Reductio ad absurdum
Videos
In logic, reductio ad absurdum, also known as argumentum ad absurdum, apagogical argument, or proof by contradiction, is the form of argument that attempts to establish a claim by showing that followi
Reductio ad absurdum, painting by John Pettie exhibited at the Royal Academy in 1884
Q.E.D.
Videos
Q.E.D. is an initialism of the Latin phrase quod erat demonstrandum, meaning "that which was to be demonstrated". Literally, it states "what was to be shown". Traditionally, the abbreviation is placed
Reductio ad absurdum
…end with the word "Contradiction!". Isaac Barrow and Baermann used the notation Q.E.A., for "quod est absurdum" ("which is absurd"), along the lines of Q.E.D., but this notation is rarely used today. A graphical symbol sometimes used for contradictions is a downwards zigzag arrow "lightning" symbol (U+21AF…
Spinoza's original text of Ethics, Part 1, Q.E.D. is used at the end of Demonstratio of Propositio III on the right hand page