Philosophy Before Socrates by Richard D. McKirahan

Although another challenging read, McKirahan does an exceptional job of explaining the thinking of these "Pre-Socratics" as they are called.  Although reading the fragments of the original philosophers themselves often frustrates, his synopses make them quite easy to comprehend.

And although much of the content of these early philosophies have long since been debunked, it's fascinating to witness their striving to figure out the world around them.  Can you imagine even trying to explain our universe without the benefit of the intervening millennia of science and research?

These brave souls were among the first to put mythology aside (often running afoul of their communities) and attempt to find more logical explanations for what we see all around us.

One amazing example is that of Leucippus and Democritus, two fifth century BC Greeks who developed the theory of atomism, the idea that all matter consists of tiny "atoms" (which is Greek for indivisible) too small to be seen.  And although the details of their atomic model were wildly incorrect, I find it amazing that they struck upon this concept more than 2,600 years ago.

Lastly I'll point out that the existence of these Pre-Socratics indicates just how far civilization had advanced by this stage.  For the first time society had advanced to the stage where a small community of people could spend their time thinking, rather than serving more functional aspects of their economies.  

Just as Egyptians were among the first to grow a surplus of crops, allowing a few to spend their time creating art and other pursuits not directly related to survival, these Greeks took it to another level by just thinking.

Civilization was on the verge of rapid acceleration in the decades that followed.