"The comportment of terrestrial bodies with respect to the earth is reducible to the comportment of the earth with respect to the remote heavenly bodies. If we were to assert that we knew more of moving objects than this their... experimentally-given comportment with respect to the celestial bodies, we should render ourselves culpable of a falsity. When... we say, that a body preserves unchanged its direction and velocity in space, our assertion is... an abbreviated reference to the entire universe. ...[S]uch ...is permitted the original author Newton, Descartes or Galileo] of the principle, because he knows... no difficulties stand in the way of carrying out its implied directions. But no remedy lies in his power, if difficulties of the kind mentioned present themselves; if, for example, the requisite, relatively fixed bodies are wanting."