The Greek philosopher Pythagoras (approx. 570—495 BCE) was the founder of the metaphysical-religious movement of Pythagoreanism.

Pythagoras is best known today for his work in mathematics, namely the Pythagorean theorem which still bears his name, but he is also said to be the first to call himself a philosopher.

However, Pythagoras was also the founder of a religious movement, which included some irrational beliefs that cannot accurately be called philosophical in their formation or expression. His early group of followers resembles a cult more than a school.

The Pythagorean Soul

Among the religious beliefs that Pythagoras, and later Pythagoreans, promoted was the immortality of the soul, as well as transmigration of the soul between different animal species. Pythagoras reportedly believed in a sort of reincarnation, that one’s soul would occupy another body after death. These were not exclusively human bodies, but rather the bodies of any sort of animal. Thus, Pythagoras had the view that all animals (and perhaps even plants) had souls. There is also evidence that he may have also believed that the sort of animal one’s soul would be transferred into depended on one’s actions, perhaps from a moral standpoint.

Pythagoras’ belief about the immortality of the soul was an influence on Plato.

Vegetarianism

Those practicing the religious and philosophical beliefs of Pythagoras were vegetarians. Pythagoreanism has a prohibition on the killing of animals because other animals, like humans, have souls — souls which may have previously been human. The immortality of the soul also gives souls a divine quality, and so animals are, for Pythagoreans, partly divine.

Pythagorean theorem

Pythagoras is best known today for his Pythagorean theorem, which describes a relation between the three sides of a right triangle. Specifically, in a triangle where one angle is a right angle (90°), the squared length of the side opposite this angle (the hypotenuse) is equal to the sides squared length of the two other sides.

The theorem is formalized as a² + b² = c², where a and b are the lengths of the two sides which make up the right angle, and c is the length of the hypotenuse.

Pythagoras is credited with the discovery and proof of this theorem, though it has also been documented that Babylonian and Indian mathematicians may have reached the same discovery earlier.