The Birth of scientific Controversies, The Dynamics of the Arabic Tradition and its Impact on ethe Development of Science : Ibn al-Haytham's Challenge of Ptolemy's "Almagest"

The so-called Copernican revolution is Kuhn's most cherished example in his conception of the non-cumulative development of science. Indeed, in his view not only has the Copernican model introduced a major discontinuity in the history of science but the new paradigm and the old paradigm are incommensurable, i.e. the gap between the two models is so huge that the changes introduced in the new model cannot be understood in terms of the concepts of the old one. The aim of this chapter is to show on the contrary that the study of the Arabic tradition can bridge the gap assumed by Kuhn as a historical fact precisely in the case of Copernicus. The changes involved in the work of Copernicus arise, in our view, as a result of interweaving epistemological and mathematical controversies in the Arabic tradition which challenged the Ptolemaic model. Our main case study is the work of Ibn al¬Haytham who devotes a whole book to the task of refuting the implications of the Almagest machinery. Ibn al-Haytham's al-Share had such an impact that since its disclosure the Almagest stopped being seen as the suitable model of the heavenly bodies. Numerous attempts have been made to find new alternative models based on the correct principles of physics following the strong appeal launched by both Ibn al¬Haytham and, after him, Ibn Rushd. The work of Ibn al-Shàtir, based exclusively on the concept of uniform circular motion, represents the climax of the intense theoretical research undertaken during the thirteenth and the fourteenth centuries by the Maràgha School (which owes its naine to the observatory of Maràgha in nonh¬western Iran). The connection point, in our view, between the works of Ibn al¬Haytham and Ibn al-Shàtir is that while the al-Shukiik gives the elenients to build a countermodel to the Ahnagesi, the work of Ibn al-Shâtir offers a mode) which takes care of the objections triggered by the work of Ibn al-Haytham. Furthennore, not only has the basic identity of the models of Ibn al-Shàtir and Copernicus been established by recent researches, but it was also found out that Copernicus used the very same mathematical apparatus which was developed by the Maràgha School over at Ieast two centuries. Striking is the fact that Copernicus uses without proof mathematical results already geometrically proven by the Maràgha School three centuries before. Our paper will show that Copernicus was in fact working under the influence of the two streams of the Arabic tradition: the well known more philosophical western stream, known as physical realism, and the newly discovered eastern mathematical stream. The first relates to the idea that astronomy must be based on physics and that physics is about the real nature of things. The second relates to the use of mathematics in the construction of models and countermodels in astronomy as developed by the Maràgha School. The case presented challenges the rote of the Arabic tradition assigned by the standard interpretation of the history of science and more generally presents a first step towards a reconsideration of the thesis of discontinuity in the history of science. Our view is that major changes in the development of science might sometimes be non-cumulative, though this is not a case against continuity understood as the result of a constant interweaving of a net of controversies inside and beyond science itself