Heraclitus was a contemporary of Pythagoras, Lao-tzu, Confucius, and Siddhartha, the Buddha; some say that the term "philosophy", love of wisdom, was first introduced by Pythagoras, who lived from approximately 580 BCE to 500 BCE. Pythagoras and his followers, the Pythagoreans, argued numbers to constitute the true nature of things and all relations to be numerical. Pythagoras is often regarded as the founder of modern mathematics and geometry. The Pythagorean Theorem states that in a right-angled triangle, the square of the hypothenuse (opposite side) is always equal to the sum of the squares of the other two sides. The Pythagoreans ... argued that earth was a sphere revolving around the sun in a predictable way.