Mediaeval empiricism: the case of Grosseteste's optics.

Abstract

THE appearance of an experimental methodology of science in Robert Grosseteste's writings is often remarked upon by historians of science and forms a convenient focal point for demonstration of the creative interplay between science and philosophy in the Middle Ages.' Grosseteste's optics has been cited as a concrete and important example of this new, experimental method.2 Despite some apparent inconsistency in application3 Grosseteste's method in optical research should in general reflect his logic of science. However, the lack of close correlation between his theory and his practice necessitates reconsideration of Grosseteste's concept of experiment. Because his optical works have borne the burden of proof to a large degree in earlier discussion, a closer examination of the optics is in order as a basis for reassessment. Robert Grosseteste's approach to the study of optics was in the tradition established by Aristotle and given a clear metaphysico-mathematical statement by Alfarabi. In the Hellenistic world a geometrical approach to optics became standard. After Euclid's Optics the tradition was confirmed by such men as Heron, Ptolemy, Heliodoros, Damianos, and Theon. But the geometrical treatment of light could be more than just a method; its metaphysical significance stemmed from the place of mathematics itself in the Neoplatonic outlook.4 When one recalls the high place accorded to mathematics in Plato's ontology and epistemology, the metaphysical suggestiveness of applying geometry to any science is clear. Following the Pythagorean rather than the Archimedean view of mathematics, Neoplatonists tended to see an ultimate reality in everything falling under the heading of this discipline. So it was that many mediaeval adherents of the emission theory of vision found support for their view in the geometrical description of the field of vision. If one could treat sight geometrically as if rays were emitted from the eye, then some reality must pertain to this notion. Of course, emission theories predate Neoplatonism and geometrical optics; Aristotle discussed their origins in Presocratic thinkers like Pythagoras and Empedocles. But strength was given to the emission theory by the success of geometry as an analytical tool in optics and by Neoplatonic emphasis on the reality of geometricals. Alkindi's De aspectibus, 7 (9th century) gave reality to a power sent out from the