Thales
From The Flat Earth Wiki
Thales of Miletus (624 BC - 546 BC) is considered by many to be "the first Greek philosopher". His denial of mythology as an acceptable explanation for natural phenomena is a fundamental component of the scientific revolution. He aimed to explain what he could see using reason.
For example, Thales theorized that earthquakes were not a result of divine intervention or other supernatural occurences, but rather that the earth floats in an ocean of water, and that earthquakes are a result of waves crashing into the earth.