Truth teller vs liar : go to Hell

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Discussion Overview

The discussion revolves around a classic logical puzzle involving two guards, one who always tells the truth and one who always lies, guarding two doors—one leading to Heaven and the other to Hell. Participants explore various strategies for determining which door leads to Heaven by asking a single question to one of the guards. The conversation includes theoretical reasoning, proposed solutions, and playful commentary.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants describe the puzzle and express their enjoyment of it, noting its historical presence in discussions.
  • One proposed solution involves asking a guard what the other would say about the door's destination, suggesting that the answer given would indicate the opposite of the truth.
  • Another participant suggests a variation of the question that aims to clarify the path to Heaven, but questions its validity based on the requirement for a yes/no answer.
  • Some participants humorously suggest alternative, non-logical methods to deal with the guards, such as physical confrontations.
  • There is a discussion about whether the guards need to know each other's identities for the proposed questions to work effectively.
  • One participant raises a concern that the guards might not necessarily know each other, which could complicate the logic of the questions posed.
  • Several participants offer different formulations of questions that could be asked, with varying degrees of agreement on their effectiveness.
  • Some express skepticism about the necessity of the puzzle, suggesting that it may be overly complicated or trivial.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best question to ask or the assumptions underlying the puzzle. Multiple competing views and proposed solutions remain, with some participants questioning the validity of others' approaches.

Contextual Notes

There are unresolved assumptions regarding the guards' knowledge of each other and the requirement for a yes/no question. The discussion reflects varying interpretations of the puzzle's parameters.

humanino
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This is a well-known logical problem. But I can't figure out where it comes from, because it seems really old. I love it.

This is what happening when one dies : one gets in a room with two doors. Each door is kept by a guard. One of the doors leads to Heaven, the other one leads to Hell. One of the guard always tell the truth, the other always lies. There is no correlation between lying/telling-the-truth and keeping the door to Heaven/Hell.

You are allowed to ask only one question to only one guy. You wish to go to heaven. You can manage to find a yes/no question.

Hint : you have no hope to determine which one tells the truth. Notice that it does not matter wether you've been a bad guy or a good guy. Von Neumman certainly managed to go to Heaven, even though he was a horrible human being :biggrin: :-p
 
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This puzzle has been posted here before...more than once !

(-1)*(1) = (1)*(-1) = -1
 
You ask one of the guards: "are your flies undone?", when he inevitably goes to look you give him a deft judo chop to the neck renderning him unconcious. You then sweep the other guard to the floor and get him into a 'Figure of Four Leglock' and apply pressure until he squeals like a pig.
 
this problem seems to be the most faviourite of all, that's why it is posted here again and again, but anyways let me give the answer and spoil the fun for those who have not seen this before.

you ask the gaurd of one of the doors," what will the other gaurd say if i asked him, where does this door lead to."
whatever the answer he gives, the opposite is the correct answer.

HOW?

figure it out yourselves.
 
humanino said:
You are allowed to ask only one question to only one guy. You wish to go to heaven. You can manage to find a yes/no question.

Do you mean manage to find ONLY a yes/no question(thus expecting only 2 possible answers, viz. yes or no.)?
If so, the question posted above is invalid. As the guard would have to give a question which is neither yes/no.

Otherwise the above question seems to be the right one to ask.
 
Okay, but it can be tweaked slightly to make it work.
 
Answer: Ask if the other person would say that this is the right path?

Lets say the door is the right path
The truth person would say that the lier would say no.
The lier woud say that the truth person would say no.

Lets say the door is the wrong path.
The truth person would say the door yes.
The lier would say the truth person would say yes.

Do the opposite of what they say.
 
Yeah, that seems right to me. Same answer i worked out while i was supposed to paying attention in chemistry lecture!
 
Or you could just ask them what color their shirt is, and if he tells the truth go through his door. :smile: :-p :-p
 
  • #10
you could shoot one down and ask the other gaurd if he is dead
 
  • #11
fizzzzzzzzzzzy said:
you could shoot one down and ask the other gaurd if he is dead

I don't think the truth teller necessarily is guarding the heaven door and vice versa.

The Bob (2004 ©)
 
  • #12
The Bob said:
I don't think the truth teller necessarily is guarding the heaven door and vice versa.

The Bob (2004 ©)

fizzzzzzzzzzzy's explanation seems pretty sound to me.
 
  • #13
Ask: What would you say if I asked you, "Does this door go to heaven?"?
The Door is the way to heaven.
The liar would say that he would say yes.
The truthful person would say that he would say yes too.

