Hippasus

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This article is about the Greek philosopher. For the mythological characters called Hippasus, see Hippasus (mythology).

Hippasus of Metapontum (Greek: Ίππασος), b. c. 500 B.C. in Magna Graecia, was a Greek philosopher. He was a disciple of Pythagoras. To Hippasus (or Hippasos) is attributed the discovery of the existence of irrational numbers. More specifically, he is credited with the discovery that the square root of 2 is irrational.

Until Hippasus' discovery, the Pythagoreans preached that all numbers could be expressed as the ratio of integers. Despite the validity of his discovery, the Pythagoreans initially treated it as a kind of religious heresy and they either exiled or murdered Hippasus. Legend has it that the discovery was made at sea and that Hippasus' fellow Pythagoreans threw him overboard.

But there are two other stories about Hippasus. The first says that Hippasus was expelled from the Pythagorean school because he published doctrines of Pythagoras, while the second says that he was drowned at sea for revealing the construction of the dodecahedron in the sphere and claiming it as his own. But since the Pythagoreans' supposed pledge to secrecy was most likely false^{[citation needed]}, the authenticity of these stories is questioned.

He was also noted as an early experimenter in acoustics and resonance. Few of his original works now survive.