Pre-Socratic Philosophy
Presocratic Philosophers
Socrates is such a well-known historical figure that he makes a very convenient marker in the annals of philosophy. There were men before and after. Those before are known as Pre-Socratics. Those contemporary start with Plato. The Pre-Socratics examined how the universe was created and what were the essential elements. It is sometimes hard to distinguish what makes their work philosophy instead of science or mathematics -- because the fields were not divided up like that yet. Famous Pre-Socratics include Thales, Pythagoras, and Empedocles.
Also see Pre-Socratic Philosophy, for background on the material discussed rather than the individual thinkers.
Timeline of Greek and Roman Philosophers
A timeline of major ancient Greek and Roman philosophers with links to more information on the individual philosophers.
Overcoming Fear of Ancient Philosophy
Perfect understanding is impossible, but an acquaintance can be useful -- even profitable.
Atomism
Atomism might sound like a branch of physics, but at the time, it was a philosophical explanation of the questions about the universe that mattered, rather than a mathematics-related scientific specialty. Atomism explained that all of nature is composed of indivisible units called atoms and the void.
Democritus of Abdera
Democritus of Abdera was an ancient Greek atomist who worked on the theory of indivisible particles.