astronomy

Gersonides

Gersonides is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Gersonides rather than just read about it. In short: Levi ben Gershon (1288 – 20 April 1344), better known by his Graecized name as Gersonides, or by his Latinized name Magister Leo Hebraeus, or in Hebrew by the abbreviation of first letters as RaLBaG, was a medieval French Jewish philosopher, Talmudist, mathematician, physician and astronomer/astrologer. He was born at Bagnols in Languedoc, France.

Key takeaways

Gersonides belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Gersonides to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Gersonides from memory before moving on to harder problems.

Reference excerpt

Levi ben Gershon (1288 – 20 April 1344), better known by his Graecized name as Gersonides, or by his Latinized name Magister Leo Hebraeus, or in Hebrew by the abbreviation of first letters as RaLBaG, was a medieval French Jewish philosopher, Talmudist, mathematician, physician and astronomer/astrologer. He was born at Bagnols in Languedoc, France. According to Abraham Zacuto and others, he was the son of Gerson ben Solomon Catalan.

Biography 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 advancement to a higher position or office. 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. Gersonides is known for his unorthodox views and rigid Aristotelianism, which eventually led him to rationalize many of the miracles in the Bible. His commentary on the Bible was sharply criticized by the most prominent scholars, such as Abarbanel, Chisdai Crescas, and Rivash, the latter accusing him of heresy and almost banning his works.

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. Ralbag's treatise strictly adhered to Aristotelian 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 decide 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 commentaries on the Pentateuch, Joshua, Judges, I & II Samuel, I & II Kings, Proverbs, Job, Ecclesiastes, Song of Songs, Ruth, Esther, Daniel, Ezra-Nehemiah, and Chronicles. He makes reference to a commentary on Isaiah, but it is not extant.

Views on God and omniscience In contrast to the theology held by other Jewish thinkers, Jewish theologian Louis Jacobs argues, Gersonides held that God does not have complete 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." 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".

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 Acharonim, 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. Rabbi Yeshayahu 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 his 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.

Talmudic works Gersonides was the author of the following Talmudic and halakhic works:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Gersonides

Start with the simplest possible case. Write down what Gersonides claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Gersonides before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Gersonides ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Gersonides

In research: Gersonides appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Gersonides in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Gersonides is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1288 births, 1344 deaths, 14th-century French mathematicians, so understanding it makes those chapters shorter.
In everyday life: Look for Gersonides outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Gersonides in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Gersonides means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Gersonides out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Gersonides in simple terms?

Why does Gersonides matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Gersonides?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Gersonides.