PETTY, SIR WILLIAM (1623-87), English statistician and political economist, born on May 26, 1623, the son of a clothier at Romsey in Hampshire, received his early education at the grammar school there. He studied for some time at Caen, and on his return to England, seems to have entered the Royal Navy. He went abroad again in 1643, and remained for three years in France and the Netherlands, pursuing his studies. In 1647 Petty obtained a patent for the invention of double writing, i.e., a copying machine. In politics he espoused the side of the parliament. In 1648 he was made deputy professor of anatomy at Oxford, and on obtaining the degree of doctor of physic, 1649, was made a fellow of Brasenose college. In 1654, observing that the admeasurement and division of the lands forfeited in 1641 had been "most inefficiently and absurdly managed," he contracted to execute a fresh survey, which he completed in 13 months. On purchasing a large estate in county Kerry, he set up ironworks in that neighbourhood, opened lead-mines and marble-quarries, established a pilchard fishery, and commenced a trade in timber. Besides the office of commissioner of distribution of the lands he had surveyed, he held that of secretary to the lord-lieutenant, Henry Cromwell, and was also during two years clerk of the council. In January 1658 he was elected to Richard Cromwell's parliament as member for West Looe in Cornwall. After the Restoration he returned to England and was favourably received and knighted by Charles II., who was "much pleased with his ingenious discourses." He obtained from the king a new patent constituting him surveyor-general of Ireland. In 1663 he in vented a double-bottomed ship, which twice made the passage between Dublin and Holyhead, but was afterwards lost. He was one of the first members of the Royal Society, and sat on its council. He died in London on Dec. 16, 1687.
Petty's Irish survey was based on a collection of social data which entitles him to be considered a pioneer in the science of comparative statistics. He was also one of the first to break away from mercantilist ideas, and he exhibits a statesmanlike sense of the elements in which the strength of a nation really consists. Roscher names him as having, along with Locke and
Dudley North, raised the English school to the highest point it attained before the time of Hume. His Treatise of Taxes and Contributions contains a clear statement of the doctrine that price depends on the labour necessary for production. Petty is much concerned to discover a fixed unit of value, and he thinks he has found it in the necessary sustenance of a man for a day. He understands the cheapening effect of the division of labour. He states correctly the notion of "natural and true" rent as the remainder of the produce of land after payment of the cost of production ; but he seems to have no idea of the "law of diminish ing returns." He has much that is just on the subject of money : he sees that there may be an excess of it as well as a deficiency, and regards the prohibition of its exportation as contrary to sound policy. But he errs in attributing the fall of the rate of interest which takes place in the progress of industry to the increase in the quantity of money. He protested against the fetters imposed on the trade of Ireland, and advocated a union of that country with Great Britain.
A complete list of his works is given in the Athenae oxonienses. The most important are: the Treatise of Taxes and Contributions (1662, 1667 and 1685) ; Political Arithmetic, presented in ms. to Charles II., but, because it contained matter likely to be offensive to France, kept unpublished till 1691, when it was edited by Petty's son Charles; Quantulumcunque, or a Tract concerning Money (1682) ; Observations upon the Dublin Bills of Mortality in 1681 and the State of that City (5683) ; Essay concerning the Multiplication of Mankind (1686); Political Anatomy of Ireland (1691). Several papers appeared in the Philosophical Transactions. See Economic Writings of Sir William Petty, ed. C. H. Hull (2 vols., 1899) ; W. Roscher, Study on Petty in Transactions of the Royal Scientific Society of Saxony (Leipzig).