Aristotle

“ If it is triangle, the quality of having two right angles belongs to the particular kinds of triangles as a result of its belonging essentially to triangle, and the demonstration in regard to triangle is a universal demonstration. ”

“ How they may be avoided Demonstration must disregard all accidental circumstances, and aim at the discovery of the essential and universal. We must not fail to notice that mistakes frequently arise from the primary universal not being really demonstrated in the way in which it is thought to be demonstrated. ”