Reconciling the observed motion of heavenly bodies with the Ptolemaic world-system requires one of two hypotheses: epicycles (فلك تدوير) or excentric spheres (فلك خارج المركز). In a remarkable pre-saging of the Copernican revolution, Maimonides follows this statement of orthodoxy with the declaration,

I will show that each of these two hypotheses is irregular, and totlaly contrary to the results of Natural Science.

In this chapter, Maimonides explains in detail why both hypotheses lead to contradictions with the Physics of Aristotle; epicycles, for example, would entail the circular motion of something around an immaterial point, which seems — under Aristotelian physics — implausible. The theory of excentric spheres would necessitate the existence of some sort of material between two successive spheres, which again seems difficult under Aristotelian physics. Maimonides summarizes these difficulties:

Consider, therefore, how many difficulties arise if we accept the theory which Aristotle expounds in Physics. For, according to that theory, there are no epicycles, and no excentric spheres, but all spheres rotate round the centre of the earth! How then can the different courses of the stars be explained? how is it possible to assume a uniform perfect rotation with the phenomena which we perceive, except by admitting one of the two hypotheses or both of them?

Quoting Ibn Bajja, Maimonides points out that accurate astronomical observations were not available to Aristotle; consequently, he did not realize how imperfect his astronomical theories were. According to Maimonides, Aristotle

did not notice [the excentricity of spheres] or hear of it; the science was not perfect in his age. If he had heard of it, he would have strongly opposed it; if he had been convinced of its correctness, he would have been greatly embarrassed as regards all that he said on the question. … the theory of Aristotle, in explaining the phenomena in the sublunary world, is in accordance with logical inference: here we know the causal relation between one phenomenon and another; we see how far science can investigate them, and the management of nature is clear and intelligible. But of the things in the heavens man knows nothing except a few mathematical calculations, and you see how far these go.

This is followed by an eloquent expression of man’s (till then) inability to grasp the order in the heavens. Quoting Psalms 115, he says

the heavens belong to the Lord; but the earth he gave over to man. سماء السموات للرب و الأرض جعلها لبني بشر

and this is because

the facts which we require in proving the existence of heavenly beings are withheld from us: the heavens are too far from us, and too exalted in place and rank … it is in fact ignorance or a kind of madness to weary our minds with finding out things which are beyond our reach.

And because the details of how the heavens work “cannot be approached by logical inference (بقياس)”, Maimonides prefers to defer to the authority of Moses, who spoke directly with God. Finally, Maimonides concludes by conceding that the solution to which he has arrived is not conclusive, but ‘embarassingly’ tentative:

another person may perhaps be able to establish by proof what appears doubtful to me. It is on account of my great love of truth that I have shown my embarrassment in these matters and I have not heard, nor do I know that any of these theories have been established by proof.

Commentary

It is clear from the text that Maimonides does not use Mosaic authority to settle scientific questions. He avoids doing this, and instead looks for answers in science. When he does not find answers, and finds a cosmology that is utterly inconsistent with the principles of physics, he believes that science cannot, therefore, conclusively answer the question of the origin of the universe. If Aristotle’s theory of the spheres is inconsistent with Ptolemy’s epicycles, and if Ptolemy’s epicycles are themselves quite necessary to explain astronomical observations, how can we accept the Aristotelian view of the universe as scientifically correct? Finding no scientifically consistent explanations, Maimonides throws his hands in the air and looks to Scripture to settle the question.