They discovered the power of human reason. To begin with, they used this power in a simple, direct way by putting forward ever bolder theories about the nature of the physical world. Philosophers like Thales and Anaximander were doing what we would now call physics. By the time of Aristotle, however, the use of reason had become highly subtle and refined. Aristotle's approach to analysing a concept or a problem is one you might find in any academic philosophy journal today.
Philosophy is all about making our beliefs consistent.
You can take in any amount of information, but that won't help if your beliefs clash with one another. The early Greek physicists looked at the Creation myths that were current at the time and saw a mass of irrational assumptions. Like the stories we tell to children about Santa Claus and the Tooth Fairy. So they set about trying to tell a story that gave a consistent account of cause and effect. It was formed, they said, from stuff.