hobbes studies 29 (2016) 9-38 © koninklijke brill nv, leiden, 2016 | doi 10.1163/18750257-02901002 brill.com/hobs Hobbes, Galileo, and the Physics of Simple Circular Motions John Henry Professor Emeritus of History of Science, University of Edinburgh, Science Studies Unit, Chisholm House, High School Yards, Edinburgh, United Kingdom, eh1 1lz john.henry@ed.ac.uk Abstract Hobbes tried to develop a strict version of the mechanical philosophy, in which all physical phenomena were explained only in terms of bodies in motion, and the only forces allowed were forces of collision or impact. This ambition puts Hobbes into a select group of original thinkers, alongside Galileo, Isaac Beeckman, and Des- cartes. No other early modern thinkers developed a strict version of the mechani- cal philosophy (not even Newton who allowed forces of attraction and repulsion operating at a distance). Natural philosophies relying solely on bodies in motion require a concept of inertial motion. Beeckman and Descartes assumed rectilinear motions were rectilinear, but Galileo adopted a theory which has been referred to as circular inertia. Hobbes’s natural philosophy depended to a large extent on what he called “simple circular motions.” In this paper, I argue that Hobbes’s simple cir- cular motions derived from Galileo’s belief in circular inertia. The paper opens with a section outlining Galileo’s concept, the following section shows how Hobbes’s physics depended upon circular motions, which are held to continue indefinitely. A third section shows the difficulty Hobbes had in maintaining a strictly mecha- nistic philosophy, and the conclusion offers some speculations as to why Galileo’s circular inertia was never entertained as a serious rival to rectilinear inertia, except by Hobbes. Keywords Thomas Hobbes – Galileo Galilei – inertia – kinematics – mechanical philosophy