Gersonides

Quick Facts

• Name: Levi ben Gershom
• Also called: Gersonides, Ralbag, Magister Leo Hebraeus
• Lived: 1288-1344
• Place: Provence, especially Orange and Avignon
• Main roles: Jewish philosopher, biblical commentator, mathematician, astronomer, astrologer
• Main tradition: medieval Jewish Aristotelianism
• Best known for: reason and Torah, divine knowledge, providence, prophecy, the Active Intellect, creation, astronomy

The Big Question

How far should a religious thinker follow reason when reason seems to revise familiar religious beliefs?

Gersonides' answer is unusually bold for a medieval Jewish philosopher: if reason really proves something, Scripture must be interpreted so it agrees with that truth. Torah and reason cannot finally contradict each other, but the literal reading of a verse may need to change.

In One Minute

Gersonides was a fourteenth-century Jewish thinker from Provence. He wrote in Hebrew and worked across philosophy, Bible commentary, mathematics, astronomy, and astrology.

His main project was to show that Judaism can be understood through disciplined reason. He admired Aristotle, read Aristotle through medieval commentators such as Averroes, and pushed the rationalist path opened by Moses Maimonides.

He is famous because he accepted hard conclusions. To protect freedom, he denied that God knows future free choices as already-set facts. To explain creation, he rejected both an eternal universe and strict creation from nothing. To explain providence, he treated God's care as working through the order of nature and intellect.

What They Taught

Gersonides taught that reason is a gift from God and must be used honestly. Revelation means divine teaching, especially in Torah. Reason means the human power to define terms, follow evidence, and test arguments. If an argument is only clever, Scripture need not bend to it. But if reason gives a real demonstration, or strict proof, then the literal reading of Scripture cannot be the final meaning.

This made him more systematic, and often more daring, than Maimonides. Maimonides sometimes leaves difficult questions partly hidden in the Guide for the Perplexed. Gersonides usually states the consequence directly. The Wars of the Lord asks what follows if Jewish teaching, philosophy, and the sciences of his day are all taken seriously.

His most controversial teaching concerns divine knowledge and human freedom. A future contingent is an event that really could go more than one way, such as whether a person will keep a promise tomorrow. Gersonides argues that if God already knew that exact future choice as a fixed fact, the choice would not be open. So he says God knows the whole rational order of the world, all possibilities in it, and all events once they are actual, but not future free choices as already settled facts.

Providence means God's care or guidance over creatures. Gersonides does not picture providence mainly as God adjusting every private event. It works through nature and intellect. A person who understands causes is better able to avoid harm. Someone who understands medicine, weather, or political danger can act more wisely than someone who ignores the order of things.

The intellect is central to his view of the soul. In medieval Aristotelian psychology, the Active Intellect is not one person's mind. It is a separate, non-bodily intelligence that helps human minds grasp universal truths. Through learning, a person builds an acquired intellect: the knowledge actually gained. Immortality, for Gersonides, is tied to this acquired intellect rather than to ordinary personality in a simple modern sense.

On creation, he chooses a middle path. He rejects Aristotle's view that the world has always existed. He also rejects strict creation from nothing. God forms the ordered world from prime matter, meaning matter with no definite shape or active qualities of its own. The world depends on God for its order and form.

His astronomy and astrology belong to the same rational project. Astrology means the claim that heavenly bodies affect events on earth. Gersonides tried to explain this naturally through light, motion, and celestial order. But human freedom could still break the pattern, so a prediction could describe a tendency without forcing a person to obey it.

Key Ideas With Examples

• Religious rationalism: religious truth and sound reason must agree. Example: if a verse seems to give God a body, a rationalist reads it figuratively because God cannot be a physical object.

• Demonstration: strict proof, not a guess or a clever interpretation. Example: a geometrical proof is stronger than a persuasive speech. Gersonides thinks theology should respect that difference.

• Future contingent: a future event that is still open. Example: if you can freely tell the truth or lie tomorrow, that choice is contingent until you make it.

• Divine knowledge: God's knowledge of the world's rational order. Gersonides says God knows possibilities and ordered patterns, but not future free choices as already fixed events.

• Providence: God's care through nature and intellect. Example: a wise person who understands causes can avoid danger better than someone who ignores them.

• Active Intellect: a separate intelligence that makes universal truths knowable to human minds. Example: a student can potentially understand a proof; when the proof becomes clear, the mind has moved from potential knowing to actual knowing.

• Acquired intellect: the knowledge a person actually gains. Example: learning mathematics or astronomy is not just useful skill; for Gersonides it helps perfect the soul.

• Prime matter: formless matter that can receive structure. Clay is only a rough analogy, because clay is already shaped and physical. Prime matter is the bare possibility of bodily form.

• Natural astrology: the medieval idea that heavenly bodies affect earthly life through natural causes. The sun causes heat and seasons; Gersonides extends that kind of causal thinking to other heavenly bodies.

Major Works

• The Wars of the Lord / Sefer Milhamot Ha-Shem (completed 1329): his main philosophical work. It treats the soul, prophecy, divine knowledge, providence, heavenly spheres, creation, miracles, and how to recognize a true prophet.

• Biblical commentaries: major commentaries on the Torah and several biblical books, including Job and Song of Songs. They often turn biblical stories into lessons about knowledge, virtue, providence, and happiness.

• Ma'aseh Hoshev (1321): a mathematical work on arithmetic, roots, series, binomial coefficients, and combinatorics. It shows his rationalism in technical practice, not just theology.

• On Sines, Chords, and Arcs (1342): a trigonometric work with a proof of the sine law for plane triangles and sine tables.

• Astronomical writings and instruments: Gersonides observed eclipses, criticized parts of Ptolemaic astronomy, and described a measuring instrument often called Jacob's staff.

Why It Matters

Gersonides matters because he shows the strength and the risk of medieval rationalism. He does not treat reason as a decorative extra added to faith. He lets it reshape doctrines about God's knowledge, providence, creation, prophecy, and the soul.

He also matters for the history of science. He measured, calculated, criticized astronomical models, and wrote technical mathematics. He was a philosopher-scientist, not just a commentator.

For philosophy of religion, he keeps a live problem sharp: can God know every future choice while those choices remain free? Gersonides says no. Many later thinkers reject that answer, but the problem remains.

Proponents, Critics, and Opponents

Gersonides develops the rationalist side of Moses Maimonides, but he is often less cautious. He inherits much from Aristotle and from Averroes' Aristotelian commentaries, while rejecting Aristotle's eternity of the world.

He belongs near the wider medieval culture of Scholasticism, where Jewish, Christian, and Islamic thinkers debated reason, revelation, science, and theology. His own works were written in Hebrew.

The strongest Jewish pushback came from Hasdai Crescas and later critics who thought Gersonides gave too much authority to Aristotelian science and weakened divine knowledge, providence, miracles, or traditional belief. Crescas especially attacks the Aristotelian framework that Gersonides represents.

Related Pages

• Moses Maimonides
• Aristotle
• Hasdai Crescas
• Scholasticism
• Aristotelianism
• Guide for the Perplexed