Abstract
This paper explores the Cartesian physics of circular motion, in particular, the long-standing puzzle concerning the possible role of a circular inertial concept in Descartes' theories. Although some commentators have claimed that Descartes' famous "rotating sling" examples favor a rotational component of "striving" towards motion, and that this aspect of his project constitutes a form of inertial thinking, it will be argued that a much stronger case for a Cartesian brand of rotational inertial motion can be constructed from a little-known correspondence, the letter to Ciermans, dated 1638.