This philosopher is considered to have been the founder of the Ionian School of philosophy.
Considered the first known scientist according to the accepted European definition of the word.
Thales was believed to have introduced geometry into the Aegean world. Interestingly, he shared the belief of animists (in non-European parts of the world), that all matter was infused with spirtual beings or gods, and transformed by them constantly.
He saw water as the basic cosmic substance from which all other substances were formed, hence rejecting mythological and religious explanations of the origins of the cosmos.
Besides geometry, he was reputed to have had deep knowledge of astronomy, mathematics, and physics. He was the author of a History of Astronomy, in which he wrote of eclipses of the sun and of the solstices (and is reputed to have been the first to have described such cosmic events).
Most of what is known of him is from the writings of Diogenes Laertius, an ancient Greek historian. He is widely known for his geometric theorems concerning triangles and semicircles.