The Pre-Socratic philosophers were early thinkers in ancient Greece who lived before Socrates. They focused on understanding the nature of the universe, existence, and the fundamental principles of reality. Key figures included Thales, who proposed that water is the essence of all things, and Heraclitus, known for his idea that change is constant. These philosophers laid the groundwork for later philosophical thought by shifting the focus from mythology to rational inquiry. Their explorations into topics like the nature of matter, the cosmos, and the origins of life influenced subsequent philosophers, including Plato and Aristotle.