Thales (620-546 BC) was born in the city of Miletus of Greek Ionia located on the coast of modern day Turkey. He was active prior to the Golden Age of Ancient Greece, but no less than a thinker of great repute given the number of prominent ancient writers, poets, philosophers, and historians who speak of him. Aristotle describes Thales as the founder of natural philosophy.
On the topic of natural philosophy. Thales is appreciated by modern historians for being the first thinker to begin looking for natural explanations for phenomena and events in the world initially attributed to the gods and heroes. Events in the world were not due to supernatural intervention but had natural causes that observation and experimentation would soon discover. Thales is also credited with the ushering in the beginning of Greek astronomy as he speculated concerning cosmological events traditionally believed to involve supernatural entities. For such reasons Thales solidified the foundations of future philosophical and scientific thought.
Although historians know Thales primarily due to his philosophical work and his cosmological thesis, he was also a noted mathematician and businessman. He traveled widely around the eastern Mediterranean and even visited Egypt where he learned the geometry he brought back to Greece. He also held a good understanding of geometry and astronomy, which included him predicting an eclipse of the sun in 585 BC. It remains a mystery how Thales managed to predict the eclipse, which modern astronomy confirms to have occurred at the time he was said to have made the prediction. Further, Thales, while in Egypt, measured the height of the Great Pyramids via their shadows and could calculate the distance ships were from shore while at sea. He also theorized the Earth’s shape, size, and support, while also attempting to determine the size of the sun and the moon, as well as the cause of earthquakes. There is a story relating how because Thales was so fixated with studying the heavenly bodies he fell into a well.
Thales was the first thinker to propose the philosophy of material monism, the idea that everything in the universe could be reduced to a single substance. As we learn from Aristotle, Thales posited water as this fundamental property. He claimed that all things, including the Earth, came into being from water, and he also said that the Earth is a disk floating or resting on top of water. As such, given his water thesis, Thales was one thinker in one group within the pre-Socratics who sought after a fundamental substance to the world. Other thinkers such Heraclitus thought this substance was fire, while Democritus posited the atom.