Levi ben Gershom (Gersonides)

Life and Works

Levi ben Gershom (c. 1288–c. 1344), known in Latin as Gersonides and in Hebrew by the acronym Ralbag, was a leading figure of medieval Jewish philosophy in Provence. He lived in a culturally mixed environment under Christian rule, where Jewish communities were in contact with both Latin scholastic and earlier Arabic philosophical traditions.

Little is known about his formal education or family background, but his writings indicate deep familiarity with Aristotle, as mediated by Arabic commentators such as Averroes, and with the Jewish philosophical legacy of Moses Maimonides. He spent much of his life in southern French centers such as Bagnols-sur-Cèze and possibly Avignon and later Perpignan, where he appears to have been active as a communal scholar and halakhist.

Gersonides wrote in Hebrew, producing an extensive corpus that ranges across philosophy, biblical exegesis, mathematics, and astronomy. His most famous philosophical work is Milḥamot Ha‑Shem (“The Wars of the Lord”), completed around 1329–1331. Structured in six “books,” it addresses topics such as the nature of the soul, prophecy, divine knowledge, providence, astrology, and the immortality of the intellect. The treatise is explicitly framed as a rational investigation, often revising or criticizing earlier authorities, including Maimonides.

In addition, Gersonides composed widely read biblical commentaries (particularly on Job, the Pentateuch, and the Former Prophets), which combine philological attention with philosophical interpretation. These commentaries circulated broadly in the Jewish world and were among the main vehicles of his influence.

His scientific works include a treatise on trigonometry and optics, and astronomical writings such as Sefer Ma’aseh Ḥoshev and De numeris harmonicis (in Latin translation). He also designed an observational instrument, later known as the “Jacob’s staff” or “Gerson’s staff”, for measuring angular distances between celestial bodies. His reputation extended beyond Jewish circles; parts of his scientific work were translated into Latin and used by Christian scholars.

Gersonides likely died around 1344, probably in Perpignan. By then, he had established himself as one of the most systematic and daring rationalist voices within medieval Judaism.

Philosophical and Theological Thought

Gersonides is often grouped with post‑Maimonidean rationalists, but his system departs from Maimonides’ on several crucial points. His philosophical project aims to show that human reason, properly exercised, can achieve demonstrative knowledge in many domains traditionally left to faith, including aspects of theology.

A central element of his metaphysics is the theory of separate intellects and the Active Intellect, derived from Aristotelian and Arabic sources. For Gersonides, human perfection consists in the acquisition of the acquired intellect, an enduring form of knowledge that links the human mind with the separate intellects and thus underwrites personal immortality. The degree of a person’s immortality corresponds to the degree and quality of the true knowledge he or she has attained.

His most controversial doctrine concerns divine knowledge and providence. To safeguard both human freedom and the rational order of nature, Gersonides argues that God knows all universals and all possible outcomes, but does not know future contingent particulars as already determined facts. Future free actions are known to God as possibilities, not as fixed events. This view is intended to avoid what he sees as logical difficulties in reconciling exhaustive foreknowledge with genuine contingency.

Similarly, he develops a distinctive account of divine providence. In his view, providence is not an arbitrary or purely miraculous intervention in the world, but is largely intellectual and law‑like. Individuals who cultivate the intellect and live virtuously come under a higher degree of providence, mediated through the Active Intellect, while others are left more exposed to the general order of nature, including chance events. Critics have viewed this as elitist, while supporters regard it as a consistent rationalization of biblical ideas of reward and punishment.

On prophecy, Gersonides maintains an intellectualist approach: prophecy requires both moral preparation and an elevated level of intellectual perfection, allowing the prophet’s imagination to receive overflow from the Active Intellect. He thus links prophecy with enhanced scientific and philosophical understanding rather than with purely supernatural inspiration.

He also takes a measured but positive stance toward astrology. While rejecting crude determinism, he allows that celestial configurations can influence the sublunary world in regular, natural ways. Astrology, properly understood, becomes part of the rational study of nature. In The Wars of the Lord, he attempts to define the limits and reliability of astrological prediction, integrating it with his views of providence and contingency.

Gersonides’ theology provoked both admiration and resistance. Some later Jewish thinkers praised his intellectual rigor, while others criticized what they saw as excessive naturalization of divine attributes and a problematic restriction of God’s foreknowledge. Modern scholarship often highlights his system as an internally coherent—and unusually bold—attempt to reconcile Aristotelian science with a scriptural religious framework.

Scientific Contributions and Legacy

In mathematics and astronomy, Gersonides made several original contributions. He worked extensively with trigonometric tables, improved methods for calculating sines, and engaged in problems of combinatorics and number theory. His interest in precise computation supported his astronomical investigations, where he sought to refine the Ptolemaic model through careful observation.

The Jacob’s staff attributed to him enabled more accurate measurement of the altitude of stars and the angular separation between celestial bodies. This instrument was used by navigators and astronomers for centuries. Gersonides’ recorded observations of eclipses and planetary motions provided data of interest to later historians of science, and some aspects of his planetary models show attempts at empirical correction of inherited schemes.

His legacy is multifaceted. Within Judaism, his biblical commentaries remained standard reference works in many communities, and his philosophical positions were known and debated, though they never achieved the canonical status of Maimonides’ Guide of the Perplexed. In the broader history of philosophy, Gersonides is regarded as a significant voice in debates over divine omniscience, human freedom, and the nature of intellect. Historians of science recognize him as an example of the close interweaving of religious, philosophical, and scientific inquiry in the late medieval Mediterranean world.

Contemporary scholars often treat Gersonides as an important case study in rationalist theology: a thinker who pushed reason as far as he believed it could go, and then reinterpreted traditional doctrines to fit the limits and findings of that rational inquiry. His work remains a subject of active research in medieval Jewish studies, history of philosophy, and history of science.