 Kilvington’s discussion of the measure of motion with respect to causes, or what we would call his ‘dynamic’ analysis, has both a physical aspect involving relations between acting and resistive potencies, and a mathematical aspect involving concepts of continuity and limits. He is interested in how a capacity is to be bounded if it is active or passive potency. Is it subject to weakening or strengthening? Is it mutable or immutable? How do we determine the boundaries of an active potency if a body moves in a medium that is not uniformly resistant?