Case 1:

if a card has a D on one side then it has a 7 on the other
if
P
then
Q

 

D
F
7
5
P
not P
Q
not Q
Card #1
Card #2
Card #3
Card #4

 

Logical answer:

Given the truth table for "if . . . then . . ."

Q is true Q is false
P is true "If P then Q" is true "If P then Q" is false
P is false "If P then Q" is true "If P then Q" is true

a proposition of the form If P then Q is falsified if and only if P is true and Q is false.

 

Since we are looking for falsifying instances -- which are cases of P and not-Q -- we need to check anything that is P (to see if it might also be not-Q), and anything that is not-Q (to see if it might also be P). Things that are not-P and things that are Q are irrelevant.

Therefore, the correct answer, in the Wason trial above, is:
"Card #1 and card #4" -- because this corresponds to the instance of P (card #1) and the instance of not-Q (card #4).

(Of course, in real experiments there are many trials, and the order of the cards is varied).

 

Case 2:

if someone drinks beer then (s)he is 21 or older
if
P
then
Q

 

beer
diet coke
23 years old
19 years old
P
not P
Q
not Q
Card #1
Card #2
Card #3
Card #4