(c.624-c.547 BCE)
Thales of Miletus is regarded as the father of philosophy, as he was the first Greek, of whom we have knowledge, to break with existing Greek systems of thinking to conjecture about the fundamental nature of matter and its transformation into myriad things. We hear from Aristotle in his Metaphysics that Thales thought this primary substance, and most fundamental "stuff" which composes the Earth and all life is water. We hear about Thales' thesis secondhand because none of Thales' writings exist today. Thus, the only knowledge we have of what Thales actually thought comes from ideas attributed to him by other thinkers. Based on these sources it is evident that Thales was a pioneer both in astronomy and geometry in addition to philosophy for which he is primarily recognized.
From Aristotle we hear that Thales supposed the Earth to be a disc that floats in a sea of water. Based on this supposition, Thales supposed the cause of earthquakes to be the sea tossing the Earth. This belief is attributed to Thales by Antius, who also named Democritus as an advocate of this explanation.
From Aristotle we also hear another idea often associated with Thales, that "all things are full of the gods." Aristotle's De Anima reads: "Some think that the soul pervades the whole universe, whence perhaps came Thales's view that everything is full of gods." (411 a7-8). It is speculated that Aristotle was given this idea by Plato. The pantheist idea that "all things are full of the gods" does seem suspect considering Thales' apparent materialism, for Thales had named water as the primary source, and not a divine being.
Another feat attributed to Thales is the foretelling of a solar eclipse which occurred on May 28th, 585 BCE. We hear about this from Herodotus, in his recount of a battle between the Medes and the Lydians. Thales is also said to have brought geometry to Greece from Egypt.
Thales' younger associate Anaximander of Miletus (c.611-c.547 BCE) is known to have rejected his teacher's idea of water as the primary substance and origin of all things. Anaximander instead said that from which all things are born and eventually return is infinite and boundless.