Reply to Saliba

Professor Saliba takes exception to views expressed in my Review Essay ("Configuring the Universe: Aporetic, Problem Solving, and Kinematic Modeling as Themes of Arabic Astronomy," Perspectives on Science 1998, vol. 6, no. 3, pp. 288-330). His criticisms and reservations mainly relate to questions about the character and the value of what I have called the hay'a project in Islamic astronomy, how and when this project was launched, and about contrasts with other projects and approaches in the same discipline, especially concerning the important question of the relations between physics and mathematics as viewed by the practitioners of the discipline. The following brief comments, kindly solicited by the editors of Perspectives on Science, are meant to sharpen some of our more serious differences, and, hopefully, to remove some misunderstandings.

First, with regard to the term hay'a, Saliba makes too much of the term itself as an indication of when a new, and specifically Islamic, project came into existence. Hay'a is a common Arabic word which means shape, figure or form; and, whether or not it originally rendered a Greek term, it could equally apply to the Aristotelian form (we now say "configuration") of the universe as a structure of homocentric spheres, and to the Ptolemaic system of eccentric spheres and epicycles. As Saliba knows, the term appears in the title of one manuscript of the Arabic version of Ptolemy's Planetary Hypotheses, and could be applied to something as simple as the figure (hay'a) of the earth. That the term came to refer to the science of astronomy in general is therefore not surprising: it emphasized a tendency, prevalent among a large number of Islamic astronomers, to view astronomical theory as primarily concerned to represent the heavens in terms of nested solid spheres obeying some physically accepted principles of motion. Thus an early use of the word hay'a does not by itself imply the motivations and features we have come to associate with celestial configurations proposed by astronomers who were acquainted with objections and demands such as those which Ibn al-Haytham had stated in the eleventh century. It is of course possible that Ibn al-Haytham was not the first to raise such objections and demands, but so far we have no knowledge, direct or indirect, of anything in the writings of earlier astronomers that compares with Ibn al-Haytham's clear, persistent and forceful strictures.

On the other hand, Saliba makes too little of the term skakk in the sense of the Greek aporia. He refers, for example, to well known Islamic corrections of certain features of Ptolemaic astronomy (as early as the ninth century or earlier), including an eleventh-century non-extant treatise called al-Istidrak 'ala Batlamyus ("Correcting Ptolemy," which Saliba translates as "Recapitulation on Ptolemy"). But an aporia, as the term is used by Aristotle and his Greek commentators, and the Arabic equivalent, shakk (doubt), widely used in the Arabic philosophical literature, is not simply an error to be deleted or corrected, but a difficulty, puzzle or problem that may have to be defined as such before it is seen to require a particular solution. In several writings listed in the references to my essay, I tried to indicate the historical importance of using the concept of aporia as a methodological tool in discussions within specialized disciplines, such as medicine and mathematics and astronomy, as well as in philosophy. In the essay itself I propose to make the historical point that a certain hay'a project began to take shape when Ibn al-Haytham used the aporetic argument to reveal a series of puzzles in what Ptolemy had presented in his Almagest and Planetary Hypotheses, with emphasis on Ptolemy's equant hypothesis. It was that fortified (not diluted!) argument, I maintained, that captured the attention of later Islamic astronomers. It is true that Ibn al-Haytham did not himself propose alternative configurations to those of Ptolemy, but he undertook to define the problem as he saw...