Kenneth Rosen Edition 7 Exercise 1.2 Question 16 (Page No. 23)

Question

An explorer is captured by a group of cannibals. There are two types of cannibals—those who always tell the truth and those who always lie. The cannibals will barbecue the explorer unless he can determine whether a particular cannibal always lies or always tells the truth. He is allowed to ask the cannibal exactly one question..

a) Explain why the question “Are you a liar?” does not work.

b) Find a question that the explorer can use to determine whether the cannibal always lies or always tells the truth.

in the below link, it mentioned double negation will work. I am not getting what is double negation here. ow the cannibal will consider as two separate question.

https://math.stackexchange.com/questions/1078866/is-this-a-correct-solution-to-determining-which-of-two-people-is-the-liar-using

Answer 1

“Are you a liar?” does not work because liar would say no and the true cannibal will also say no so we cannot find who is telling truth and who is lying.

The question If i were to ask whether you are a liar,would you answer yes would work here double negation means

for the question are you a liar the original answer of the liar cannibal be yes name it p first it would answer no so this will be first negation of p and for second question would you answer yes it would say no another negation of p so both negations get canceled and it would be an yes

Comments

@gatecse 

 Sir, in vpria's answer for part b) given, "for second question would you answer yes it would say no. "

I think it should be like "for second question, would you answer yes? it would be yes." because we assume cannibal is liar. 

am I right, Sir????? 

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The given answer for part b is correct.

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@gatecse yes, sir given answer is correct but my doubt in a particular line. 

According to you, what is the answer of "would you answer yes?"