Then motion is simply the transfer of one body from the vicinity of other bodies which are in immediate contact with it, and which are regarded as being at rest, to the vicinity of other bodies.
Like his explanation of space, Descartes' explanation of motion is a revision of the common conception. Motion is not an action separate from bodies themselves, but, rather, it is a property of bodies. Motion is a function of the position of a body relative to other bodies. It is, therefore, derivable from the principles of geometry, as Descartes thinks all physics must be.