"The sum of the three angles of every plane triangle is equal to two right angles. The mathematical truth enunciated in the above theorem is not new It has been known for more than two thousand years. ...Our reason for believing that Thales was not ignorant of the theorem under consideration is found in the beautiful demonstration by which he proved that every angle in a semicircle is a right angle. This appears to have been regarded as the most remarkable of the geometrical achievements of Thales, and it is stated that on inscribing a right angled triangle in a circle he sacrificed an ox to the immortal gods."