above the natural spring.[1] Again, in the 'Political Arithmetick' he seems to miss the full application of his own doctrines as to the origin of value, and maintains the advantage of foreign trade because it produces 'not only wealth at large, but more particularly abundance of silver, gold and jewels, which are not perishable articles, but are wealth at all times and all places, whereas abundance of wine, corn, fowls, flesh, etc., are riches but hic et nunc; so as the raising of such commodities, and the following of such trade, which does store the country with gold, silver, jewels, etc., is profitable before others;' and his analysis of the influence of supply and demand on value, to be noticed further on, is partly vitiated by the recognition of an inherent value in some articles as such, which he thinks must be wealth at all times and places.[2]
To acknowledge these shortcomings is only to acknowledge that Sir William Petty, though far in advance of his time, had not shaken himself entirely free from the influences of the errors which the mercantile system had accepted from the purely prohibitory system, viz., that wealth consists of the precious metals, and that a system of revenue and trade is to be deemed good or bad, according as it can be shown to promote the influx of those metals into a country or not. There is always a temptation to believe, when certain general conceptions seem present to the mind of an author, that the logical basis of those conceptions must have been present also; but this is an error which the student of economic history has to avoid.
Progress in economic science in the seventeenth century was gradual and tentative, and Petty's grasp of logical method does not require to be exaggerated in order to make him take a high place in the ranks of the founders of the science. It was no mean achievement for any writer in the seventeenth century to have discerned the great theoretic truth on which free trade depends; to have clearly realised that the highest
