Is it ensure, insure, or assure?
Greek mathematician whose book, Elements, was used continuously until the 19th century. In it he organized and systematized all that was known about geometry. Euclid's systematic use of deductions and axioms was widely regarded as a model working method and influenced mathematicians and scientists for over two thousand years.
An ancient Greek mathematician; the founder of the study of geometry. Euclid's Elements is the basis for modern school textbooks in geometry. One of the basic statements, or postulates, of Euclid's geometry is that if a line and a point separate from it are given, only one line parallel to the first line can pass through the point.
Note: Albert Einstein used other approaches to geometry to derive the theory of relativity. These “non-Euclidean geometries” deny Euclid's postulate about parallel lines.