Born at Gayhurst, Buckinghamshire, this Roman Catholic courtier with a reputation as an alchemist was a proponent of the view that physical phenomena can (and should) be explained only in terms of mechanistic qualities like size, shape, and motion. This is, of course, a Cartesian (and Hobbesian) view, but Digby (1644, 344, 346) claimed that it was also Aristotle's view and that the more traditional understanding of Aristotle (as offering scientific explanations that appeal to substantial forms) was a result of the distortions of the Scholastics. Digby believed that Roman Catholicism was the only guarantor of truth, but since he was persuaded that the mechanical philosophy of Descartes or Thomas Hobbes was correct, he decided that Aristotle's authority could be maintained only by interpreting it as an essentially mechanistic philosophy (Henry 2009, 43–75). Digby was active in intellectual circles, publishing the mammoth Two Treatises in 1644, among other tracts. He was an early member of the Royal Society, and over the course of his career he engaged with Hobbes, Robert Boyle, Marin Mersenne, Pierre Gassendi, Isaac Beeckman, Walter Charleton, and Descartes, among other important figures (Dobbs 1971, 25; Westfall 1958, 16).

The Two Treatises, Digby's landmark work, attempts to illustrate the power of mechanistic science but also aims to buttress the view that souls are immaterial. Digby's thought here is this: if all physical phenomena can be explained in terms of matter in motion, but certain aspects of rational souls cannot be so explained, then rational souls are not “physical phenomena.” If rational souls cannot be explained by matter in motion, then they are immaterial. Furthermore, if destruction or (in Aristotle's phrase) “passing away” is explained in terms of a dissolution and scattering of the particles of matter that constitute a thing, then souls must be immortal because, being immaterial, they are not constituted of material atoms (1644, preface).

Digby and Descartes (AT VIIIA 25, CSM I 210–11) agree that the existence of mental qualities cannot be explained in terms of qualities like size, shape, and motion and are thus the qualities of an entirely different substance.