Pre-Socratic philosophy

The Pre-Socratic Greek philosophers were active before Socrates. The popular usage of the term come from Hermann Diels' work "Die Fragmente der Vorsokratiker" ("The Fragments of the Pre-Socratics", 1903).
Most of what we know about the pre-Socractic philosophers come from quotations by later philosophers and historians. While most of them produced significant texts, none of the texts have survived in complete form.
The standard reference works in English are:
The fundamental idea which motivated most of the presocratics (as they are called) is naturalism. This is the idea that questions about life and the world can be answered without using myths, and that "the natural world is the whole of reality". 
With the Greeks we see rational thought and scientific reasoning emerge from the mists and myths of a pre-scientific age, not suddenly, but slowly and gradually.