In Wason's selection task, subjects see four cards showing symbols like E, K, 4, and 7, and are told that each card has a letter on one side and a number on the other. The task is to choose the cards that need to be turned over in order to determine whether the following rule is true or false: “If a card has a vowel on one side, then it has an even number on the other side.” Most subjects choose E alone, or E and 4, while the correct answer is E and 7, because “Any odd number on the other side of E falsifies the rule in exactly the same way as would any vowel on the other side of 7” [Wason and Johnson-Laird, 1972]. According to First-Order Predicate Logic (FOPL), the “rule” to be tested is (for-all x) (Vowel(x) → Even(x)). This proposition is false if there is at least one card that satisfies Vowel(x) but not Even(x), otherwise it is true. Therefore, to determine the truth value of the rule means to check all cards that may falsify the rule. What the subjects show is a tendency to find cards that verify the rule. In FOPL, cards verifying the rule make no contribution to its truth value. If the 4 card indeed has a vowel on the other side, it does not mean the rule is true. If the E card and 7 card do not make the rule false, the rule is true, and the 4 card does not need to be turned over.
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