 The issues that arise in this simple theory help to frame the debates between later physicists and philosophers concerning the nature of motion; in this article, we will focus on the theories of Descartes, Leibniz, and Newton. In the companion entry on  absolute and relational space and motion: post-Newtonian theories,  we study the approaches followed by Mach, Einstein, and certain contemporary researchers. We will see that similar concerns pervade all these works: is there any kind of privileged sense of motion: a sense in which things can be said to move or not, not just relative to this or that reference body, but ‘truly’? If so, can this true motion be analyzed in terms of motions relative to other bodies – to some special body, or to the entire universe perhaps? (And in relativity, in which distances, times and measures of relative motion are frame-dependent, what relations are relevant?) If not, then how is the privileged kind of motion to be understood, as relative to space itself – something physical but non-material – perhaps? Or can some kinds of motion be best understood as not being spatial changes – changes of relative location or of place – at all?