Thales is considered the first person to practice math for its own sake and not for some practical reason (for instance, for building projects or to record economic transactions). Thales studied math because he wanted to understand math, and for no other reason.
He was also the first person to formally proof a theorem, meaning that he purposefully set out to show with absolute certainty that his theorems were true, starting with self-evident axioms (assumption) and using accepted logical steps.
Earlier mathematicians often gave specific numerical examples to make it plausibe their theorem was valid in general. In cases where rigorous logic was used, such as in India, there was no conventional structure of proof consistently invoked as essential to the validation of mathematical theorems.
His most famous proof is of a theorem now called Thales' theorem, in which he proved that a triangle with one side being the diameter of a circle and all corners of the triangle on the circle, always has an angle of 90 degrees (the proof can be found in my book in small steps understandable for the general public).
"The Great World History Book"