Person

Gersonides

Provençal Jewish philosopher, biblical commentator, mathematician, astronomer, and Talmudist; the most systematic Jewish Aristotelian after Maimonides and one of the most influential rabbinic commentators on Tanakh. Known in Hebrew as Ralbag (acronym for Rabbi Levi ben Gershon) and in Latin as Leo Hebraeus. His magnum opus Sefer Milhamot ha-Shem (Wars of the Lord, c. 1329) takes up six metaphysical-theological questions Maimonides left underdetermined and pushes Jewish rationalism in often-controversial directions. His commentaries on the Bible (running to thousands of pages) and his mathematical and astronomical work (De Sinibus, Chordis et Arcubus, c. 1342, on trigonometry; significant astronomical observations) made him one of the most active scientific minds of the 14th century.

Position in the codex's framework

Gersonides occupies a distinctive position in the medieval Jewish thought map: more rationalist than Maimonides on some questions (he affirms the eternity of prime matter, though not of the universe simpliciter, which Maimonides denies; he limits divine foreknowledge of contingent future events to preserve free will), and a key witness to non-strict-24-hour readings of Genesis 1 in the rabbinic-philosophical tradition.

In the Genesis-1 interpretation map (see Genesis Interpretation Spread), Gersonides reads the six days as ordering-of-formation rather than as strict chronological days; he holds, with Maimonides, that the simple-24-hour-day reading is not the only legitimate rabbinic option. The codex cites him alongside Maimonides and Nachmanides as a medieval Jewish witness against the "Jews always read Genesis literally" claim.

Key positions

Eternal prime matter, created cosmos: contra Maimonides, Gersonides argues that prime matter is eternally co-existent with God (as the substrate from which form is imposed), while the organized universe with its forms is created in time. This is closer to a Platonic-demiurgic position than to the standard creatio ex nihilo of the rabbinic mainstream and was a source of considerable controversy in his reception.
Divine foreknowledge limited: Gersonides argues that God knows all necessary and general truths, but does not know which contingent option an agent will freely choose. The position is sometimes called "Gersonidean limited omniscience" and prefigures Open Theism in some respects (though Gersonides grounds the limitation in metaphysical structure, not in temporal becoming). It is rejected by mainstream rabbinic theology following Crescas and others.
Genesis 1 as ordering-of-formation: Gersonides reads the six days as the natural / philosophical ordering required for the cosmos to come into its present structure, not as a chronological log of strictly successive 24-hour periods. He treats day-language as accommodation to teach the ratio of cosmic order, not as a calendar entry.
Astronomy and mathematics: invented the Jacob's staff (cross-staff) for celestial measurement; produced corrected tables of celestial motion; argued for observational refinement of Ptolemaic models. His astronomical / trigonometric work was widely cited in 14th-16th-c. Latin scholarship.

Major works

Sefer Milhamot ha-Shem (Wars of the Lord, c. 1329), six books on: immortality of the soul, prophecy, divine knowledge of contingents, providence, celestial motion / the heavens, creation of the world
Commentary on the Torah (Perush al ha-Torah), verse-by-verse rabbinic commentary, c. 1329-1338
Commentaries on the Former and Later Prophets and Writings
Sefer ha-Heqesh (commentary on Averroes's commentary on Aristotle's Prior Analytics) and other Aristotelian commentaries
De Sinibus, Chordis et Arcubus (c. 1342), trigonometry, translated into Latin for Pope Clement VI
Ma'ase Hoshev, early algebra and combinatorics treatise

Reception

Within Judaism: controversial. The mainstream rabbinic tradition (Hasdai Crescas, Isaac Abarbanel) rejected Gersonides's limited-divine-foreknowledge position and his eternal-prime-matter doctrine. His biblical commentaries, however, were widely used and remained influential through the early modern period (Rashi and Ramban are more popular; Gersonides is the philosophical commentary).
In Latin scholasticism: his trigonometric work was translated into Latin and influenced 14th-c. Christian astronomy. His philosophical positions on divine foreknowledge were known to and engaged by Latin scholastic readers.
Modern: Gersonides is recognized as one of the most original medieval Jewish thinkers; the Cambridge edition of Wars of the Lord (Seymour Feldman, 1984-1999) made him accessible to English-language philosophy of religion.