Suppose that I have a pack of cards each of which has a letter written on one side and a number written on the other side. Suppose in addition that I claim the following rule is true:
- If a card has a vowel on one side, then it has an even number on the other side.
Imagine that I now show you four cards from the pack:
Which card or cards should you turn over in order to decide whether the rule is true or false?
Only about 5% of the educated population give the correct answer, which is E and 7. If you said (only) E, you saw that the rule in question would be overthrown were there to be an odd number on the other side of this card -- but you failed to note that if the 7 card has a vowel on the other side, this too is a case that shoots down the rule.