Euclid was a mathematician whose third century B.C. textbook *Elements* served as the western world's unchallenged standard for two millennia. Nothing is known about Euclid's life or physical appearance, and what little is known about his career comes from inferences in later sources. It is generally agreed that he taught geometry in Hellenistic Egypt, at Alexandria during the reign of Ptolemy I, between 305 and 285 B.C. He is credited with the thirteen volumes of *Elements*, a work that accumulated mathematical knowledge and codified it into a deductive system of proofs. Euclidean geometry was *the* geometry until the 19th century, when mathematicians began to challenge Euclid's assumptions about parallel lines, for example, when considering measurements over very large distances of, say, billions of light years.

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