An understanding of the precise nature of the scientific method of Sir William Petty has proved elusive to historians of economic thought, in no small part because of a lack of Petty's own characterization of his scientific approach. This research clarifies the nature of Petty's method, as to whether it was primarily inductive or deductive, and to what extent it relied on empirical foundations. The paper employs a two-pronged analysis. First, it examines the main sources of Petty's method: the works (...) of Sir Francis Bacon and Thomas Hobbes, and the synergistic influences of the Hartlib Circle, the Royal Society, the Dublin Philosophical Society, and the Mersenne group. Second, four of Petty's most noted contributions to political economy are deconstructed in order to identify his scientific method. This research concludes that Petty relied almost exclusively on deduction in his scientific approach and that his analysis does not reveal any inductive reasoning. When data was available, Petty constructed his economic theories on empirical foundations. (shrink)