In the east, and around the same time, Abū 'Ubayd al-Jūzjānī (d. ca. 1070), the student of Ibn Sīnā (Latin Avicenna d. 1037), also left us a small treatise On the Construction of the Spheres. In this treatise he mentioned that he had discussed with his teacher, Ibn Sīnā, the famous Ptolemaic absurdity, which was by then known as the problem of mu'addil al-masīr (Equator of Motion, or equant for short).[185] The fact that both such texts existed, one from Al-Andalus, the farthest western reaches of the Islamic world at the time, and one from Bukhara, the farthest east, and the fact that the second text comes from the philosophical circle of Ibn Sīnā and not from the circle of astronomers and mathematicians, could only mean that the cosmological issues that were perceived to have plagued Ptolemaic astronomy were by then circulating in widespread intellectual and geographical circles; they were no longer restricted to the elite of astronomical theoreticians. The equant problem itself, which had the longest staying record, is none other than the physical absurdity of proposing that a physical sphere could move uniformly, in place, around an axis which did not pass through its center. This absurdity permeated almost all of the models, which were proposed in Ptolemy's Almagest. What the texts of al-Andalus and Bukhara suggest is that by the eleventh century that proposition was apparently widely recognized as a physical impossibility.
In his own rather humorous story Abū 'Ubayd informs us that when he discussed the proposed solution for this Ptolemaic absurdity of the equant, with his teacher Ibn Sīnā, he was told by Ibn Sīnā himself that he had also resolved it, but refrained from giving out the solution in order to urge the student to find it for himself. In the very next sentence the student went on to say that he did not believe that his teacher had ever resolved that problem.
The anecdote, legendary as it may be, is still indicative of the kind of problems those custodians of the "foreign sciences", the philosophers in particular, were competing to solve, and the challenges they were facing, as well as the fame they hoped to acquire if they could rid the Greek astronomical tradition of its absurdities. The anecdote also indicates that if the philosophers were already aware of this joint reading of the Ptolemaic texts (the
Almagest and the Planetary Hypotheses) in which such problems would arise, this must mean that the astronomers were obviously much more deeply entrenched in that perspective. And the debates of the latter must have informed the former.
For the good fortune of the astronomers, it also appears that their debates over such kinds of issues were socially condoned. They did not only transcend their circles to reach the circles of the philosophers, but they probably also gave rise to the likelihood of rebutting the incoming Greek tradition altogether, since they were being critical of it.
Such discussions had no direct bearing on the other more socially controversial perception of the Greek tradition: its willingness to harbor those astrological sciences that were not as widely accepted as the theoretical critiques seem to have been. For our immediate purposes, however, it is important to document the tradition of the critiques themselves in order to demonstrate the sophistication of that tradition, and its wider implication on the very formation of Islamic science.
Again in the same century, and still in the east, we find the prolific polymath and famous astronomer Abū al-Raiḥān al-Bīrūnī (d. ca. 1048) who also had something to say about the physical absurdities of the Ptolemaic system. This, despite the fact that Bīrūnī's main astronomical production was really geared toward the mathematical observational part of astronomy and paid much less attention to the cosmological aspects of the discipline. In his Ibṭāl al-buhtān bi-īrād al-burhān (Disqualifying Falsehood by Expounding Proof), which seems to have been lost but which was quoted by the astronomer Quṭb al-Dīn al-Shīrāzī (d. 1311), Bīrūnī had this to say about the Ptolemaic description of the latitudinal motion of the planets: "As for the motions of the five epicyclic apogees in inclination, as it is commonly known, and is mentioned in the Almagest, those would require motions that were appropriate for the mechanical devices of Banū Mūsā, and they do not belong to the principles of Astronomy."[186] That was Bīrūnī's polite way of saying that Ptolemy's discussion of the planetary latitudes was not astronomy proper, and that it amounted to nothing. Such was the extent of criticism of Ptolemy, even by people who had a vested interest in defending him against his detractors. And yet they could not remain silent about the Ptolemaic absurdities, probably because they apparently felt that they had a greater interest in competing amongst themselves by demonstrating that they could outsmart Ptolemy.
The best-preserved and most elaborate text in the genre of shukūk was a criticism of Ptolemy that was leveled by another polymath, by the name of Ibn al-Haitham (d. ca. 1040 Latin Alhazen), who was also a contemporary of the astronomers mentioned above, and whose work on Optics was the only work that was known in the Latin West and which earned him his well deserved fame. His critique of Ptolemaic astronomy is contained in an Arabic text which has survived, but which was apparently never translated into Latin. The text in question is his extensive al-Shukūk 'alā Baṭlamyūs (Dubitationes in Ptolemaeum) \Shukūk\,[187] in which he took issue with several of Ptolemy's works in which he found fault.
The three Ptolemaic works in question included the Almagest, the Planetary Hypotheses, and the Optics. Their mere grouping is a clear indication that those works were read together, in a comprehensive manner, and not in isolation as is sometimes claimed.[188] For Ibn al-Haitham, the common thread that connected the three books together is that they all contained problems or doubts (shukūk) that revealed contradictions that could not be explained away (lā ta'awwul fīhā).[189] This phraseology also indicates that every effort was already made to give Ptolemy the benefit of the doubt. Problems were obviously explained away wherever that was possible,[190] and only those absurdities that could not be justified were attacked. Ptolemy's books were taken up in the following order: the Almagest, which had the lion's share, to be followed by the Planetary Hypotheses, and then the Optics. In the sequel I will take a few select examples from this treatise in order to illustrate the kind of issues that attracted the attention of Ibn al-Haitham.
In his critique of the Almagest, Ibn al-Haitham passes very quickly over the early chapters of that book, and commences the real critique with the Ptolemaic description of the model for the lunar motion. In it Ptolemy assumes that the motion of the moon, on its own epicycle, is measured from a line that passes through the center of the epicycle, but is directed, not to the center of the world, around which the motion of the epicycle itself is measured, nor to the center of the sphere that carries the epicycle, called the deferent, but to a point, called the prosneusis point (nuqṭat al-muḥādhāt) by Ptolemy. In the Ptolemaic model, this point falls diametrically opposite to the center of the deferent from the center of the world. In his overall assessment of this model Ibn al-Haitham clearly said that it was basically fictitious and that it had no connection to the real world it was supposed to describe. He singled out the soft spot in the model with the following remark: "The epicyclic diameter is an imaginary line, and an imaginary line does not move by itself in any perceptible fashion that produces an existing entity in this world."[191] Furthermore: "Nothing moves in any perceptible motion that produces an existing entity in this world except the body which [really] exists in this world."[192] Later on, he went on to affirm once more: "no motion exists in this world in any perceptible fashion except the motion of [real] bodies." And then he concluded this section by stating that a single epicycle could not possibly move the moon by its own anomalistic motion and at the same time move in such a way that its diameter will always be directed toward the prosneusis point. That would entail that a single sphere was supposed to move in two separate motions by itself, which was impossible.
187
Ibn al-Haitham (d. 1049),
188
See the literature reviewed in F. Jamil Ragep, "Duhem, the Arabs, and the history of Cosmology",