Euclid (Εύκλείδες, c. 300 BCE) was a Greek mathematician. He worked in Alexandria at the Museum founded by Ptolemy I. He systematized the geometric and arithmetic knowledge of his times in thirteen Books—Euclid's elements (Στοιχεία).
Nothing else is known of Euclid's life. That he lived and worked in Alexandria in the days of Ptolemy I is reported to us by the Greek philosopher Proclus (c. 410–485 CE) in his "summary" of famous Greek mathematicians. Probably Euclid is older than Archimedes (c. 290/280–212/211 BCE).
Besides the Elements, Euclid wrote numerous other books. We have the texts of four others, Data, On Divisions of Figures, the Phaenomena, and the Optics. Data is a collection of 94 geometric propositions, On Division of Figures deals with problems of dividing a given figure by one or more straight lines into others with desired properties of shape and area. It was used by Leonardo of Pisa in his Practica geometriae of 1220. The Optics is the first Greek treatise on perspective, with geometrical propositions on the rectilinear propagation of light. The Phaenomena is an introduction to mathematical astronomy. Since it closely resembles the style of the earlier treatise "On the Moving Sphere" by Autolycus (c. 330 BCE), historians have concluded that Euclid did not necessarily invent the form of his works.
From Greek commentators one learns of several books that have been lost. They include Pseudaria (on fallacies); Porisms (on conditions determining curves); Conics (which was superseded by a similar work of Apollonius); Surface Loci (perhaps dealing with cones, spheres, and cylinders, or with curves on these surfaces); Elements of Music (possibly including the Pythagorean theory of harmony); and Catoptrics (on the properties of mirrors). A surviving Catoptrics bearing Euclid's name is in reality a later compilation—possibly by Theon of Alexandria (c. 350 CE)—but it probably is based upon an authentic Euclidean work of the same name and in the same form. Arabic writers also attribute to Euclid various treatises on mechanics including books on the balance and on specific gravity.