The Door is not the way to heaven.
The liar would say that he would say no.
The truthful person would say he would say no too.

This is an alternative solution.
 
  • #14
fizzzzzzzzzzzy said:
you could shoot one down and ask the other gaurd if he is dead
But then you'd only know who's the truth teller and who's the liar...you still wouldn't know which door is the door to heaven and which door is the door to hell. And you only get one question.
 
  • #15
jcsd said:
You ask one of the guards: "are your flies undone?", when he inevitably goes to look you give him a deft judo chop to the neck renderning him unconcious. You then sweep the other guard to the floor and get him into a 'Figure of Four Leglock' and apply pressure until he squeals like a pig.

I prefer this method! :smile:
 
  • #16
pat777 said:
Ask: What would you say if I asked you, "Does this door go to heaven?"?
The Door is the way to heaven.
The liar would say that he would say yes.
The truthful person would say that he would say yes too.

The Door is not the way to heaven.
The liar would say that he would say no.
The truthful person would say he would say no too.

This is an alternative solution.

It does not satisfy the criterion that only one person be asked.
 
  • #17
Gokul43201 said:
It does not satisfy the criterion that only one person be asked.

It does, no matter who you ask you always get the right answer.
 
  • #18
LastOneStanding said:
But then you'd only know who's the truth teller and who's the liar...you still wouldn't know which door is the door to heaven and which door is the door to hell. And you only get one question.

OMG, it's a good thing I learned this before dying. Think of all the poor people who got cheated...
 
  • #19
gerben said:
It does, no matter who you ask you always get the right answer.

Oops, sorry, I misread it, I guess.
 
  • #20
3 Years Later...

Two Men. One "The Liar" who always lies. One "The Truth" who always tells the truth. You are not told who is who. So, find out who is who by asking only one question.

Your question should be: What would the other man say if I asked him who you are?
(Although you do not presently know who is who)

'The Liar' would say, "That man would say I'm 'The Truth'." Because the liar must always lie.

'The Truth' would say, "That man would say I'm 'The Liar'." Because the truth must always tell the truth.

Unfortunately in todays society we face the problem that people don't always lie, and don't always tell the truth. Usually it's a mixture of both.

But I do suppose God can't lie, and Satan never tells the truth. So aslong as you are directly questioning Satan and God, there should be no problem at all.
 
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  • #21
AlwaysLost said:
Two Men. One "The Liar" who always lies. One "The Truth" who always tells the truth. You are not told who is who. So, find out who is who by asking only one question.

Your question should be: What would the other man say if I asked him who you are?

It does not work.
Not necessarily the guards know each other. So, the liar doesn't have to know the other is a truth teller, and vice-versa.

The correct question has to be something like
"would you say 'yes' if someone had asked you whether this is the way to heaven?"

:smile:

Rephrasing Gokul,
(-1) * (-1 ) = (1) * (1) = 1
 
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  • #22
I see what you mean.

I suppose we are hoping that the two men know who the other is.

Not a bad thing, the whole thing is up **** creek if they don't.
 
  • #23
Do the guards have to answer yes or no, or must the question be such that they can only reply yes or no?
 
  • #24
I do believe we are making this more difficult than it should be, I'm sure we are all wise enough to concoct our own watertight versions. if you need someone elses advice on such a trivial point then maybe you are part of that generation of people who only asks questions and never figures out the answer for themselves. seems as though we are all typing for typings sake. Pointless.
 
  • #25
humanino said:
This is a well-known logical problem. But I can't figure out where it comes from...:

Clearly, it was devised by Satan.
 
  • #26
the second soln is definitely more elegent as u need to ask only one guard and the guards need not know each others identity... it is the soln to the more broader problem

truly (-1)*(-1) = 1 ;)
 
  • #27
Why don't you just ask "Am I male?"
 
  • #28
neu said:
Why don't you just ask "Am I male?"

Because you would conclude nothing at all.

Read again the first post:
humanino said:
...There is no correlation between lying/telling-the-truth and keeping the door to Heaven/Hell.
 
  • #29
humanino said:
There is no correlation between lying/telling-the-truth and keeping the door to Heaven/Hell.

Wait, the guard to hell always lies, the guard to heaven always tells the truth, so how can there be no correlation between lying/truth telling and keeping door to Hell/Heaven?
 
  • #30
neu said:
Wait, the guard to hell always lies, the guard to heaven always tells the truth, so how can there be no correlation between lying/truth telling and keeping door to Hell/Heaven?


You should read the first post.
 

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