- Gersonides. A Portrait of a Fourteenth-Century Philosopher-Scientistby Ruth Glasner
This book is a gem: small, highly dense, and precious. It visibly is the fruit of many years of studying Gersonides' works and, more important, of thinking about them. Ruth Glasner mostly examines Gersonides' work in science and identifies the evolution of his thought. Gersonides straddled two disciplines: natural philosophy and astronomy. Committed to a realist epistemological stance, Gersonides sought to create a "unified theory" of both. Glasner describes the fruit of his efforts to do so.
Chapter 1 delineates the incompatibility between the normative Aristotelian theory and mathematical astronomy. This sets the stage for Gersonides' scientific endeavor. Next, in Chapter 2, comes an "outline" of Gersonides' scientific biography. Glasner emphasizes that Gersonides' Wars of the Lord, begun in 1317, consists of different strata written over some 12 years (the work on the astronomical section continued at least until 1340). Asserting that over this period Gersonides' views evolved and on some points even changed radically, she divides his life into three periods: the early period, ending in 1324/5; the middle period, until 1328; and the last period, until Gersonides' death in 1344. [End Page 283]
The early period includes the writing of many philosophical supercommentaries. Gersonides also began writing a supercommentary on the Metaphysics, but left it unfinished (it is lost today) and switched to writing biblical commentaries. Glasner persuasively argues that this switch from one literary genre to another (in 1324/5) was a radical break and had profound causes: through minute analyses of scattered comments on the value and status of metaphysics, she shows (chapter 6) that Gersonides gradually developed "a growing sense of unease with metaphysics" (p. 74), a result of the low degree of verifiability of its premises and arguments. This led him to abandon the supercommentary on Metaphysicsand instead expound his notion of science and of the scientific methods through commentaries on biblical books, notably the books of wisdom ascribed to King Solomon (p. 75). This is one of the book's major innovative insights. During the late period, Gersonides devoted himself exclusively to astronomy, abandoning natural science and philosophy. This late period is also that of "increasing interaction with the Christian environment" (p. 18).
Chapter 3 discusses Gersonides' physics. It brilliantly shows that Gersonides was from the outset committed to the idea that physics and astronomy describe different aspects of one and the same reality and that they must therefore be compatible. He profoundly modified the received physics so that it would be in agreement with celestial motions.
Chapter 4 is a concise overview of the history of the idea that the celestial bodies have souls or intellects and describes the implications of these conceptions for the views of celestial motions, especially for understanding how the planets participate in the daily motion. Gersonides posited the existence of a sort of quasi-body that "does not preserve its shape" and that transmits motion from one sphere to that beneath it. Glasner points out that "Gersonides regarded the hypothesis of the body that does not preserve its shape as a great accomplishment," because "it provided him not only with a basis for his 'eccentric cosmology' but also for his theory of creation" (p. 45). For Gersonides, the "body that does not preserve its shape," whose existence he regarded as definitely established, was the cornerstone of the "synthesis of [his] new astronomy, cosmology, and the theory of creation" (p. 46). The "mechanical" account of the motions of the heavenly bodies implied that "separate intellects are not needed qua movers" (p. 47). However, Gersonides did not abandon the notion that the heavenly bodies are "animated," although, "unlike Ptolemy, he used the hypothesis sparingly" (p. 50). [End Page 284]
Chapter 5 examines Gersonides' scientific method. Glasner notes that Gersonides "had opted for the eccentric hypothesis many years before he could determine the solar eccentricity (in 1335)," and explains that he became convinced of the truth of this hypothesis because "he examined all the hypotheses known to him (which he considered to...