Main Knights And Knaves Discussion

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peixe200
Topic
02:02:39 AM May 2nd 2017
After some hard thinking, I concluded that the first solution to this problem, "If I asked you if the door you're guarding leads to where I want to go, would you say 'yes'?", doesn't work. The problem is that this solution assumes that the correct door is being guarded by the honest man. If the situation was reversed (that is, if the liar was guarding the correct door), then the honest man would say "no" and the liar would say "yes", and you'd think the liar was telling the truth. In another hand, if you already knew which person (the honest man or the liar) was guarding which door, then all you'd have to do was figure out who is telling the truth (by asking "what is 2+2?" or some other obvious question). So this solution is either wrong or completely pointless.
jormis29
03:55:13 AM May 2nd 2017
edited by jormis29
It works if you ask the lying guard too since he says the opposite of what he say if you just asked him what is he guarding

If just you ask the lying guard "does the door lead where I want" and he is guarding the correct door, he would say no but if you ask the "If I asked you if the door you're guarding leads to where I want to go, would you say 'yes'?", he would have to say the opposite so it is a lie and vice versa
peixe200
08:01:22 AM May 2nd 2017
"It works if you ask the lying guard too" No, it doesn't, and I'm already assuming the question is being made to both guards.

The problem is that you don't know which guard is guarding which door, and therefore you can't know what would the guards answer if you asked them "does the door you're guarding lead to where I want". They could both answer "yes" or answer "no". But you don't know that.

If you already knew what would be their answer to this question, then you'd already know which guard is guarding the correct door. So all you'd have to do was figure out which guard is which, by asking "what is 2+2", and go through the correct door. So the question would be unecessary. That's what I'm trying to explain.
jormis29
04:49:40 PM May 2nd 2017
edited by jormis29
It doesn't matter which guard you ask because since the lying guard would have to say the opposite of what he say if you ask him the simple question (therefore say what he is actually guarding) and the truthful guard just speaks the truth, they both end up speaking the truth with one question

It works because the lying guard would have to lie about the lie (the opposite of the opposite) and end up telling the truth
jormis29
04:57:21 PM May 2nd 2017
edited by jormis29
There are four scenarios if asked the question as "If I asked you if the door you're guarding leads to where I want to go, would you say 'yes'?" to a single guard

  • If the lying guard is guarding the correct door, he would have to say he is guarding the correct door since it is the opposite of the answer he would give if you asked him "does the door lead where I want"
  • If the lying guard is guarding the wrong door, he would have to say he is guarding the wrong door since it is the opposite of the answer he would give if you asked him "does the door lead where I want"
  • If the truthful guard is guarding the correct door, he would say he is guarding the correct door because that is truthfully what he would say if you asked him "does the door lead where I want"
  • If the truthful guard is guarding the wrong door, he would say he is guarding the wrong door because that is truthfully what he would say if you asked him "does the door lead where I want"
ScroogeMacDuck
Topic
12:29:45 PM Aug 10th 2016
Someone just found a new possible answer today:

Ask to each one "What does two plus two make?" For obvious reasons, this will tell you who is the lying guard and who is the truthful guard. You are then morally justified in punching the liar through the door. When you do, watch what happens/watch through the door.
AshleyY
Topic
01:31:22 PM Nov 6th 2011
I've just added the Orqwith/Doom Patrol one to Puff of Logic, and it looks correct to me:
Rebis: I've come to ask the question. One of you must have the answer. Why is there something instead of nothing?
Priest in Black: I am a liar and I do not know why there is something instead of nothing.
Priest in White: I am an honest man and I do not know why there is something instead of nothing.
Rebis: Tell me then, the Priest in Black, why is there something instead of nothing?
Priest in Black: There is something instead of nothing.
Rebis: Then you can't possibly exist.
Why is it "completely wrong"?

Icalasari
Topic
04:19:07 PM Feb 19th 2011
Shouldn't this have an alt title added? I never would have thought to search for Knights and Knaves
92.40.181.150
Topic
02:43:29 AM Apr 28th 2010
Is it certain that Raymond Smullyan invented this type of puzzle? I first came across it in the early sixties, and it was old then.

It's known that Smullyan invented the "knights and knaves" scenario, in order to avoid the Unfortunate Implications of the original "two primitive tribes" scenario, along with the variations involving insanity and vampirism (but not the "me no speak English" variant), but the puzzle itself probably goes back to Lewis Carroll's time.
62.64.143.194
08:16:24 AM May 10th 2010
Robin Adams: I don't think it is known for certain who invented them.

The earliest mention I've been able to find is a letter by the computer scientist John Mc Carthy in Scientific American in 1957. Mc Carthy gives the "da" and "bal" variant (he uses "pish" and "tush"), but he writes as if the original problem was well-known by then.

Smullyan was born in 1919, so it's possible that he invented the puzzle in time for it to be well-known by 1957, but not very likely.

Lewis Carroll wrote at least one fairly similar puzzle in 1895: the Five Liars. Five people - A,B,C,D,E - each make two statements, of the form "Either B or D tells a Truth and a Lie; either C or E tells two Lies". A few years later, he wrote to his friend Professor Wilson "I hope you will be pleased to know I have decided to abandon my 'Liar' Problems; the metaphysical difficulties are too appaling!", which suggests that sort of puzzle was not common, and maybe was invented by him.
Vilui
04:55:03 PM May 13th 2010
Yes, I'm going to remove the ridiculous claim that Smullyan invented this type of puzzle from the page. As well as the above facts, Martin Gardner was a friend of Smullyan and would certainly have credited him if he had invented it.
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