**Euclid** was a Greek mathematician, often referred to as the “Father of Geometry”. He was active in Alexandria during the reign of Ptolemy I (323–283 BC). His *Elements* is one of the most influential works in the history of mathematics, serving as the main textbook for teaching mathematics (especiallygeometry) from the time of its publication until the late 19th or early 20th century.^{} In the *Elements*, Euclid deduced the principles of what is now called Euclidean geometry from a small set of axioms. Euclid also wrote works on perspective, conic sections,spherical geometry, number theory and rigor.

“Euclid” is the anglicized version of the Greek name Εὐκλείδης, meaning “Good Glory”.^{
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Very few original references to Euclid survive, so little is known about his life. The date, place and circumstances of both his birth and death are unknown and may only be estimated roughly relative to other figures mentioned alongside him. He is rarely, if ever, referred to by name by other Greek mathematicians from Archimedes onward, who instead call him “ό στοιχειώτης” (“the author of Elements”). The few historical references to Euclid were written centuries after he lived, by Proclus c. 450 AD and Pappus of Alexandria c. 320 AD.^{
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Proclus introduces Euclid only briefly in his *Commentary on the Elements*. According to Proclus, Euclid belonged to Plato’s “persuasion” and brought together the *Elements*, drawing on prior work by several pupils of Plato (particularly Eudoxus of Cnidus, Theaetetus and Philip of Opus.) Proclus believes that Euclid is not much younger than these, and that he must have lived during the time of Ptolemy I because he was mentioned by Archimedes (287-212 BC). Although the purported citation of Euclid by Archimedes has been judged to be an interpolation by later editors of his works, it is still believed that Euclid wrote his works before those of Archimedes.^{
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Proclus later retells a story that, when Ptolemy I asked if there was a shorter path to learning geometry than Euclid’s *Elements*, “Euclid replied there is no royal road to geometry.” This anecdote is questionable since it is similar to a story told about Menaechmus and Alexander the Great.^{
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In the only other key reference to Euclid, Pappus briefly mentioned in the fourth century that Apollonius “spent a very long time with the pupils of Euclid at Alexandria, and it was thus that he acquired such a scientific habit of thought” circa 247-222 BC.^{
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A detailed biography of Euclid is given by Arabian authors, mentioning, for example, a birth town of Tyre. This biography is generally believed to be completely fictitious.^{
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Partly due to the lack of biographical information that is somewhat atypical for the period (extensive biographies are available for most significant Greek mathematicians for several centuries before and after Euclid), it has been proposed that Euclid was not, in fact, a historical character, and his works were written by a team of mathematicians (not unlikeBourbaki) who took the name Euclid from the historical character Euclid of Megara. However, this hypothesis is not well accepted by scholars and there is little evidence in its favor.^{}^{
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Although many of the results in *Elements* originated with earlier mathematicians, one of Euclid’s accomplishments was to present them in a single, logically coherent framework, making it easy to use and easy to reference, including a system of rigorous mathematical proofs that remains the basis of mathematics 23 centuries later.

There is no mention of Euclid in the earliest remaining copies of the *Elements*, and most of the copies say they are “from the edition ofTheon” or the “lectures of Theon”, while the text considered to be primary, held by the Vatican, mentions no author. The only reference that historians rely on of Euclid having written the *Elements* was from Proclus, who briefly in his *Commentary on the Elements* ascribes Euclid as its author.

Although best known for its geometric results, the *Elements* also includes number theory. It considers the connection between perfect numbers and Mersenne primes, the infinitude of prime numbers, Euclid’s lemma on factorization (which leads to the fundamental theorem of arithmetic on uniqueness of prime factorizations), and the Euclidean algorithm for finding the greatest common divisor of two numbers.

The geometrical system described in the *Elements* was long known simply as *geometry*, and was considered to be the only geometry possible. Today, however, that system is often referred to as *Euclidean geometry* to distinguish it from other so-called *non-Euclidean geometries* that mathematicians discovered in the 19th century.