Q.E.D.


Q.E.D. is an abbreviation for the Latin quod erat demonstrandum meaning ‘[that] which had to be demonstrated’.

‘Q.E.D.: a Mathematician’s way of saying “I win”.’ – Urban Dictionary

The abbreviation of the phrase is traditionally placed at the end of a mathematical proof or philosophical argument to denote the conclusion of the demonstration. The abbreviation thus signals the completion of the proof.

The phrase is a translation into Latin from the Greek ὅπερ ἔδει δεῖξαι ‘what was required to be proved’. The phrase was used by many early Greek mathematicians, including Euclid and Archimedes.

The phrase has also been used outside mathematics and philosophy for comic effect.

For instance, in Thomas Dolby’s 1988 song Airhead, he imagines a conversation with the titular young woman and says “quod erat demonstrandum, baby”, to which she squeals the eager reply “ohhh, you speak French!”

Also, in chapter six of Douglas Adams’ The Hitchhiker’s Guide to the Galaxy, the abbreviation is included in the following exchange:

The argument goes something like this: “I refuse to prove that I exist,” says God, “for proof denies faith, and without faith I am nothing.”
“But,” says Man, “the Babel fish is a dead giveaway, isn’t it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don’t. QED.
“Oh dear,” says God, “I hadn’t thought of that,” and promptly vanishes in a puff of logic.
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3 thoughts on “Q.E.D.

  1. Ah, an aficionado of the Late Douglas Adams – all four books of his trilogy grace the shelves of my library.

  2. I don’t know if there is a formal phrase for the opposite of Q.E.D., but if there were, this Adams quotation would have to be said to fit it:

    “Imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’ This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, frantically hanging on to the notion that everything’s going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise.”

  3. I have yet to read the follow-ups to The Hitchhiker’s Guide, but indeed, he was a great writer.

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