Hobbes was an elder contemporary of Descartes and a member of Mersenne's circle after he fled to Paris (1640) in anticipation of the English Civil War. Hobbes composed the Third Objections to Descartes’ Meditations and made a close study of Descartes’ Dioptrics after having been sent a copy of the Discourse on Method in 1637 by Sir Kenelm Digby. Criticisms of this work were taken up by Hobbes in a Latin optical treatise in the late 1630s. They were also summarized in a lost letter of 1640 from Hobbes to Mersenne. Echoes of these criticisms are noticeable in letters between Descartes and Mersenne in 1641. Although Hobbes is sometimes claimed to be, like Descartes, a philosopher interested in the refutation of skepticism (Popkin 1979; Tuck 1988, 1989), there is very little textual or other evidence for this interpretation (Sorell 1993), and the main common ground between them is a strong anti-Aristotelianism. Hobbes's earliest full-length philosophical treatise, The Elements of Law (1640), contains a strong attack on the Aristotelian theory of sense perception, and the political philosophy for which Hobbes is best known today is a wholesale rejection of Aristotle – from the definition of man as a political animal, to the idea that citizenship is exercised through deliberation and judgment, to the idea that what Aristotle calls “tyranny” is a perversion of kingship.

The most sustained engagement between the two philosophers comes in Hobbes's mostly uncomprehending Third Objections to the Meditations and Descartes’ mostly exasperated Replies. Hobbes's two main targets – which probably reflect his preoccupations when he criticized the Dioptrics – are Descartes’ immaterialism and (as Hobbes thinks) traditionalism. Hobbes approves of the cogito but disagrees strongly with its corollary: that the “I” designates a thinking thing (AT VII 172–73, CSM II 122). Hobbes first accuses Descartes of being confused in speaking interchangeably about a thinking thing and an “intellect” or “reason.” There are no such things orsubstances as intellects, Hobbes thinks, only acts or powers of intellection on the part of subjects (AT VII 172–73, CSM II 122), and, for all Descartes shows, subjects are corporeal. Indeed, Hobbes adds, the lesson of the wax argument in the Second Meditation is that whatever undergoes change is a body (AT VII 173, CSM II 123), so that Descartes is inconsistent when he claims that the “I” of the cogito is incorporeal.