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Levi ben Gershon ("Levi son of Gerson"), better known as Gersonides or the Ralbag (1288 - 1344), was a rabbi, philosopher, mathematician, astronomer, astrologer, and Talmudist. He is best known for his philosophical work Sefer Milhamot Ha-Shem, (The Wars of the Lord), written between 1317 and 1329, which synthesized an Aristotelian cosmology with Jewish theology. An accomplished mathematician and skillful astronomer, he firmly believed that reason could answer any philosophical question and explain any phenomenon. He promoted the view that an “Active Intelligence” mediated between God and man, and that immortality could be achieved by the soul’s acquiring knowledge that mirrored the Active Intellect by containing “a conception of the rational order obtaining in all individuals” (things). He believed that an omniscient God knew all the possible choices open to an individual human being in any given circumstances, but did not know in advance which choice the individual would make.


Gersonides also wrote several biblical commentaries which became a fundamental part of Jewish theology, but his philosophical ideas were rejected by the orthodox Jewish community as being heretical. He wrote works on arithmetic, trigonometry, geometry and logic, and devised an instrument to measure the angular distance between celestial objects. His work on astronomy and astrology, its original Hebrew still in manuscript form, was translated into Latin in 1342 at the request of Pope Clement VI.


Levi ben Gerson was born in 1288 at Bagnols in Languedoc, France. As is the case with the other medieval Jewish philosophers little is known about Gersonides’ life. His family had been distinguished for piety and exegetical skill in Talmud. His father is thought to have been Gershom ben Salomon de Beziers, a notable mentioned in medieval histories. Gersonides was known in the Jewish community for his commentaries on the Bible, but he never seems to have accepted any rabbinical post. He may have married a distant cousin; it is not known whether he had any children. He is known to have been at Avignon and Orange during his life, and is believed to have died on April 20, 1344, though Zacuto asserts that he died at Perpignan in 1370.


Philosophical and religious works

Gersonides wrote several commentaries on the portions of Aristotle then known through the commentaries of Averroes; some of these are printed in the early Latin editions of Aristotle’s works. He is best known for the important treatise, Sefer Milhamot Ha-Shem, (The Wars of the Lord), which he wrote between 1317 and 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.

Among the Jewish community Gersonides was well known as a Halakhist (one who deals with the intricacies of Jewish law) and his biblical commentaries were well-received. A commentary on the Book of Job, completed in 1325, became one of the earliest Hebrew books to be published (Ferrara, 1477). A complement to book four of Milhamot Ha-Shem, the commentary discusses the problem of divine providence. Each character in the Book of Job represents a different theory of divine providence; Gersonides' own position is a restatement of Elihu's theory that providence is not directed to particulars but rather to groups of individuals, or universals. Gersonides was also the author of a commentary on the Pentateuch and other exegetical works. Each commentary began with an explanation of the meaning of the biblical text and was followed by a philosophical interpretation.

Gersonides also wrote a logical treatise, Sefer Ha-heqesh Ha-yashar (On Valid Syllogisms, 1319), examining problems associated with Aristotle's modal logic as developed in the Prior Analytics. Though it was translated into Latin at an early date, Gersonides' name was not attached to it.

Gersonides differed from other Jewish philosophers in his profound knowledge of mathematics and his scientific interest in astronomy and astrology, which he felt gave him the tools to resolve difficult questions of cosmology. He regarded astronomy as a means of understanding God; his understanding of astronomy and mathematics served as the basis for his explanation of philosophical questions. Gersonides was the first Jewish philosopher to use an analytic, scholastic method. His style was dry and concise and avoided the literary flourishes and enhancements used by Maimonides. He was a consistent philosopher: where philosophical theory conflicted with Jewish tradition, he took the philosophical view.

While Gersonides’ biblical commentaries became central to Jewish theology, his philosophical works met with opposition. Hasdai Crescas attacked them on philosophical grounds; orthodox Jews considered his ideas to be heresies and even perverted the title Wars of the Lord to “Wars With the Lord.”

