Probably the ancient Egyptians who discovered that the diagonal of a unit square was not a rational number. And then discovered other such numbers.
There is no single mathematician. It has been known since ancient times that the length of the diagonal of a unit square is sqrt(2) which is irrational.
It was known from ancient times that pi is irrational. However, that fact was proven in 1761 by the Swiss scientist Johann Heinrich Lambert.
Pythagoras was an ancient philosopher that attributed most of his work to Plato. He however, never went to jail.
The history of rational numbers goes way back to the beginning of historical times. It is believed that knowledge of rational number precedes history but no evidence of this survives today. The earliest evidence is in the Ancient Egyptian document the Kahun Papyrus. Ancient Greeks also worked on rational numbers as a part of their number theory. Euclid's elements dates to around 300 BC. Indian mathematicians also worked on rational numbers. This is documented in different texts but the most important is probably the Sthananga Sutra which dates back to around the second century BC.
They were known to the ancient civilisations. Irrational numbers are supposed to have been discovered by Hippassus of Metapontum, a member of the Pythagorean school in ancient Greece (5th Century BC).
Probably the ancient Egyptians who discovered that the diagonal of a unit square was not a rational number. And then discovered other such numbers.
Yes, Euclid was a philosopher of Ancient Greece.
Hipparchus, ancient greek philosopher
An ancient Greek mathematician in the era of Pythagoras is said to have been murdered by his fellow secret society members for discovering the (now well-known) proof that square root of 2 is irrational.
Plato was the ancient Greek philosopher who was the focus of study for Italian humanists.
Confcius
Socrates.
A philosopher from ancient Egypt.
Confucius.
Homer was a poet and storyteller, not a philosopher - his role was entertainment.
The ancient philosopher known in the Western world as Confucius (whose original name was close to Kung Fu Tse) was relied upon by the government of ancient China.