Casting out nines

arithmetic tradition · medieval

A centuries-old arithmetic technique that uses the sum of digits (modulo 9) to check calculations.

To check a+b=c without redoing it: add the digits on each side and compare the remainders modulo 9. It's the same 'reduction' numerology uses, applied to a purely arithmetic purpose.

Here is the most honest connection between number and 'meaning': casting out nines was used to check calculations, not to predict the future. It's the evidence that the numerological mechanism is real in mathematics — it's the reading that is symbolic.