Levi ben Gershom (Gersonides)
also called Gersonides, Leo De
Bagnols, Leo Hebraeus, or (by acronym) Ralbag
born 1288, Bagnols-sur-Cèze, Fr.
died 1344
French Jewish mathematician, philosopher, astronomer, and
Talmudic scholar.
In 1321 Levi wrote his first work, Sefer ha-mispar (“Book
of the Number”), dealing with arithmetical operations,
including extraction of roots. In De sinibus, chordis et
arcubus (1342; “On Sines, Chords, and Arcs”) he presented an
original derivation of the sine theorem for plane triangles
and tables of sines calculated to five decimal places. On
the request of Philip of Vitry, bishop of Meaux, he composed
a book on geometry, preserved only in Latin translation, De
numeris harmonicis (1343; “The Harmony of Numbers”),
containing commentaries on the first five books of Euclid
and original axioms.
Influenced by the works of Aristotle and the 12th-century
Islāmic philosopher Averroës, Levi wrote Sefer ha-hekkesh
ha-yashar (1319; Latin Liber syllogismi recti; “Book of
Proper Analogy”), criticizing several arguments of
Aristotle; he also wrote commentaries on the works of both
philosophers.
Although Levi’s biblical commentaries are complex, he
presupposed an audience familiar with these commentaries,
medieval astronomical literature, and the works of Averroës
when he wrote (1317–29) his major work, Sefer milḥamot
Adonai (“The Book of the Wars of the Lord”; partial trans.
Die Kämpfe Gottes, 2 vol.). Divided into six parts, the work
treats exhaustively of the immortality of the soul; dreams,
divination, and prophecy; divine knowledge; providence;
celestial spheres and separate intellects and their
relationship with God; and the creation of the world,
miracles, and the criteria by which one recognizes the true
prophet. In the fifth part, he describes “Jacob’s staff,” an
instrument that he used to measure the angular distance
between celestial bodies.
Levi’s work has often been criticized because of his bold
expression and the unconventionality of his thought, which
continued to exercise wide influence into the 19th century.
