Petty, Sir William (1623–87), physician, scientist, and administrator, was born 26 May 1623 in Hampshire, England, third child of Anthony Petty, engaged in the textile trade, and his wife, Francesca. Petty may subsequently have exaggerated the obscurity of his origins in order to dramatise his rise. His father ensured that the boy received a conventional schooling. Thus equipped, he became a mariner. After an accident, he was abandoned in Normandy in 1637. There he repaired to the Jesuit college in Caen, which perfected his mathematical skills and whetted his voracious intellectual appetite. These studies were resumed in London. By 1643, with England engulfed in civil war, he again repaired to the Continent. His tour, intended to acquire medical qualifications, encompassed Amsterdam, Leyden, Utrecht, and Paris. He met Hobbes (a fellow exile in Paris), Gassendi, Mersenne, and other virtuosi. Back in England by 1646, he continued his medical studies at Oxford, graduating as doctor of physick in 1649. As the university was purged of royalists, Petty, pliant in both his political and religious principles, was intruded into a fellowship at Brasenose College and, in 1651, elected professor of anatomy. In this capacity he gained wider celebrity when he resuscitated a corpse cut down from the gallows and destined for dissection, a feat that he publicised in print. The continental travels had familiarised him with the experimental and deductive methods favoured by savants, and with some of the outstanding practitioners. Now he gravitated to the circle interested in experiment, speculation, and material improvement, centred on Samuel Hartlib. To Hartlib, Petty in 1647 addressed his first published project, on the advancement of learning.
Petty's medical expertise brought him to Ireland in September 1652 as physician to the army there. The post gave him a comfortable competency – a daily allowance of £1 – which he supplemented through private practice. It also enabled him to display his organisational talents. Soon he was in demand for multifarious administrative and supervisory tasks as slowly peace returned to the island. Never reticent about his own prowess, in December 1654 he was entrusted by the authorities with planning and managing the redistribution of forfeited lands in what became known as the ‘Down survey’. Having supplanted the incumbent surveyor-general, Benjamin Worsley (qv), in the undertaking, Petty inevitably attracted controversy. His combative style added to the tension. Aggrieved soldiers and frustrated civilians blamed Petty for the delays and disappointments in what they were granted, and contrasted his large profits with their own meagre returns. In addition, since 1655 he had identified himself closely with the new governor of Ireland, Henry Cromwell (qv), who was distancing himself from radicals and sectaries. Cromwell appointed Petty an additional clerk of the Irish council and employed him as his own secretary. In 1659 Petty secured election to the Westminster parliament, being returned for the Cork boroughs of Kinsale and Bandon and for the Cornish seat of West Looe. He chose to represent the latter. His presence in the parliament was required not just as an Irish expert, but to defend himself from an anticipated attack on his conduct of the Down survey and on Henry Cromwell's administration.
As confusion enveloped Britain and Ireland, Petty lingered in England. He joined the group of republicans and utopians, ‘The Rota’, which hoped to forward its ideals. The restoration of Charles II posed Petty problems similar to those faced by numerous other collaborators with and beneficiaries of the Cromwellian order. In particular, as recipient of more than 18,000 acres scattered across five Irish counties, he watched nervously as a fresh land settlement was devised. In 1661 he was elected to the Irish house of commons; returned for both Enniscorthy and the Kilkenny town of Inistioge, he preferred to sit for the second. With his experience and penetration, it was natural that he should be consulted by the governments in Dublin and London. But, to his chagrin, although included in the novice Irish council of trade, he no longer enjoyed the power that Henry Cromwell had accorded him.
One outlet for his thwarted energies was the Royal Society of London, of which he was a founding fellow. His prominence in this world of useful and applied knowledge led to his being knighted in 1662. Another vent was his Irish property: particularly on his extensive holdings in Kerry, he aimed to demonstrate how his rational system would quickly improve that underdeveloped region, and enrich himself. To these ends, he poured money into iron-making and fisheries, but with disappointing results. In his ventures, he applied the doctrine that all should and could be reduced to ‘number, weight, and measure’. This rigorously statistical method was expounded in his writings. Notable were his contributions to an analytical demography first of London, and then of Dublin and Ireland. His extended treatment of Ireland – A treatise of taxes and contributions (1662) and The political anatomy (published posthumously in 1691) – mingled orthodox description with quantitative and qualitative evaluation. He advocated nostrums, some prophetic, others wildly impractical. He proposed, for example, that Ireland be legally united with England, and that peoples be transferred between the two kingdoms. Hopes that he might be heeded rose when James II's (qv) accession presaged a reconsideration of the assumptions behind England's Irish policies. Furthermore, Petty's religious scepticism made him happy enough to urge toleration for the Irish catholics: a measure that a catholic monarch could be expected to applaud. Nevertheless, when Petty died on 16 December 1687, there were few signs that he had recovered official influence.
Probably the richest commoner in Ireland, with an annual rental of between £5,000 and £6,000, he resolutely refused a peerage. His widow, Elizabeth, a daughter of Sir Hardress Waller (qv), whom Petty married in 1667 (earlier she had been married to Sir Maurice Fenton), was ennobled as Baroness Shelburne, in the Irish peerage. At the same time (1688), his eldest son and heir, Charles, received a barony. From 1659 Petty divided his time between London and Dublin. Yet, despite some London properties, Ireland supplied the bulk of his wealth. Latterly, in 1684, he was able to realise one of his long-held ambitions through the newly founded Dublin Philosophical Society. Petty was elected as its first president. Many of its activities, although guided by the young William Molyneux (qv), revived or continued what Petty and his confrères from the Hartlib circle had wanted in the 1650s. More lasting was the Down survey, which he had conceived and supervised (and defended). Modified but not overthrown by subsequent settlements, it established the essential contours of land ownership for the late seventeenth century and beyond. An additional product of these labours were country maps of Ireland, which were published in 1684 as Hibernia delineatio. Above all, Petty pioneered the statistical treatment of Ireland. The basis of many figures, calculations, and extrapolations in his ‘political arithmetic’ remains questionable. Nor did he avoid the prejudices common among more obviously subjective analysts. His information and insights may have had less impact on policy than he wished. Yet his writings continue to illumine the topography and societies of seventeenth-century Ireland.
His published works are listed in G. Keynes, A bibliography of Sir William Petty (1971). A portrait by John Closterman is at Bowood House, Wiltshire; another, reputedly of the young Petty, is at Brasenose College, Oxford. The largest collection of his papers is now in the BL, Add. MSS 72850–72908. An essential guide is provided by Frances Harris, ‘Ireland as laboratory: the archives of Sir William Petty’ in M. Hunter (ed.), Archives of the scientific revolution: the formation and exchange of ideas in seventeenth-century Europe (1998), which supplements and to some extent replaces M. D. Slatter, Report on the literary, personal and official correspondence and papers of Sir William Petty (1980). More correspondence with his agents in Ireland is in McGill University Library, Osler MS 7612; an account book of the 1650s is TCD MS 2947. Correspondence and other materials are also to be found among Hartlib's papers in Sheffield University Library, and in the Royal Society of London. The papers of the Dublin Philosophical Society were calendared by K. T. Hoppen, Anal. Hib., xxx (1982), issued in microfiche, and latterly published in hardcopy by the Irish Manuscripts Commission (2008).