five
Ex convento successit nummus, atque ob hanc causam vocabitur (a Graecis) nempe a lege, quia non natura, sed lege valeat, sitque in nostra potestate eum immutare, inutilemque reddere.
Coin succeeded to the agreement, and for this reason it will be called (by the Greeks) namely, by law, because it is valid not by nature, but by law, and it is in our power to change it and render it useless.
Also — Chapter Six, Book One of Politics
Bishop Covarruvias:
Si non natura ipsa, sed a Principe valorem numisamata accipiunt, & ab ipso legem revocante inutilia effici possunt, profecto non tanti estimatur materia ipsa auri vel argenti, quantum numus ipse; cum si tanti estimaretur natura ipsa non lege praetium haberet.
English:
If, not by nature itself, but by the Prince, the coins receive their value, and by him revoking the law they can be rendered useless, surely the material itself of gold or silver is not so esteemed as the coin itself; since if nature itself were valued so much, it would not have a value by law.