Greek philosophy is usually introduced through Socrates, which puts the story about a century late. It starts on the coast of Asia Minor, in a trading city called Miletus, with a question that sounds like physics rather than philosophy: what is everything actually made of?
Thales answered water. He was wrong, and it hardly matters. What mattered was the shape of the answer: a natural cause for a natural world, offered without invoking a single god, and open to being argued with. His successors argued with it immediately, which is the point. Anaximander said the source must be something boundless rather than any one substance; Anaximenes said air. Within two generations there was a practice of correcting your teacher in public.
You cannot step into the same river twice.
The turn toward the human
The early thinkers asked what is real. Socrates asked how one should live, and asked it by questioning people in the Athenian marketplace until their confident answers came apart. He wrote nothing. He was executed by his own city in 399 BCE, and almost everything that followed in Greek philosophy is in some way a response to that death.
Plato, his student, built a philosophy in which the visible world is a shadow of something more real. Aristotle, Plato's student, turned back to the visible world and began classifying it: animals, constitutions, arguments, tragedies. The two of them set an argument running about where truth is found, above the world or in it, that has never entirely stopped.
After Athens
When Alexander's conquests unsettled the city-state, philosophy changed its question again: not what is real, nor how should a citizen live, but how does a person stay steady when the world is beyond their influence. Four answers competed. The Stoics said align yourself with nature and mind only what is yours. The Epicureans said remove fear and keep good friends. The Skeptics said suspend judgement and find peace there. The Cynics said strip everything away and see what is left. These are practical philosophies, written for people who no longer expected to shape events.
Most of it is gone
It is worth saying plainly that we have lost the majority of it. Heraclitus, Parmenides, Democritus and Empedocles survive as fragments, quoted by later writers who were often arguing against them. What we call their philosophy is reconstructed from sentences preserved by opponents. The atlas keeps those fragments with the chain of hands that carried each one, because the transmission is part of the evidence.
Volume I of the atlas follows this from Miletus to Athens and out across the Hellenistic world, with the places on the map and the lives on the timeline. Greek philosophy is easier to hold when you can see how short the distances were and how few people it took.