Fun with Logic! I demand that you get excited about this!

In this post I will show how logic could save your life! It can help you find the door to "the castle beyond the goblin city," or it could save you from certain death! In this video Sarah uses her reason to deduce which door leads to the castle, and which door leads to certain doom.





The dialog I have written out:



You can't ask us, you can only ask one of us. It's in the rules!
Blue guy: And I should warn you that one of us always tells the truth, and one of us always lies. That's a rule too. HE always lies (gesturing to red guy)
Red Guy: I do not! I tell the truth!
Blue Guy: OH what a lie…. Lol
Red Guy: He's the lier!
Sarah asking Red Guy: Alright, answer yes or no. Would he (blue guy) tell me this door (behind red guy) leads to the castle?
Red Guy: Uh… Yes?
Sarah: Then the other door leads to the castle, and this door leads to certain death.
Red Guy: How do YOU know? He could be telling the truth.
Sarah: But then you wouldn't be. So if you told me that he (blue guy) said yes, I know the answer is no.
Red Guy: But *I* could be telling the truth!
Sarah: But then HE would be lying. So if you told me that he said yes, I know the answer would still be no.
Guy:  Is that true?
Other guy:  I don't know, I've never understood it!  lol


Which guard is the liar and which guard is the truthteller?  By all reason, we cannot know.  But we can use logic to discover which door leads us to good and which will lead us to evil....

 
Logically Sarah's argument is thus:
  
Ask guard A the following: "If I asked guard B which was the door to Hell, what door would he point to?"

If A is the truthteller and B is the liar:


B must point at the heaven door if asked to indicate the door to hell (because he must always lie).  A knows this, and always tells the truth, and therefore indicates the door to heaven.
  
If A is the liar and B is the truthteller:


B must tell you which is the door to hell (because B must always tell the truth).  A knows this, and must lie, so A must indicate the door to heaven.

(Sarah simply reverses the argument a bit, by asking if B would indicate door A's as leading to the Castle, and she assumes this door leads to certain death)

In either case, while we cannot determine which is the liar and which is the truthteller, the Heaven door is indicated by simple logic.

Now let us imagine for the sake of argument that the doors to Heaven and Hell are guarded by two equally opposing deities...  One deity is entirely good, and thus always tells the truth.  One deity is entirely evil, and therefore always lies...  You lack the information necessary to determine which deity is good and which is evil, and thus you must use your reason to discover the door leading to salvation.  ;)  




Phaedrus out!