reductio ad absurdum, is a mode of demonstration employed by mathematicians when they prove the truth of a proposition by demonstrating that the contrary is impossible, or leads to an absurdity. It is in this manner that Euclid demonstrates the fourth proposition of the first book of the Elements, by showing that the contrary involves a manifest absurdity, viz. "That two straight lines can inclose a space."

ABSINTHIUM. See Artemisia, Botany Index.