Quod erat demonstrandum

Cogito ergo sum Logic and rhetoric

Key articles

Logical fallacy Syllogism Argument

General logic

Precautionary principle Fear, uncertainty, and doubt Nazi analogies پاسخ کورتیر دوگانگی مرتن تقلیل به هیتلر

Bad logic

Genetic fallacy Flying carpet fallacy Appeal to gravity Argumentum ad lunam گزاره‌ی صاحب‌نظر Countless counterfeits fallacy v - t - e

Quod Erat Demonstrandum (normally abbreviated to Q.E.D.) is a Latin phrase that means literally "that which was to be demonstrated."

In normal English, this means "thus it is proven", or "so there, nyer nyer nyer"; the Latin equivalent of a mic drop. It is used formally after the last line of a mathematics proof, and colloquially in discussion or argument in a similar fashion.

Confusion

Due to the scary nature of Latin, many people have been misled into thinking it actually stands for "quite easily done", which stems from math teachers who — thinking they are clever — tell this joke without identifying it as such.

This ignorance is actually a godsend (or, alternatively, a curse), as it allows the educated to correct their cretinous peers in as condescending a manner as they feel necessary.

As a fallacy

See the main articles on this topic: Danth's law and Begging the question

While fancily ending a syllogism of one's proposal with "Q.E.D." totally sounds like something a college dean — learned at the topic at hand — might do, doing so is actually not the least bit indicative of said argument being logically acceptable.

Here's an example of the "Q.E.D. stamp" being applied to a blatantly fallacious argument:

P1: 10% of car thefts are committed by left-handed people.

P2: Half of all polar bears are left-handed.

C: There's a 5% chance your car was stolen by a polar bear.

Q.E.D.

As such, treat unexamined claims of Q.E.D for what they most likely are — mere pretentious Latin shorthand for shouting "Nailed it!!" while touring the room for high-fives.

This logic-related article is a stub.
You can help RationalWiki by expanding it.