Pythagoras

As we discuss ancient Greece, I remembered one interesting philosopher from the time period, Pythagoras, who in addition to (maybe) making formulas about right triangles, also founded a weird number based cult. Pythagoras was born around 569 BC, and interacted with some of the earliest Greek philosophers, such as Thales. Influenced by some of these philosophers as well as the practices of Egyptian priests, he founded what was basically a cult that was equal parts philosophical, mathematical, and religious. He established a communal, secretive group based around his teachings, at the center of which was the idea that the universe is based on numbers and proportions. Some of the group's practices included strict vegetarianism, not wearing leather or other animal products, not eating beans, and doing lots of geometry (it must have been a tough pitch to get people to join).

His beliefs about numbers are apparently based on the fact that things like music and astronomy are based on ratios, so the whole universe might as well be too. His love of ratios did lead to important contributions in recognizing their importance in those fields, probably helping develop Greek music theory, for example. As far as Pythagoras' actual contributions to mathematics, it's difficult to tell what he did himself, as the society he founded was communal and work on problems was shared openly, so we don't really know if Pythagoras really proved the Pythagorean theorem. Beyond a few geometry problems the Pythagoreans solved, their main contribution to math was probably as one of the first groups to view numbers as abstract objects, divorced form physical meaning (The concept of the number 2 vs. counting 2 physical objects), as well as their belief in other perfect mathematical objects like triangles and circles that could exist abstractly, outside of physical representations. They probably took this too far with worshipping numbers and claiming they made up the entire universe, but these contributions were still important. So if you ever wondered in math class who this Pythagoras guy actually was, the sort of surprising answer is the founder of a number-based, religious cult from 2500 years ago.