Where this idea comes from
Guess the secret rule
In 1956 Robert Abbott invented Eleusis, a card game where one player makes up a secret rule and the others find it by playing cards. In 1960 the English psychologist Peter Wason gave people the numbers 2, 4, 6 and asked them to find his rule: most only tested numbers they expected to fit.
One thing to hold: Three numbers on a card — 2, 4, 6 — and a rule you have to find.
Why it matters: A test that could prove your guess wrong teaches you more than one that can only agree with it.