Of course the apparent loss of the treatise in which Muḥammad b. Mūsā b. Shākir made his argument does not help us determine if he had a solution to the problem or not. The present study of that treatise is based on a fragmentary quotation from the work of an astronomer who lived centuries after Muḥammad b. Mūsā b. Shākir.
For the anonymous Andalusian (c. 1050) author of Kitāb al-istidrāk, whose extant work Kitāb al-hay'a is still preserved in Hyderabad, India,[269] the more global question was to firmly pinpoint the status of the new astronomy in whose writing he was now participating. In a critical passage on how this new astronomy ought to be pursued, he says:
The one who works in this craft must obtain the [mean] motions, that are taken as principles, from the observations, and then consider through geometry how these motions could take place, and which configuration would fit them best. In his search he should not abandon the principles of this craft, which he should accept from natural philosophy. Accordingly, he should not depart from spheres and circular uniform motions and pass on to bodies that are not spherical or not circular. And if he were able to discover many configurations for the same planet, all of them yielding the same observable results of the particular motions, he should then chose that which is simpler and easier, in a manner appropriate to celestial bodies, as was already done by Ptolemy who, in the case of the sun, opted for the eccentric model, that described only one motion, instead of the epicyclic one, which would have necessitated two.[270]
For this anonymous author, then, the astronomical universe within which all the planetary motions had to be understood and fitted, was a strictly Aristotelian universe that had its own premises that the astronomer was not allowed to violate. And while praising Ptolemy, the author used this language as an implicit critique of Ptolemy who did just that. According to all the authors of books on doubts (shukūk), Ptolemy definitely departed from the premises of that Aristotelian universe, and thus deserved to be criticized so severely by them.
Furthermore, the anonymous Andalusian author intended to stress that it was not only the principle of a spherical universe, with spheres moving uniformly, that had to be observed, but that the representation of the particular configurations of that universe also had to be consistent with the nature of that universe. In other words, he wished to advocate the main message of the hay'a writers, which could be summed up in the new requirement of consistency that all astronomical theories had to be subjected to. Simply stated, this consistency requirement demanded that the mathematics, used by the astronomer to describe the phenomena that one observed in the physical universe, must at no point depart from the mathematical characteristics of that universe. In these representations, for example, if one dared to accept the concept of a sphere that moved uniformly, in place, around an axis that did not pass through its center, then one might as well accept the absurdity of representing a sphere with the figure of a mathematical triangle.
In this context, one can define the main feature of the new astronomy of hay'a as an astronomy that was obsessed with this consistency between the premises of the field and all the ensuing constructions the field required.
The last section of the quotation underlines the importance of another aesthetic principle that was already known to the Greek authors, and which had nothing to do with observational astronomy proper, namely, that of the principle of simplicity and ease. Ptolemy himself already articulated that principle, in so many words, when he explained, in book III of the Almagest, why he opted for the eccentric model for the sun rather than the epicyclic one.
Other astronomers and philosophers working in the Islamic domain had other axes to grind with Aristotle himself, and sometimes with Ptolemy as a representative of that philosophy. After all, it was Ptolemy who had already started the debate by his unspoken options for the solar model. Both options violated Aristotelian cosmology. The first posited the existence of eccentrics whose centers by definition did not coincide with the center of heaviness around which everything moved, as was required by Aristotle. The second option assumed the existence of epicycles, out in the celestial realm, which had their own centers of motion, again contrary to what Aristotle recommended.
In the case of the sun, Ptolemy satisfied himself with the eccentric model and said nothing of the other option, except that it was an option. But in the case of the other planets, Ptolemy had no such simple options. He had to accept both models: the eccentrics as well as the epicycles. In this, every other astronomer working in the Islamic domain, with the exception of Ibn al-Shāṭir who rejected the eccentrics all together, followed him.
Under the circumstances, it becomes understandable why would someone like Averroes, who lived some two centuries before Ibn al-Shāṭir, object so vehemently to the astronomy of his days, when he said, "to propose an eccentric sphere or an epicyclic sphere is an extra-natural matter (amrun khārijun 'an al-ṭab')."[271] He then went on to say:
The epicycle sphere is in principle impossible (gharu mumkinin aṣlan), for the body that moves in a circular motion has to move around the center of the universe (markaz al-kull) and not outside it.[272]
He followed that with a more damning statement:
The science of astronomy of our time contains nothing existent (laysa minhu sha'un maujūdun), rather the astronomy of our time conforms only to computation, and not to existence (hay'atun muwāfiqatun li-l-ḥusbān Iā li-l-wujūd).[273]
As has already been noted, it was Ibn al-Shāṭir who took these objections seriously, and who responded to the issue of the eccentrics by banishing them out of his system. But in the case of the epicycles he tossed the ball back to the Aristotelian yard to ask about the very nature of the ether as we have also said.
Then there was the issue of the Aristotelian spheres themselves, whether they would move by their own volition or be forced to move by something else.[274] The problem arose from the fact that the planets themselves do not have the same kind of motion, and seemed to exhibit individual motions of their own. But according to Aristotle, there were no such motions without movers that caused them in the first place. Thus every planet must have a sphere that caused its motion. And because of the complexity of those motions the spheres got multiplied, and so on.
These motions of the spheres led to a lively discussion that apparently started with 'Urḍī (1266) in the thirteenth century and continued well into the sixteenth century with the works of Ghars al-Dīn b. Aḥmad b. Khalīl al-Ḥalabī (d. 1563). The essence of the debate is to point to the paradox in the Aristotelian thinking about those spheres. If those spheres moved of their own volition, as they seemed to do, then how could one anticipate their motions, and predict where the planets would be at a specific time? If the spheres, on the other hand were forced to move in predictable motions, then could they exhibit this variety of motions that we witness in the celestial realm? 'Urḍī interjected:
If we were to admit that the mover of a planet could speed up and slow down, then we would have no need of constructing a configuration (hay'a), and his own astronomy (hay'a) (i.e. Ptolemy's) would be in vain. Any assumption that a planet would have more than one sphere would be an unnecessary excess, which is impossible.[275]