Pre-Socratic philosophy is a title that applies generally to the philosophical views of ancient Greek philosophers up until the time of Socrates (469–399 B.C.). Several thinkers comprise the group known as the “Pre-Socratic philosophers” including Thales, Pythagoras, Anaximander, Anaximenes, Xenophanes, Empedocles, Democritus, Parmenides, and Heraclitus.
In general, Pre-Socratic philosophy was concerned with offering rational and logical explanations as to what the world was made of and how it worked. That is why the Pre-Socratic philosophers are often referred to as “cosmologists” (cosmos means “world). Thales, for instance, argued that the primary substance of the world was water. Anaximander said it was fire. Anaximenes said air, and Xenophanes said earth. Empedocles said reality consisted of all four elements. Some of the Pre-Socratics anticipated future scientific discoveries and theories. Democritus, for example, posited a view of atoms and held that atoms and void were the only two realities. Anaxagoras promoted a view similar to that of the Big Bang theory of today. Anaximander held to a crude form of the theory of evolution.
With the exception of Parmenides (b. 515 B.C.), most of the writings of the Pre-Socratic philosophers have been lost. Much of what we know today about them comes from the writings of Aristotle. The Pre-Socratics have usually been lumped together as a group, but in the last two centuries some of them have emerged as important philosophers in their own right. With the coming of Socrates and the Sophists philosophy turned away from cosmology to ethical and social matters.