ABSTRACT
It is said that Plato set the following problem for his students: what uniform and ordered motions must be assumed for each of the planets to account for its apparently irregular motions across the night sky? This has been known as Plato's problem.1 It was a significant event in the intellectual history of human civilization, not so much because this challenging problem had engaged some of the best minds of Europe for almost two millennia, but because it was probably the first-ever significant suggestion that observed phenomena should be regarded as mere appearances, to be explained in terms of postulated reality. In response, Johannes Kepler (1571-1630), following the lead of Ptolemy, Copernicus, et al., proposed that the planets' real motions were in the form of ellipses around the sun. It was the combination of these elliptic motions of the planets and the earth's own motion that gave rise to the observed retrograde loops. What is the logic of this type of explanation? .
