Q.E.D ( Quod erat demonstradum) Which means “ that which was to be demonstrated” Usage for the abbreviation Q.E.D. is found from the 17th century. The philosopher Spinoza famously used Q.E.D. at the end of an argument in his 1677 Ethics. Scientists use this Greek word as a means of showing off ( a foot note to others saying I proved what I set out to do).