Quṭ,b al-Dī,n Shī,rā,zī,in his Ikhtiyā,rā,t Muẓ,affarī,in Hayʾ,a (theoretical astronomy), believed that according to Ptolemy in the introduction of Almagest, any entity that is not necessary for astronomy should be omitted. This principle seems close to the definition of the notion of simplicity and parsimony in philosophy of science. Although several times the “, simplicity”,is brought up and applied, the phrase “, omitting what is not necessary”,is not mentioned explicitly in the Almagest. It seems that Isḥ,ā,q ibn Ḥ, unayn or Thā,bit ibn Qurra added this phrase to the Arabic translation according to their own understanding of the content of the book. 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’,nonPtolemaic 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.