Consider, Philo says, the following arithmetical curiosity. If you take any multiple of 9 (like 18, 27, 36, etc.) and add up the digits (1 + 8, 2 + 7, 3 + 6, etc.), you always get 9 back again. To the mathematically naive, this might appear a matter of chance. To the skillful algebraist, by contrast, it is immediately seen to be a matter of necessity.
