Levi ben Gershom ( _he. לוי בן גרשום), better known as Gersonides or the Ralbag ["Ralbag" is the acronym of "Rabbi Levi Ben Gershom", with vowels added to make it easily pronouncable - the normal traditional Jewish practice with the names of prominent Rabbis.] (1288-1344), was a famous rabbi, philosopher, mathematician, astronomer/astrologer. He was born at Bagnols in Languedoc, France.


As in the case of the other medieval Jewish philosophers little is known of his life. His family had been distinguished for piety and exegetical skill in Talmud, but though he was known in the Jewish community by commentaries on certain books of the Bible, he never seems to have accepted any rabbinical post. It has been suggested that the uniqueness of his opinions may have put obstacles in the way of his preferment. He is known to have been at Avignon and Orange during his life, and is believed to have died in 1344, though Zacuto asserts that he died at Perpignan in 1370.


Philosophical and religious works

Part of his writings consist of commentaries on the portions of Aristotle then known, or rather of commentaries on the commentaries of Averroes. Some of these are printed in the early Latin editions of Aristotle’s works. His most important treatise, that by which he has a place in the history of philosophy, is entitled "Sefer Milhamot Ha-Shem", ("The Wars of the Lord"), and occupied twelve years in composition (1317–1329). A portion of it, containing an elaborate survey of astronomy as known to the Arabs, was translated into Latin in 1342 at the request of Pope Clement VI.

"The Wars of the Lord" is modeled after the plan of the great work of Jewish philosophy, the "Guide for the Perplexed" of Maimonides. It may be regarded as a criticism of some elements of Maimonides' syncretism of Aristotelianism and rabbinic Jewish thought. The "Wars of the Lord" review:

:1. the doctrine of the soul, in which Gersonides defends the theory of impersonal reason as mediating between God and man, and explains the formation of the higher reason (or acquired intellect, as it was called) in humanity—his view being thoroughly realist and resembling that of Avicebron;:2. prophecy;:3. and 4. God's knowledge of facts and providence, in which is advanced the theory that God does not know individual facts. While there is general providence for all, special providence only extends to those whose reason has been enlightened;:5. celestial substances, treating of the strange spiritual hierarchy which the Jewish philosophers of the middle ages accepted from the Neoplatonists and the pseudo-Dionysius, and also giving, along with astronomical details, much of astrological theory; and:6. creation and miracles, in respect to which Gersonides deviates widely from the position of Maimonides.

Gersonides was also the author of a commentary on the Pentateuch and other exegetical and scientific works.

Views on God and omniscience

In contrast to the theology held by the majority of Orthodox Judaism, Gersonides held that God limited his own omniscience concerning foreknowledge of human acts. "Gersonides, bothered by the old question of how God's foreknowledge is compatible with human freedom, holds that what God knows beforehand is all the choices open to each individual. God does not know, however, which choice the individual, in his freedom, will make." (Louis Jacobs, "God, Torah, Israel: Traditionalism without Fundamentalism")

Another neoclassical Jewish proponent of self-limited omniscience was Abraham ibn Daud. "Whereas the earlier Jewish philosophers extended the omniscience of God to include the free acts of man, and had argued that human freedom of decision was not affected by God's foreknowledge of its results, Ibn Daud, evidently following Alexander of Aphrodisias, excludes human action from divine foreknowledge. God, he holds, limited his omniscience even as He limited His omnipotence in regard to human acts". ("Philosophies of Judaism", Julius Guttmann, JPS, 1964. P.150, 151)

:"The view that God does not have foreknowledge of moral decisions which was advanced by ibn Daud and Gersonides (Levi ben Gershom) is not quite as isolated as Rabbi Bleich indicates, and it enjoys the support of two highly respected Achronim, Rabbi Yeshayahu Horowitz ("Shelah haKadosh") and Rabbi Chaim ibn Attar ("Or haHayim haKadosh"). The former takes the views that God cannot know which moral choices people will make, but this does not impair His perfection. The latter considers that God could know the future if He wished, but deliberately refrains from using this ability in order to avoid the conflict with free will." ("Tradition: A Journal of Orthodox Jewish Thought", Vol. 31, No.2, Winter 1997, From "Divine Omniscience and Free Will", Cyril Domb, p.90-91)

Rabbi Isaiah Horowitz explained the apparent paradox of his position by citing the old question, "Can God create a rock so heavy that He cannot pick it up ?" He said that we cannot accept free choice as a creation of God's, and simultaneously question its logical compatibility with omnipotence.

