Socrates (470 – 399 BCE) was an ancient Greek philosopher that fundamentally reshaped Western philosophy. At a time when natural philosopher was a big focus, Socrates shifted focus toward human life and ethics. He was a big proponent of “know thyself”, encouraging people to look inward to identify the rightness of their thoughts and actions.
He was not a big writer, and spent most of his time having debates with people. These debates were his way to understand himself and others better, encouraging a culture of reasoning and logic. You may be familiar with these discussions through the Socratic method. With all of this, he set the foundations for rationalistic thought, especially as it pertains to morality and ethics.
Most of his teachings nowadays are immortalized by his more devoted student, Plato.