 Leibniz, later, articulated a more general “equipollence of hypotheses”: in any system of interacting bodies, any hypothesis that any particular body is at rest is equivalent to any other. Therefore neither Copernicus’ nor Ptolemy’s view can be true—though one may be judged simpler than the other—because both are merely possible hypothetical interpretations of the same relative motions. This principle clearly defines (what we would call) a set of reference frames. They differ in their arbitrary choices of a resting point or origin, but agree on the relative positions of bodies at any moment and their changing relative distances through time.