See further discussion in "Free will In Jewish thought".

Views of the afterlife

Gersonides posits that people's souls are composed of two parts: a material, or human, intellect; and an acquired, or agent, intellect. The material intellect is inherent in every person, and gives people the capacity to understand and learn. This material intellect is mortal, and dies with the body. However, he also posits that the soul also has an acquired intellect. This survives death, and can contain the accumulated knowledge that the person acquired during their lifetime. For Gersonides, Seymour Feldman points out, "Man is immortal insofar as he attains the intellectual perfection that is open to him. This means that man becomes immortal only if and to the extent that he acquires knowledge of what he can in principle know, e.g. mathematics and the natural sciences. This knowledge survives his bodily death and constitutes his immortality." (Gersonides, Trans. Seymour Feldman "Wars of the Lord", Book 1, p. 81, JPS, 1984)

Works in mathematics and astronomy/astrology

Gersonides wrote "Book of Numbers" in 1321 dealing with arithmetical operations, including extraction of roots. Also, in 1342, he wrote "On Sines, Chords and Arcs", which examined trigonometry, in particular proving the sine law for plane triangles and giving five figure sine tables.

One year later, at the request of the bishop of Meaux, he wrote "The Harmony of Numbers" which is a commentary on the first five books of Euclid.

He is also credited invariably to have invented the Jacob's staff ['s_staff1.htm] , an instrument to measure the angular distance between celestial objects. It is described as consisting :"... of a staff of 4.5 feet (1.4 m) long and about one inch (2.5 cm) wide, with six or seven perforated tablets which could slide along the staff, each tablet being an integral fraction of the staff length to facilitate calculation, used to measure the distance between stars or planets, and the altitudes and diameters of the Sun, Moon and stars."

Levi observed a solar eclipse in 1337. After he had observed this event he proposed a new theory of the sun which he proceeded to test by further observations. Another eclipse observed by Levi was the eclipse of the Moon on 3 October 1335. He described a geometrical model for the motion of the Moon and made other astronomical observations of the Moon, Sun and planets using a camera obscura.

Some of his beliefs were well wide of the truth, such as his belief that the Milky Way was on the sphere of the fixed stars and shines by the reflected light of the Sun. Gersonides was also the earliest known mathematician to have used the technique of mathematical induction in a systematic and self-conscious fashion and anticipated Galileo’s error theory [ [ science] ] .

One of the Moon's features, the Rabbi Levi crater, was named after him.

Gersonides believed that astrology was real, and developed a naturalistic, non-supernatural explanation of how it works. In "Philosophies of Judaism", Julius Guttman explains that for Gersonides, astrology was:

founded on the metaphysical doctrine of the dependence of all earthly occurrences upon the heavenly world. The general connection imparted to the prophet by the active intellect is the general order of the astrological constellation. The constellation under which a man is born determines his nature and fate, and constellations as well determine the life span of nations....The active intellect knows the astrological order, from the most general form of the constellations to their last specification, which in turn contains all of the conditions of occurrence of a particular event. Thus, when a prophet deals with the destiny of a particular person or human group, he receives from the active intellect a knowledge of the order of the constellations, and with sufficient precision to enable him to predict itss fate in full detail.....This astrological determinism has only one limitation. The free will of man could shatter the course of action ordained for him by the stars; prophecy could therefore predict the future on the basis of astrological determination only insofar as the free will of man does not break through the determined course of things.

In Modern Fiction

Gersonides is an important character in the novel "The Dream of Scipio" by Iain Pears, where he is depicted as the mentor of the protagonist Olivier de Noyen, a non-Jewish poet and intellectual. A (fictional) encounter between Gersonides and Pope Clement VI at Avignon during the Black Death is a major element in the book's plot.


*"Gersonides". "The Encyclopaedia Judaica". Keter Publishing.
*Eisen, Robert (1995). "Gersonides on Providence, Covenant, and the Chosen People: A Study in Medieval Jewish Philosophy and Biblical Commentary". State University of New York.
*Guttman, Julius (1964). "Philosophies of Judaism", p.214/215. JPS.
*Feldman, Seymour. "The Wars of the Lord" (3 volumes). Jewish Publication Society.


External links

* [ Stanford Encyclopedia of Philosophy]
* [ Milhamot HaShem Fulltext] (PDF)
* [ Detailed bibliography of works on and by Gersonides]

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