"When Newton wrote his Mathematical Principles of Natural Philosophy and System of the World, he distinguished the phenomena to be saved from the reality he postulated. He distinguished the "absolute magnitudes" that appear in his axioms from their "sensible measures" which are determined experimentally. He discussed carefully the ways in which, "the true motions of particular bodies [may be determined] from the apparent," via the assertion that "the apparent motions... are the differences of true motions.""