Wait, they're not - at least, in the example of the article. Allow me to digress: There is a figure with 4 cards, showing: all orange, all blue, the number 2,

Wait, they're not - at least, in the example of the article. Allow me to digress: There is a figure with 4 cards, showing: all orange, all blue, the number 2, the number 1. The only condition is that an even number must have an all blue flipside. This condition is either irrelevant or satisfied for the blue card. For all other cards, we don't know. The card with "1" may have the number "4" on the flip side. So we need to check ask other cards. TL;DR: only a blue face guarantees compliance with the rule, all other cards need to be flipped. In the second case, the unstated assumption that the cards all have one side worth digits and one side with symbols is still unstated, but a lot stronger. (These are things to check for age-alcohol compliance, so logically they must contain both age and alcoholic state of consumption.)