If you were to ask the lying guard for ‘an example of a credit card number, expiry date and security code that weren’t the details of the other guard’, they would necessarily have to give the correct details, which the truth guard would pass on
Couldn’t they just lie and give you a non-working example?
Or are you hinging your question on a variation of “don’t tell me the other guard’s credit card information”, which you expect the lying card to contradict and tell you?
You’re asking for an example of details that aren’t those of the other guard. If they give you a non-working example, then the lying guard has been truthful. Which is a contradiction.
Consider the question ‘what is an example of something that is not a cat?’. Any answer that is not a cat is truthful; therefore the lying guard must give an example of a cat.
Ah, ok, so to clarity, you could also ask, “tell me anything that isn’t the other guard’s number,” and they have to tell you exactly the other guard’s number.
![Two Guards Riddle. [11]](https://lemmy.world/pictrs/image/d5137530-1a9a-4e36-9cdc-f4de97e02362.jpeg){kind=link}
Couldn’t they just lie and give you a non-working example?
Or are you hinging your question on a variation of “don’t tell me the other guard’s credit card information”, which you expect the lying card to contradict and tell you?
You’re asking for an example of details that aren’t those of the other guard. If they give you a non-working example, then the lying guard has been truthful. Which is a contradiction.
Consider the question ‘what is an example of something that is not a cat?’. Any answer that is not a cat is truthful; therefore the lying guard must give an example of a cat.
Ah, ok, so to clarity, you could also ask, “tell me anything that isn’t the other guard’s number,” and they have to tell you exactly the other guard’s number.