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Thales of Miletus (c. 624–546 BCE)

    Thales of Miletus (c. 624–546 BCE) is regarded as one of the seven great pre-Socratic philosophers. whose work establish the foundations of Western science. He argued that phenomena are due to "natural causes" that could be investigated through systematic observation, argument, and mathematical reasoning. Thales is associated with the aphorism “Know thyself."  

    He is credited with one of the first geometric proofs, known as Thales' Theorem [below]. This is a universal conclusion derived by deductive reasoning, rather than a simple observation or measurement or a single case.  If AC is the diameter of a circle with its center at O, and B is any point elsewhere on the circle, then the angle ∠Aµþ°ä is necessarily a right angle (90^o). The proof is that the angle ∠B´¡°ä is necessarily the same as the angle ∠Aµþ°¿, because both are the base angles of an isosceles triangle. Similarly, ∠A°äµþ is the same as âˆÌ̵þ°¿. Because the angle of a triangle add up to 180^o , 2a + 2ß = (2)(a + ß) = 180 , thus (a + ß) = 90  QED.