The Cambridge Platonist, More, was one of the most influential figures in the early reception of Cartesianism in England, as both admirer and critic of Descartes. He was one of the first to advocate the teaching of Descartes’ philosophy in English universities and is credited with coining the term “Cartesianism.” More's interest in Cartesian philosophy is first registered in his long poem, Democritus platonissans (1646). In 1648 he entered into correspondence with Descartes (1648–49), possibly at the instigation of Samuel Hartlib. He was attracted by Descartes’ natural philosophy, which he regarded as the best account of the phenomena of nature in the post-Aristotelian context of the mid-seventeenth century. Much as he admired Descartes’ natural philosophy, he did not accept Descartes’ physics in every detail but sought to enlarge the metaphysical dimension of Cartesianism.

In his letters, he argues that that all substance, both corporeal and incorporeal, is extended and that God Himself is res extensa. Since both corporeal and incorporeal substance are extended, the operative distinction between them is solidity (or what he called impenetrability), bodies being extended and impenetrable, while incorporeal substance (soul or spirit) is extended and penetrable. He also took issue with Descartes over his denial of the possibility of a vacuum, rejecting final causes, and denial that animals have souls. He raised the problem of transmission of motion from one body to another, if motion like shape is merely a mode of body. He points up areas where Descartes is not fully self-consistent, or has dissembled his position, notably in positing the indefinite extent of the universe but not its infinity. He also criticized his account of refraction, and his vortical theory of celestial motions (see vortex). Descartes responded to More's letters in detail, though he evidently regarded More's arguments as too anthropomorphic for his liking.