William Petty (1623–87)—medical doctor, Oxford professor of anatomy, Gresham College professor, surveyor of Ireland for the Cromwellian Protectorate, Fellow of the College of Physicians, founding member of The Royal Society, and inventor of political arithmetic—is now most particularly remembered for his contributions to the formation of political economy. When Petty left Ireland for the last time in 1685 and returned to London, he took with him a very large collection of unpublished papers and correspondence. These largely remained in the property of his descendants—together with some important later additions—until 1993, when they were sold to The British Library. Petty's collected, published economic writings were edited and republished a century ago, together with some other materials (Hull 1899). In the 1920s Petty's descendant, the sixth Marquis of Lansdowne (H.E.W. Petty-Fitzmaurice), published two collections of manuscripts from what by then had become known as “The Petty Papers” archive (Lansdowne 1927, 1928). All these volumes have subsequently been republished in facsimile a number of times.