# Truth teller vs liar : go to Hell !

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 :tongue2:

Gokul43201
Staff Emeritus
Gold Member
This puzzle has been posted here before...more than once !

(-1)*(1) = (1)*(-1) = -1

jcsd
Gold Member
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."

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.

Gokul43201
Staff Emeritus
Gold Member
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. :rofl: :tongue2: :tongue:

you could shoot one down and ask the other gaurd if he is dead

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

fizzzzzzzzzzzy's explanation seems pretty sound to me.

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.

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.

jimmy p
Gold Member
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!! :rofl:

Gokul43201
Staff Emeritus
Gold Member
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.

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.

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...

Gokul43201
Staff Emeritus
Gold Member
gerben said:
It does, no matter who you ask you always get the right answer.

Oops, sorry, I misread it, I guess.

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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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?"

Rephrasing Gokul,
(-1) * (-1 ) = (1) * (1) = 1

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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.

Do the guards have to answer yes or no, or must the question be such that they can only reply yes or no?

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.

This is a well-known logical problem. But I can't figure out where it comes from....:

Clearly, it was devised by Satan.