 Descartes’ use of the terms “tendency” and “striving” in his rotating sling example should not be equated with his previous concept of a determination of motion. A determination is confined to a body’s actual motion, whereas a body’s tendency towards motion only occurs at a single instant. He states: “Of course, no movement is accomplished in an instant; yet it is obvious that every moving body, at any given moment in the course of its movement, is inclined to continue that movement in some direction in a straight line,…” (Pr II 39). In another passage in the Principles, Descartes identifies these strivings as a “first preparation for motion” (Pr III 63). Hence, while determinations necessitate a span of several instants, tendencies towards motion are manifest only at single instants. This is a crucial distinction, for it partitions Cartesian dynamics into two ontological camps: forces that exist at moments of time, and motions that can only subsist over the course of several temporal moments. In many parts of the Principles, moreover, Descartes suggests that quantity of motion is the measure of these bodily tendencies, and thus quantity of motion has a dual role as the measure of non-instantaneous bodily motion as well as the instantaneous bodily tendencies (see Pr III 121).