The truth/lie riddle :
You have two identical paths before you. One leads to your goal, the other to certain doom. There is a guardian of some sort beside each path, usually a troll or gargoyle or soldier. You are informed that one of these soldiers always tells the truth, while the other always tells lies, but you don’t know which is which and you can only ask them ONE question.
The Liar Paradox has been discussed continually in philosophy since the middle of the 4th century BCE. The most ancient attribution is to Eubulides of Miletus. He said, "A man says that he is lying. Is what he says true or false?" An ancient gravestone on the Greek Island of Cos was reported by Athenaeus to contain this poem about the paradox:
O Stranger: Philetas of Cos am I,
'Twas the Liar who made me die,
And the bad nights caused thereby.
Theophrastus, Aristotle's successor, wrote three papyrus rolls about the Liar Paradox, and the Stoic philosopher Chrysippus wrote six, but their contents are lost in the sands of time.
In the New Testament of the Bible, Saint Paul warned, "One of themselves, even a prophet of their own, said the Cretans are always liars." Paul, however, gave no indication he recognized anything paradoxical about the Cretan's remark.
There are many versions of the Paradox in addition to Buridan's and the Liar generated from. Some liar paradoxes begin with sets of two or more sentences:
The following sentence is true.
The previous sentence is false.
Read more about the logic and history behind the paradox/riddle here:
http://www.iep.utm.edu/p/par-liar.htm#H1
