Value in Medieval Islamic Science:Ptolemy and Ibn al-Haytham

Historians of medieval Islamic science attach great importance to Ibn al-Haytham’s (d. 1040 CE) book Doubts Concerning Ptolemy. There is, however, considerable disagreement over how to interpret it. Some have argued that it represents a straightforward criticism of the inconsistencies between the physical theses Ptolemy of Alexandria (d. ca. 170 CE), the outstanding scientist and philosopher of the Hellenistic period, professes to adhere to and the mathematical models he sets out. This view says Doubts does not take a stand on the relation of physics to mathematics, but is motivated by a desire to reconcile inconsistencies in astronomical theory. Another influential interpretation sees Ibn al-Haytham as a leading medieval representative of a group of realist astronomers and cosmologists who oppose the purported instrumentalism of Ptolemy. Against the instrumentalists, who say, in effect, that mathematical models are useful concepts that facilitate our engagement with the world, realists insist they are about real objects in the celestial realm. There are problems with both views and I will argue that a functionalist interpretation holds the key to understanding the critical project Ibn al-Haytham pursues in Doubts. I maintain that he is, in fact, concerned with fundamental questions about mathematics and the physical world and that the functional status of mathematical models is a major theme. While Ptolemy and Ibn al-Haytham fall more or less neatly into the realist camp, I argue that the disagreement captured in Doubts relates to the function of mathematical models in astronomy. The dispute, I claim, concerns the epistemic values these models must embody to be accepted by the community of astronomers. The criticisms of Ptolemy articulated in Doubts with such force, therefore, are informed by Ibn al-Haytham’s beliefs about the intellectual virtues good scientific theories must possess and what he believes to be Ptolemy’s failure to make his models live up to the standards of modelling excellence.