Popper was very interested in how athens became so great. his first explanation was culture clash. people like xenophanes and pythagoras came and brought with them different ideas, and this led to questioning conventions and fruitful disagreements. but he did not think this was a full explanation.
today i found out another part of the story, which Popper worked out late in life. it is that Pisistratus (a tyrant of athens) had homer written down as books. before that, homer was an oral tradition, and books were individual things guarded by priests. well, homer got written down in lots of copies, and athens became literate, and everyone read it. then private individuals had some other poetry written down, and sold it, and that was popular too. this paved the way for people to write books for the purpose of commercial publication (the first being, Popper thinks, _On Nature_ by Anaxagoras). and so Athens had the first *book market*, where many books could be purchased cheaply. this also led to competition among writers to make better books.
Popper mentions a nice confirming fact. He found records of large shipments of papyrus from egypt to athens, starting in a year when Pisistratus was in power. he also found several mentions of the book market in surviving books from the time.
You can find this theory in _In Search of a Better World_. look for the chapter title mentioning books *and* the one after it.