Scientific methodology in medieval astronomy and cosmology: The case of Levi ben Gerson (1288–1344)

We distinguish between method and methodology and propose to illustrate the claim that methodology is essential to the acquisition of scientific knowledge in all periods. The case in point is Levi ben Gerson (d. 1344) and his creative response to the methodologies in astronomy and cosmology implicit in the works of Ptolemy (second century). We begin by outlining Ptolemy’s methodological framework in astronomy, in which one key point is his dependence on his own observations in the first instance, and then on those of others that are consistent with them. In cosmology Ptolemy sought to make his geometric models physical and to account for the transmission of motions to the planets. We then closely examine Levi’s ingenious adaptation of Ptolemy’s methodologies, which included new planetary models, new cosmological principles, as well as dependence on his own observations for determining planetary parameters, and descriptions of new observational instruments. Levi insisted that the planetary models had to account for observations of both positional data and physical characteristics, in contrast to the theories of his predecessors. We conclude with an analysis of Levi’s methodology, emphasizing what made his practice so innovative. Methodology was (and is) the engine of scientific change.