The Greeks Told Us Where They Got It
Thales studied in Egypt. Herodotus said geometry was Egyptian. Plato has a priest call the Greeks children. The Greek miracle is a later invention, and the Greeks are on record contradicting it.
The story of Western civilisation begins, we are told, in Greece. Philosophy, logic, geometry, medicine, history, drama and democracy, all of it arriving more or less from nowhere in a few hundred years around the Aegean, in what gets called the Greek miracle.
There is an awkward problem with the miracle, and it is not that modern scholars have debunked it.
It is that the Greeks did not believe it either.
They were, in fact, embarrassingly clear about where their knowledge had come from, and they wrote it down, repeatedly, in texts we still have and still assign to undergraduates. The forgetting happened later, and it was not done by them.
Start with Thales of Miletus, traditionally the first philosopher, the man who said everything is water and thereby, in the standard account, launched rational inquiry as such.
The ancient sources are unanimous that Thales studied in Egypt, and that he learned his geometry there. His famous prediction of the solar eclipse of 585 BCE almost certainly rested on Babylonian astronomical records, since Greek astronomy at that date had no capacity to predict eclipses at all. The founding figure of the Western tradition made his reputation using imported methods, and nobody at the time thought this worth concealing.