Thales of Miletus ( THAY-leez; Greek: Θαλῆς; c. 624/623  – c. 548/545 BC) was a Greek mathematician, astronomer, statesman, and pre-Socratic philosopher from Miletus in Ionia, Asia Minor. He was one of the Seven Sages of Greece. Many, most notably Aristotle, regarded him as the first philosopher in the Greek tradition, and he is otherwise historically recognized as the first individual known to have entertained and engaged in scientific philosophy. He is often referred to as the Father of Science.

Thales is recognized for breaking from the use of mythology to explain the world and the universe, instead explaining natural objects and phenomena by offering naturalistic theories and hypotheses. Almost all the other pre-Socratic philosophers followed him in explaining nature as deriving from a unity of everything based on the existence of a single ultimate substance instead of using mythological explanations. Aristotle regarded him as the founder of the Ionian School of philosophy and reported Thales' hypothesis that the originating principle of nature and the nature of matter was a single material substance: water.

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 also the first known to whom a mathematical discovery has been attributed.