 The identification of space and matter poses a puzzle about motion: if the space that a body occupies literally is the matter of the body, then when the body – i.e., the matter – moves, so does the space that it occupies. Thus it doesn’t change place, which is to say that it doesn’t move after all! Descartes resolved this difficulty by taking all motion to be the motion of bodies relative to one another, not a literal change of space.