Our information about Plato’s predecessors in philosophy, who we call Presocratic philosophers, is limited by the vast abyss between our worlds - the Holy Roman empire took down with it a large share of the literature and the material culture of its age. As the rest of the West succumbed to Christian powers, imitations of surviving literature by monks were all that remained of antique philosphy. They are most of what we have today from that age.
Doxography involves identification of fragments, which are quotations from Presocratic philospohers by later commentators, and testimonia which are reports or paraphrases by later writers. However doxographers have to be careful in sieving out unfaithful accounts, which is almmost always difficult.
Presocratics
The Presocratics differ in their interests from Socrates in that they were interested in the the origins of the earth and nature while Socrate was interested in ethics. Hence Aristotle called them phusikoi, or physicists.
Thales of Miletus
THe first of the Presocratics was probably Thales of Miletus. Our doxographic sources suggest he was not just a philosopher, but a mathematician and an engineer. why his ideas continue to survive is probably sheerly because of his ingenuity. Thales was probably the first Miletian philosopher to not base his ideas off myth or religion; instead he conducted an honest investigation into the nature of the world. He relied instead on observation and reason. Thales was therefore the first person who can probably be credited for looking for a causation for motion, for interaction between things, like a magnet’s power to move iron.