Gersonides was a highly original Jewish philosopher, scientist and biblical exegete, active in Provence in the first half of the fourteenth century. Ruth Glasner explores his impressive achievements, and argues that the key to understanding his originality is his perspective as an applied mathematical scientist. It was this perspective that led him to examine Aristotelianism from directions different from those usually adopted by contemporary scholastic scholars. Gersonides started on his way, as he himself claims, as a ‘mathematician, natural scientist, and philosopher’, who believed in his power to solve the main problems of medieval science. He ended up concentrating on his work as a mathematical astronomer, developing techniques of observation and computation, and somewhat less optimistic about the prospect of scientific knowledge.