The Principle of Simplicity in the Hayʾa Books : A Revision to the Motivations behind the Development of non-Ptolemaic Planetary Models

Abstract

Abstract: Quṭb al-Dīn Shīrāzī believed that according to Ptolemy in the introduction of Almagest, any entity that is not necessary for this astronomy should be omitted. This principle seems close to the definition of the notion of simplicity and parsimony in the philosophy of science. the phrase “omitting what is not necessary” is not mentioned explicitly in the Almagest. It seems that Isḥāq or Thābit added this phrase to the Arabic translation. The Islamic astronomers mentioned “simplicity” within their discussion of the solar model again and again and believed that a model with fewer orbs is acceptable for the sun because it is “simpler”. Nevertheless, the Maragha astronomers’ non-Ptolemaic models include more orbs than the number of the orbs in the Ptolemaic models. The non-Ptolemaic models were produced to solve the contradiction between Ptolemaic models and the Aristotelian natural principle of uniform motion. Although there were at least two other Aristotelian natural principles for the celestial motions which were in contrast with Ptolemaic models, Maragha astronomers did not pay attention to these contradictions. It is possible that they felt that supposing more orbs with uniform motion is simpler or more elegant than fewer orbs with non-uniform motion.