Ancient Greek Philosophy Part 1 - Thales of Miletus
Thales of Miletus(624–546 BC) was considered by Aristotle, among others, to be the first philosopher in the Greek tradition. Thales (along with Socrates and many others) is often accociated with the phase "Know thyself." This phrase is perhaps the best known of the 147 Delphic Maxims inscribed into the temples of Delphi. I find it to be one of the fundamental tenets of philosophy. Without it, philosophy can become little more than a quest to justify one's own opinion. Knowing when your own mind ends and reality begins is tantamount to clearly perceiving reality. Alternatively, Thales is quotes as saying "the most difficult thing in life is to know yourself." I agree with this less. Knowing youself is a stepping stone to knowing the world. Thus I consider knowing the world to be much more difficult than knowing youself.
Thales of Miletus lived in a time when humans had just stumbled across the concept of metaphysics. Metaphysics had recently replaced mythology as the primary method for explaining why and how the world exists. A regular topic of discussion at the time was attempting to identify the universe's quintessential substance. It was thought that the material world was ultimately all made up of one thing. Thales thought this universal substance was water. Anaximenes, a contemporary philosopher of the same Milesian school as Thales, thought it was air. Others had other opinions.
This question has still not been fully answered by modern science. Half a century ago we thought the answer was atoms. Then we thought it was quarks. But we are still searching for that elusive elementary particle or structure of the universe. What would the Milesian philosophers think if we went back and told them we've decided that the quintessential substance of the universe is string? String theory is just the most recent in a long line of attempts to answer this fundamental question of metaphysics posed so long ago.
Sadly, none of the philosophical works of Thales have survived to the current day, but he is still remembered for his advancements in the field of geometry, including the Thales' theorems.
Part 2 - Heraclitus of Ephesus