Chimera Demons Logic Puzzle

[Zombra]

Putting this here as I sprung it on my P&P group in our 'Chimera' campaign. Let's see if you are smarter than they are.

Two demons guard a portal. They will allow you to pass if you solve the following puzzle.

Here are the conditions:

1. One demon may only speak the truth; the other may speak the truth or lie. Which is which is a mystery.
2. You may ask only one question, and you may only ask one of the demons.
3. The demon asked must give a direct answer if he can.

To solve the puzzle, you must prove which demon is the truthful one and which is the potential liar.

There are no tricks, lies, or other chicanery in the puzzle itself. It can be solved with a yes/no question of seven words (other types of questions may work; I don't know).

Good luck. PM me if you give up. No one I know has solved it yet.

 

[FeelTheRads]

http://en.wikipedia.org/wiki/Knights_and_Knaves

 

[Zombra]

http://en.wikipedia.org/wiki/Knights_and_Knaves

Yes, this was inspired by Smullyan's work, but it is not a true knights-and-knaves puzzle. In Smullyan's terminology, one demon is a "knight" (may only speak the truth) and the other is "normal" (may lie or tell the truth). Familiarity with knight-knave puzzles should help in nailing down the solution to this one. I never saw this particular puzzle in any of his books, though.

 

[27]

It's 3 am here so a quick thought before going to bed:

Spoiler

What would the other demon say if asked 'are you the truthful one?'?
Lying would say: 'yes' or 'no', assuming that their lie would have to be a direct opposite of the truth.
Truthful would say: 'yes or no'.

 

[Zombra]

I like the way you think, 27, but a lie doesn't have to be the exact opposite of the truth. If my favorite color is blue, and I lie about it, I don't have to say "orange" (which would be the opposite of blue on a color wheel) - I can lie and say red, yellow, purple, any color but blue. If asked your question, the 'normal' demon could give the same answer as the truthful one.

 

[Alex]

If you ask "Would the other demon answer my question truthfully", the truth speaking demon won't be able to give a direct answer, since he can't know. But the other will be forced to say either yes or no, since he can give a direct answer.

Edit: Or cast banish on them.

 

[Zombra]

Nice one Alex! I believe that one works! It's not what I was thinking of, but I don't see any flaws.

 

[Zombra]

And my solution ...

Spoiler

"Is your answer to this question 'no'?"

Astonishing!

On reflection, the same roblem could be made even simpler* - there don't need to be two demons. There can just be one - either he always tells the truth ... or he doesn't. The critical part is that either way he must answer directly if he can.

*Of course simpler doesn't mean easier!