soil are what give rise to rent, and when we have found some way of arithmetically estimating these, we have reached an expression for natural rent. In the same way, but not so fully worked out in his writings, he reaches expressions for natural price, for natural measure of exchange, natural rate of wages (338). In the case of interest his use of the word nature is instructive because the advocates of prohibiting usury from Aristotle down also appealed to nature. Money was by nature unproductive, and no charge could be made for its use. Petty takes a directly opposite position. When he says interest is a reward or recompense for forbearing the use of your money, he assumes the contradictory of the mediæval view. There is always a productive use for money. It can be invested in land, and the rent of land gives the standard for the natural rate of interest.
It is clear that Petty's studies in natural science had a great influence on his conception of nature. His method is indeed from Hobbes, but in his conception of the validity of natural laws he diverges from that thinker. Petty shows decided traces of that reverence for natural conditions which characterizes Locke and the philosophic school of the next century. If one has an optimistic view of nature it is probable that one will use arguments against the existing state of things, if they seem to violate the natural order. Petty's writings show evidences of this tendency. He tells us it is against nature to prohibit the export of money (Quantulumcunque). In another place (35) he remarks that "making civil and positive laws against the laws of nature" is vain and fruitless.
