Sir William Petty FRS (26 May 1623 – 16 December 1687) was an English economist, physician, scientist and philosopher. He first became prominent serving Oliver Cromwell and the Commonwealth in Ireland. He developed efficient methods to survey the land that was to be confiscated and given to Cromwell's soldiers. He also remained a significant figure under King Charles II and King James II, as did many others who had served Cromwell.
Petty was briefly a Member of the Parliament of England and was also a scientist, inventor, and merchant, and was a charter member of the Royal Society. It is for his theories on economics and his methods of political arithmetic that he is best remembered, however, and to him is attributed the philosophy of "laissez-faire" in relation to government activity. He was knighted in 1661. He was the great-grandfather of The 1st Marquess of Lansdowne (better known to history as The 2nd Earl of Shelburne), who served as Prime Minister of the Kingdom of Great Britain, 1782–1783.