Pre-Socratic philosophers marked the beginning of Western philosophy in ancient Greece. They sought rational explanations for natural phenomena, challenging traditional beliefs and laying the groundwork for later thinkers like Socrates, Plato, and Aristotle. Key figures include Thales, Anaximander, Heraclitus, and Parmenides. Their ideas on the fundamental nature of reality, change, and the universe had indirect implications for political thought, emphasizing reason and universal principles over traditional authority.