Often called the “Father of Geometry,” Euclid’s influence is so pervasive that his book, The Elements, was the primary math manual for over 2,000 years. It was not a “textbook” in the modern sense, as it did not have problem sets, word problems, or diagrams for practice, but it was a foundational reference and logical guide that students had to read, memorize, and demonstrate.

In the Western and Islamic worlds, Euclid’s Elements was the standard geometry text for almost every educated person until the 19th century. Before Euclid, mathematical knowledge was often a collection of disconnected rules and observations, but Euclid introduced the axiomatic method, which begins with a small set of “self-evident” truths called axioms and postulates and uses deductive reasoning to build an entire system of complex theorems.

Beyond geometry, Euclid also made significant contributions to number theory. In The Elements, he provided the first proof that prime numbers are limitless, which remains a cornerstone of arithmetic today. His “Euclidean algorithm” is an efficient method for finding the greatest common divisor of two integers, and students still learn it today, while modern computer scientists and cryptography experts put it to use.

Euclid left a timeless foundation, although modern mathematicians now know that his rules of flat, three-dimensional geometry do not apply everywhere in the universe. Instead of being flat and predictable, space can be curved, warped, or twisted because of gravity and other factors altering the properties of distance, angles, and parallel lines. Nevertheless, Euclidean geometry remains the most accurate way to describe the flat space of everyday human experience.