Known by students today for the Pythagorean theorem (the square of the length of the hypotenuse of a right triangle is equal to the sum of the squares of the lengths of the other two sides), Pythagoras (c. 580–c. 500 B.C.) was a Greek philosopher and mathematician who lived in the sixth century B.C. and whose followers kept Pythagoreanism alive into the middle of the fourth century B.C. Religious in nature (some have referred to the society as a “cult”), Pythagoreans believed in the functional and even mystical significance of numbers and made considerable advances in mathematics and astronomy. Pythagoras left no writings, which has prompted some scholars to believe that no such person ever existed, but rather, any doctrines ascribed to him are actually attributable to a group of people. Whatever the case, the Pythagorean legacy is real. It includes: the word *calculus* (Pythagoreans used lines, triangles, and squares made of pebbles to represent numbers; the Latin word for pebble is *calculus);* the rendering of astronomy and music in numerical patterns (which were studied as mathematical subjects); and the first suggestion (as early as the sixth century B.C.) that Earth is spherical (not flat) and that Earth, the moon, and the planets revolve around the sun.