7

THE DRIVE TO DEFEND (D4)

We shall defend our island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender.

—WINSTON CHURCHILL

Here's a mental experiment. Given a set of cards that have letters on one side and numbers on the other, the idea is to test whether the following statement is true: “If a card has a D on one side, it has a 3 on the other.” Which of these cards would you need to turn over?

D  F  3  7

Most people choose either the D card or the D card and the 3 card. The correct answer is D and 7. The 3 card is irrelevant; the rule said that D's have 3's, not that 3's have D's. The 7 card is ...

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