Pythagoras
Greek mathematician and philosopher in the 6th century B.C., known for the Pythagorean Theorem.
We have no known texts by Pythagoras, but he founded one of the most influential and devoutly followed schools in pre-Socratic Greek thought.
Pythagoras is sometimes considered to be the pupil of Anaximander and is reputed by very early sources to have visited Thales in his twenties, just before Thales died. There is no account of the specifics of the meeting, other than the report that Thales recommended that Pythagoras travel to Egypt in order to further his philosophical and mathematical training. There is certainly evidence that the Egyptians had advanced further than the Greeks of their time in Mathematics and Astronomy and it is now widely believed that Egyptians used the Pythagorean Theorem in some of their architectural projects before the 6th century B.C.
It is sometimes difficult to determine which ideas are original to Pythagoras and which are latter additions by his followers. However, there is general agreement that Pythagoras either developed the Pythagorean Theorem himself or at the very least introduced it to Greek thought. In addition to the Pythagorean Theorem, it there is general agreement that the numerical ratios which determine the musical scale trace back to a discovery by Pythagoras himself, since this plays a key role in many other areas of the Pythagorean tradition, and since there is no evidence of earlier Greek or Egyptian musical theories.
Pythagorean thought was dominated by mathematics, but it was also profoundly mystical. In the area of cosmology there is less agreement about what Pythagoras himself actually taught, but most scholars believe that the Pythagorean idea of the transmigration of the soul, is too central to have been added by a later follower of Pythagoras. On the other hand it is impossible to determine the origin of the Pythagorean account of substance. It seems that the Pythagorean account begins with Anaximander's account of the ultimate substance of things as "the boundless." Another of Anaximander's pupils, Anaximenes, who was a contemporary of Pythagoras, gave an account of how Anaximander's "boundless" took form, through condensation and refraction. On the other hand, the Pythagorean account says that it is through the notion of the "limit" that the "boundless" takes form. This is important, because it reveals one of the key beliefs of the Pythagorean school -- namely that all things are in essence numbers.