The reputation. Aristotle, Herodotus and Proclus all name Thales as the founder of Greek deductive geometry and natural philosophy. His eclipse of 585 BCE is fixed by astronomy.
The theorems. The five results credited to him are plausible but documented centuries later (Eudemus via Proclus). The attributions are tradition, not contemporary record.
Did he "prove"? Whether Thales gave formal proofs, or whether deduction crystallised later and was projected back onto him, is genuinely argued by historians.
Take any circle and its diameter AB. Pick any point P on the arc and join it to both ends. Thales' Theorem: the angle at P is always exactly a right angle — drag P anywhere and it never budges from 90°. A Babylonian would have measured a few and trusted it. Thales proved it, and you can watch the proof: drop the radius OP and the figure splits into two isosceles triangles whose base angles must sum to a right angle.
The gap Thales crossed: a recipe tells you what to do and is confirmed by trying it; a proof tells you why it can't fail and is confirmed by reason alone. Measure ten thousand triangles and a doubter can still ask about the ten-thousand-and-first. Prove it once and the question is closed for all of them, including the ones no one will ever draw. Five theorems are traditionally his — the first deductions in history:
A diameter cuts a circle into two equal parts — the first recorded geometric proof.
If two sides are equal, the angles opposite them are equal.
Two crossing lines make opposite angles that must match.
The theorem above — said to have so delighted him he sacrificed an ox.
Enough to reconstruct a triangle — and, the story goes, enough to find the distance to a ship at sea from the shore.
"Megiston topos: apanta gar chorei." — Space is the greatest thing, for it contains all things. — attributed to Thales
Thales was practical before he was abstract. He is said to have predicted the solar eclipse of 28 May 585 BCE, which fell during a battle between the Lydians and the Medes and frightened them into peace — one of the oldest precisely datable events in history. He measured the height of a pyramid from the length of its shadow, at the hour his own shadow equalled his height (similar triangles, used in the field). And he made the first move of science: explaining the world without the gods — proposing that water is the arche, the single underlying stuff of all things. Wrong answer; revolutionary question.
Gate kept on. Thales wrote nothing that survives — possibly nothing at all. Everything here comes through later hands: Aristotle (two centuries on), and Proclus (a millennium on) quoting the lost history of Eudemus. So the theorems and proofs are the tradition's Thales; some historians think formal proof matured later and was credited back to the founder. He is the firm first of the reputation for deductive reasoning, the traditional first of the theorems, and a debated first of proof in the strict sense. The romance and the record don't perfectly align — and saying so is the point of this whole domain.