Bayesian Ptolemaic Psychology

Ptolemy’s device, the epicycle on deferent, allowed him to account very accurately for motions with respect to the fixed stars of the sun, the moon, and the five observable planets. Indeed, thanks to Harold Bohr, we now know, as Ptolemy did not, that any periodic motions can be approximated arbitrarily well by iterations of epicycles. So far as the data concern only semi-periodic motions with respect to the fixed stars, Ptolemy’s framework can fit anything. Copernicus’ framework is not so generous; it requires strict relations among the observable motions of the sun and the planets, and Kepler saw in those relations the very explanatory virtues of Copernican theory, and the comparative explanatory defects of Ptolemaic theory. In Kepler’s hand, the Copernican identification of observable features of solar/planetary motion—angles of separation, oppositions,