Euclid — "A number is a multitude composed of units."

"There are infinitely many prime numbers."

"A semicircle is the figure contained by the diameter and the circumference cut off by it. And the center of the semicircle is the same as that of the circle."

"Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle that which has two of its sides alone equal, and a scalene triangle that which has its thr…"

"A plane angle is the inclination of the lines to one another, when two lines meet one another, but are not in the same straight line."

"The only purpose of the 'Elements' is to demonstrate mathematically certain fundamental propositions."