Thales of Miletus; Greek: Θαλῆς (ὁ Μιλήσιος), Thalēs; c. 624 – c. 546 BC) was a pre-Socratic Greek philosopher and mathematician from Miletus in Asia Minor and one of the Seven Sages of Greece. Many, most notablyAristotle, regard him as the first philosopher in the Greek tradition. Aristotle reported Thales' hypothesis that theoriginating principle of nature and the nature of matter was a single material substance: water.

Thales attempted to explain natural phenomena without reference to mythology. Almost all of the other Pre-Socraticphilosophers follow him in attempting to provide an explanation of ultimate substance, change, and the existence of the world without reference to mythology.

In mathematics, Thales used geometry to calculate the heights of pyramids and the distance of ships from the shore. He is the first known individual to use deductive reasoning applied to geometry, by deriving four corollaries to Thales' Theorem. He is the first known individual to whom a mathematical discovery has been attributed.