Socrates (c. 470–399 BC)

Cicero later said that Socrates brought philosophy from heaven to earth. That's right: the pre-Socratic philosophers were often concerned with observations of nature, including astronomy (so they were concerned with "heaven"). They were the pioneers of what later became natural science, which was a great achievement. Socrates, on the other hand, brought a new element to philosophy: he conducted practical conversations with citizens on the streets of Athens. He asked them questions, making them think and realize how much they actually think they know but don't know anything. This critical questioning is still an important part of philosophy and science today. Socrates' method is a model for today's didactics and professional advice. Socrates was also the teacher of Plato, who, alongside Aristotle, became the most influential philosopher of antiquity. Socrates was executed in old age because he was unwilling to renounce his teachings. He died a martyr.