Others raised further theoretical questions that could be read together with the philosophical questions that will be touched upon later on as we discuss the relationship of science to philosophy in the case of astronomy. In the present context of the encounter with the Greek tradition those questions gain a special significance as they touched upon the philosophy of science in a more focused sense. That is they tried to determine the domain in which one astronomer was justified in raising objections to the work of another. What was it exactly that one was allowed to object to, and what kind of evidence one was required to bring to the argument in order to make the case? What was the role of the observations in astronomy and what was an acceptable account of them? For this type of questioning the best representative was the Damascene astronomer Mu'ayyad al-Dīn al-'Urḍī, whose name was mentioned several times already. In his extensive treatise, Kitāb al-Hay'a,[227] which could be read in its entirety as a comprehensive statement of objections to Ptolemaic astronomy, he isolated such issues in particular when he attempted to reform, for example, the proposed Ptolemaic configurations for the planet Mercury. After enumerating the various spheres, their motions, and their relative positions with respect to one another, he went on to say:
The conditions resulting from the observations just mentioned — I mean the ones from which these conditions are known—are only the motions of the deferent's apogee and perigee. As for the directions of these motions, these were not necessitated (by the observations), rather they were simply given by Ptolemy.
Had these motions been in the manner which he had adopted, and hadn't they contradicted the principles, then he would have achieved his purpose.[228]
By questioning the relationship between the observations and the kind of results one was allowed to deduce from them, 'Urḍī wished to direct the attention of his reader to the activity of model building itself. What part of it was dictated by the observations and what part was left to the astronomer himself to construct? And in the case of the model for the planet Mercury, 'Urḍī had this to say:
This total (configuration) resulted from many factors: The observations, the proofs which are based on observations, the periodic motions, the configuration (hay'a) that he [meaning Ptolemy] had conjectured, and the directions of the (various) motions (involved). In regard to the observations, the proofs, and the periodic motions, nothing of them could be criticized, for nothing had come to light, which would contradict them.
As for the method of conjecture (ḥads), he (i.e. Ptolemy) has no priority in it, (especially) after his mistakes have been clearly exposed. If anyone else ever finds something, which agrees with the principles, as well as the particular motions of the planet which were found by observation, then that person would have a greater claim to the truth.
When we saw the error of this opinion, and sought to rectify it, as we did in the case of the remaining planets, we found out that we could perfect it if we reversed the directions of the two previously mentioned motions—I mean the motions of the director and the deferent orb.[229]
In very clear terms, this demonstrates the type of engagement that 'Urḍī had sought to achieve with the Ptolemaic tradition. To some of its parts, especially the observational aspects, he had no objections to make because he had no observations of his own to bring to bear. The periods of the planets, he also had to accept as he also did not have better ancient sources to deduce his own. Ptolemy's mathematics was also superb, especially after it had been already updated by the generations of astronomers who worked on it since the ninth century. But when it came to conjecture (ḥads), 'Urḍī's term for theorizing, there was no reason to prefer Ptolemy's theories over others, especially when those theories could not account for the observations and yet remain faithful to the Aristotelian cosmology that was already accepted by Ptolemy. It is not that astronomers like 'Urḍī were blaming Ptolemy for abandoning Aristotle, and that they were so enamored by the latter to wish to re-install him through the adoption of his cosmology, but what they found objectionable was the theoretical contradiction between Ptolemy's acceptance of a set of principles on the one hand, no matter whose principles, and his contradicting those very principles when he came to describe the mathematical constructs that represented the same principles. On that level of theorization they felt that Ptolemy should have no priority over them. In fact they felt better qualified to theorize simply because they avoided the contradictions that plagued Ptolemy's astronomy. And yet their alternative models accounted for the observations just as well as Ptolemy's models could. Virtuous as he was, Ptolemy's authority could not overcome his inability to theorize properly.
At this stage, the Islamic astronomical tradition had obviously reached such maturity that it could profitably raise issues that were not raised before. It could contemplate problems, relationships, theoretical strategies, that were not dreamt by Ptolemy. The confidence in the new foundations of science gave these astronomers the ability to go beyond the criticism of Ptolemy, and to dare to oppose his models with their own, by either redeploying the same mathematics that he used, or by devising some of their own to replace it. This confidence also allowed them to look at the universe from a different perspective and to lay down new rules for the science that would eventually describe it. Such matters that were being explored in that manner touched the foundation of every science and were no longer restricted to astronomy alone. Again they should be borne in mind when we discuss the relationship of science to philosophy later on.
In the present context, and even at the expense of some overlapping with the later chapter, these theoretical issues should be highlighted here as well. The theoretical lines that were developed in response to the Greek astronomical tradition also gave rise to the debate over the admissibility of eccentrics and epicycles among the celestial spheres, a debate that was not in essence a debate over the violation of the physicality of the spheres as was discussed so far, but a discussion over whether in principle the celestial realm admitted such configurations at all. The origin of the problem was already locatable in several of the Aristotelian works, but most notably in the De Caelo, where Aristotle proved, with impeccable philosophical rigor, not only that the whole universe was spherical, but also that the Earth was at its center. And if one did not have an Earth there, one had to assume an Earth as the fixed point of any moving sphere, besides being the ultimate point of heaviness of the universe.[230] The argument was therefore of a necessary nature and not a merely convenient option to place the Earth at the center of the universe or not. Astronomers could debate as much as they wanted whether the observable phenomena could be explained by the assumption of a fixed Earth at the center of the universe, or by a revolving Earth around its own axis or around the sun. Some of them did raise these very possibilities in pre-Aristotelian and post Aristotelian times as well as during Islamic times, as was done by Aristarchus of Samos (c. 230 B.C.), for example, and by Bīrūnī (1048) centuries after him. They did acknowledge that the same phenomena could either be explained by a fixed Earth at the center or by a moving one. But that did not change the Aristotelian cosmological conditions one bit. According to Aristotle that "theoretical" Earth had to be motionless at the very center of the universe in the same way every moving sphere must have a motionless point at its very center.
230
See the similar texts in the translation of Leggatt quoted before and add to it, Aristotle,