Sefer Milhamot Ha-Shem, (The Wars of the Lord)

The Wars of the Lord is modeled after Maimonides’great work of Jewish philosophy, the Guide for the Perplexed, and may be regarded as a criticism of some elements of Maimonides' syncretism of Aristotelianism and rabbinic Jewish thought. The Wars of the Lord discussed topics on which Gersonides felt Maimonides had been inconclusive or insufficiently clear. In the introduction to Milhamot, Gersonides enumerated six questions which he hoped to examine: Is the rational soul immortal? What is the nature of prophecy? Does God know particulars? Does divine providence extend to individuals? What is the nature of astronomical bodies? Is the universe eternal or created? Gersonides firmly believed that reason could ultimately answer every question, and was not satisfied until he had discovered a reasonable explanation for every phenomenon. Where Jewish tradition conflicted with reason, he recommended a less literal interpretation, saying, "the Law cannot prevent us from considering to be true that which our reason urges us to believe."

Sefer Milhamot Ha-Shem comprises six discussions, each relating to one of the six questions:

1. Doctrine of the Soul.
2. Prophecy and the Omniscience of God.
3. and 4. Providence, God's Omniscience and Knowledge of Particulars
5. Celestial Substances.
6. Creation and Miracles



When persecution forced many Jews to leave Spain during the thirteenth century, Provence, in France, became a center for Jewish cultural and intellectual activity. The popes at Avignon were tolerant toward the Jews, and scholarship flourished. Provençal Jews learned of Aristotle through Averroes, the twelfth-century Spanish Muslim philosopher, whose commentaries had been translated from Arabic into Hebrew, and mainstream Jewish philosophy began to incorporate the thought of Averroes and Aristotle.

Gersonides spoke Provençal, but wrote all of his works in Hebrew. It is possible that he read Latin; he appears to have been familiar with contemporary Scholastic discussions. Gersonides was influenced by Averroes and Aristotle, and by Moses Maimonides, his greatest Jewish philosophical predecessor.

Omniscience of God

Gersonides apparently tried to reconcile Aristotle, who said that God did not know particulars (details of individual human lives) and Maimonides, who said that God did know particulars, by positing that God knew particulars, but only as they related to the natural order, for example, He knew individual persons only through knowing the species humanity.

In contrast to traditional Orthodox Jewish theology, Gersonides held that God limited His own omniscience concerning foreknowledge of human acts. God knew all the choices open to an individual in a particular situation, but He did not know which of the available alternatives the individual would choose. In this way Gersonides protected contingency and human free will. To explain how prophecy was possible when even God did not know the outcome of future events, Gersonides claimed that a prophet received only general knowledge of the future and was himself responsible to apply this knowledge to particular circumstances and events. Prophets were more attuned than ordinary persons to receive these messages from the Active Intelligence, and were in a historical position to apply them to specific events.

In book four, Gersonides explained that divine providence was also of a general nature, pertaining to a “species” or group rather than to a particular individual in that group. Divine providence was bestowed on a person according to the general species with which he was affiliated, for example the “species” of rabbis or philosophers.


Gersonides’ view of the soul resembled those of the Islamic philosopher Avicenna and of Avicebron. He defended the theory that an impersonal “reason” or “Active Intellect” mediates between God and man, and explained the formation of the higher reason (“acquired intellect”) in man. He posited that the soul is composed of two parts: a material, or human, intellect; and an acquired, or agent, intellect. The material intellect, inherent in every person, has the capacity to understand and learn; it is mortal, and dies with the body. The acquired intellect survives death, and contains the accumulated knowledge acquired during a person’s lifetime.

Gersonides agreed with Alexander of Aphrodisias that immortality consisted in the intellectual perfection of the material intellect, but differed in his concept of intellectual perception. Immortality was achieved when the content of the acquired intellect mirrored the Active Intellect by containing “a conception of the rational order obtaining in all individuals."

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: 81, JPS, 1984)


Gersonides rejected Aristotles’ arguments for the eternity of the universe, advancing evidence that the beginning of the universe must be due to the action of a superior agent. He maintained that though the universe had a beginning, it would exist for eternity, because the heavenly bodies, which were the source of life and motion, were not material and therefore not subject to decay. He criticized both creation “ex nihilo” and the theory of the existence of a primordial cosmic substance. Instead, he proposed that there had existed an inert form of matter which had only a potential existence until the moment that God bestowed essence, form, motion and life on it. All “sublunary beings” and heavenly substance proceeded from this matter, except for separate intelligences which emanated directly from God.

