Anaximander of Miletus (c. 610–c. 540 BC)
Anaximander was a Greek philosopher of the Milesian School, student of Thales, and possibly the first person to speculate on the existence of other worlds. Anaximander held that the fundamental essence of all things is not a particular substance, like water, but apeiron, or the infinite. It seemed reasonable to him, given this boundless creative source extending in all directions, that there might be an indefinite number of worlds existing throughout time, worlds that "are born and perish within an eternal or ageless infinity." The pluralism he taught, therefore, was not of a multitude of planets in space but of an endless temporal succession.
Anaximander also pioneered the notion that the Earth is not flat, suggesting it was cylindrical and that it floated free, unsupported, at the exact center of the Universe, with people living on one of the flat ends. As for the Sun, he said it was as large as the Earth – an audacious theory at that time.