Mathematician:Levi ben Gershon

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French Jewish philosopher, Talmudist, mathematician, physician and astronomer/astrologer.

Notable for publishing an early proof using the principle of mathematical induction.

Anticipated Galileo's error theory.

One of the first astronomers to estimate the distance of the fixed stars to a reasonable degree of accuracy (of the order of $100$ light years).

Refuted Claudius Ptolemy's model of astronomy by direct observation, paving the way for the new model of Nicolaus Copernicus more than $2$ centuries later.

Was involved in a lively debate about Euclid's $5$th postulate, and whether it could be derived from the other $4$.




  • Born: 1288 in Bagnols now Bagnols sur C├Ęze, Provence, France
  • Died: 20 April 1344 in Avignon, France

Theorems and Definitions


  • 1317 -- 1329: Sefer Milhamot Ha-Shem ("The Wars of the Lord")
  • 1321: Ma'aseh Hoshev ("Work of Calculation", "Art of Calculation" or "Art of the Computer", or, by means of a Hebrew pun: "Clever Work")
(often confused with Sefer ha-Mispar ("The Book of Number"), by Abraham ibn Ezra)
  • 1342: A portion of Sefer Milhamot Ha-Shem containing a survey of astronomy translated into Latin
  • Two geometry books:
    • A commentary and introduction to the first five books of Euclid, but not presented axiomatically
    • Science of Geometry of which only a fragment has survived
  • A number of non-mathematical works

Also known as

Some sources give his name as Levi ben Gerson, and others as Levi ben Gershom.

Better known by:

the Greek form of his name: Gersonides
his Latinized name (Magister) Leo Hebraeus
in Hebrew as RaLBaG (by the abbreviation of first letters of Rabbi Levi Ben Gershon with vowels added for ease of pronunciation)

Less commonly he can be seen referred to as Gersoni, Leo de Bagnols, Leo de Balneolis or Leo Judaeus.

It could be complicated identifying someone specifically before the invention of surnames.