Mathematics and Astronomy

Gersonides wrote Book of Numbers in 1321 dealing with arithmetical operations, including extraction of roots. 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, a commentary on the first five books of Euclid.

He invented an instrument to measure the angular distance between celestial objects, Megalle ‘amuqqot ("Revealer of Profundities"), called Bacullus Jacobi ("Jacob's staff") by his Christian contemporaries. 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."

After observing a solar eclipse in 1337, Gersonides proposed a new theory of the sun which he proceeded to test by further observations. He observed the eclipse of the Moon on October 3, 1335, and 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 conclusions were inaccurate, 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.

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

The Wars of the Lord includes the finest study of trigonometry available in Western Europe at the time it was written, and 136 chapters devoted to astronomy and astrology. This section, also referred to as Sefer Tekunah, was not included in the publication of Sefer Milhamot Ha-Shem and still remains in manuscript form; however it was translated into Latin in 1342 at the request of Pope Clement VI. Gersonides exposed flaws in the systems of Ptolemy and Al-Bitruji, and elaborated his own view of the universe, supported by his astronomical observations. The work was completed in 1328 but was later revised with observations made up until 1340. The ninety-ninth chapter contained astronomical tables, and was frequently quoted by Pico della Mirandola (1463-1494).


During the twelfth and thirteenth centuries most Jewish and Christian philosophers accepted natural astrology, believing that the celestial bodies affect events on earth to at least some extent. Astrology, closely bound up with astronomical observations, was treated as a science until the seventeenth century. Gersonides was concerned with questions of astrological determinism, the extent to which the heavens exerted an influence over human actions, particularly those entailing human choice. He disagreed with Maimonides that there was no connection between the Neoplatonic “lunar” and “sub-lunar” spheres, asserting that both spheres were material and were therefore governed by the same principles. Gersonides argued that the heavenly bodies existed for the sake of the sublunar (earthly) universe, and guaranteed the perpetuation of life on earth. (Maimonides did not allow the possibility that a greater entity could exist for the sake of a lesser one, but Gersonides maintained that it was not inappropriate that the more noble exist for the sake of the less noble.) Gersonides listed twenty-seven problems of astronomy which he said could only be explained by assigning divine astrological purposes to the heavenly bodies. He believed that the "law, order and rightness" of the universe implied that the stellar influences emanated from God, the “First Separate Intellect.” The “agent intellect” functioned as the link between the celestial bodies and human affairs. Each celestial body had an “intellect” of its own which had knowledge of its own movement and influence, but not of the movements of the other heavenly bodies. A prophet or diviner could receive this knowledge of individual heavenly bodies through communication with the agent intellect, and could then interpret exactly how the various influences might act on each other and on earthly affairs. The wide variety of mixtures of stellar influences (ribbui hayahasim) guaranteed variety on earth. A person could, however, by exercising his freedom of choice, shatter a course of action which had been preordained for him by the celestial bodies.


  • Adlerblum, Nima H. A Study of Gersonides in His Proper Perspective, Kessinger Publishing, 2006.
  • Eisen, Robert. Gersonides on Providence, Covenant, and the Chosen People: A Study in Medieval Jewish Philosophy and Biblical Commentary, (S U N Y Series in Jewish Philosophy) University of New York Press, 1995.
  • Eisen, Robert. The Book of Job in Medieval Jewish Philosophy, Oxford University Press, USA, 2004.
  • Gershom, Levi Ben. The Wars of the Lord, (3 volumes) Jewish Publication Society of America, 1984.
  • Samuelson, N. M. Gersonides Wars of the Lord Treatise Three: On Gods Knowledge, (Mediaeval Sources in Translation). Institute of Mediaeval Studies, 1977.
  • Staub, Jacob J. The Creation of the World According to Gersonides, (Brown Judaic Studies 24). Scholars Press, 1982.
  • Wigoder, Geoffrey; Fern Seckbach. The Encyclopaedia Judaica, Encyclopaedia Judaica: The Most Comprehensive Authoritative Source on the Jewish World. Brill Academic Publications, 1999.

External links

All links retrieved December 11, 2013.

General Philosophy Sources


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