THE’riddle of the two doorsalso known as the riddle of the gods two guardians, It’s a seemingly simple but insidious logic game.
The situation is this: we find ourselves in front of two doors, each of which is guarded by a guardian. We know that one of the two doors leads to the salvation, while the other to death, and we also know that one of the two guardians he always says truth, while the other mind Always. The problem is that we do not know which of the two guardians is sincere, nor which of the two doors leads to salvation. For their part, however, the guardians know perfectly well which of the two is telling the truth and who is lying, and which door is the right one.
Now, we have one and only one question availablewhich we can do to one of the two guardians so as to find out which door will take us to safety.
What is the right question to ask, remembering that we don’t know which guardian is lying and which is telling the truth?
The solution to the riddle of the two doors or the two guardians
Let’s start by considering that we cannot choose who to ask the question to – whether to the sincere guardian or the lying one – because we don’t know how to distinguish them. We must therefore ask a question that can provide us with the answer we want, whether the guardian answers us with a lie or if he tells us the truth. This tells us what to ask “What is the door of salvation?” that’s not the right question: if it were the sincere guardian who answered us, we would be safe, but if it were the liar who gave us the answer, we would have failed in our aim.
However, we know that the two guardians know each other’s naturetherefore the sincere person knows that the other is lying, and vice versa. And that’s where it lies solution: in asking one of the two guardians to tell us what the other would respond if we asked him to tell us the door that leads to salvation, and then choose the opposite door.
How would the other guardian respond if we asked him what is the door that leads to salvation?
By asking this question, the answer it will lead us to salvation regardless of the guardian who answers us. In fact, if it were the truthful person who answered us, he would tell us the truth about what the other person, that is the liar, who would point to the wrong door, would answer. If instead it were the liar who spoke, he would lie to us about what the truthful one would answer, who would indicate the right door, therefore also giving us the wrong door as an answer.
At this point, we are sure that by asking the above question, each of the two guardians it would show us the door of deathand to save us we just need to choose the door opposite to the one indicated.
A trickier solution: the power of double negative
Exists another solution This riddle is known to few and often difficult to understand, but not if you know the “secrets” of logic.
We initially observed that asking the question “What is the door of salvation?” he wouldn’t have helped us, but incredibly one of his small variationwould have given us the right answer. If we asked one of the two guardians this question
What would you answer if I asked you “What is the door to salvation?”
we would be sure to receive the door of salvation as an answer.
But how is this possible? What is the difference between one question and another? The difference lies in asking the question twice within the same question, which leads to the solution thanks to a double negative. Let us understand this better: If the above question were asked to the sincere guardian, there would be no difference with the question “What is the door of salvation” and we would be given the right answer; if instead it was the lying guardian who answered, the cards on the table change completely compared to the first question.
It is true, the liar to the question “What is the door of salvation?” he would answer us the wrong door, but if we ask him “What would you answer if I asked you…” we are asking him to tell us the answer: if he said the wrong door, he would be telling us the truth about what he would answer… but he is a liar and lies Always! For this, he will tell us the opposite of what would answer, therefore indicating us precisely the right door!
