Ancient Greek philosophy was a tradition of philosophical inquiry that began in the 6th century BCE and continued throughout the Hellenistic period. It was characterized by the use of reason and critical thinking to examine fundamental questions about existence, ethics, knowledge, and values. Key figures in ancient Greek philosophy include Thales, Pythagoras, Heraclitus, Parmenides, Empedocles, Anaxagoras, Socrates, Plato, and Aristotle. These philosophers made significant contributions to fields such as metaphysics, ethics, politics, and epistemology, and their ideas have had a lasting impact on Western thought.