Solution
The correct response was to turn the cards showing 8 and brown, but no other card. Remember how the proposition was stated: "If the card shows an even number on one face, then its opposite face is red." Only a card which has an even number on one face and which is not red on the other face can invalidate this rule. If we turn over the card labeled "3" and find that it is red, this does not invalidate the rule. Likewise, if we turn over the red card and find that it has the label "3", this also does not break the rule. On the other hand, if the brown card has the label "4", this invalidates the rule: it has an even number, but is not red. The interpretation of "if" here is that of the material conditional in classical logic.
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