Hi Kai,

Implicit in this riddle is the idea that truth and lie are absolutes. The uncertainty you and Raghunathan address is introduced when truth and lie are made relative. That is to say when truth or lie depend on how we perceive or know the facts to be. In order to introduce this uncertainty, one must also assume that the liar lies because he chooses to lie. Moreover, one must assume that the teller of truth chooses to tell the truth. This would be useful if the point of the riddle was about the morality of honesty and deceit, but it isn't.

If one wants to, one can surely include the caveat that the two men know each other, just as one could spend many paragraphs describing elaborate contraptions by which the instant and inevitable death might occur. But then someone else could contrive a loophole whereby one might encounter and still escape the (no longer) inevitable death, all leading to a Godelesque expansion of the riddle. In effect, if not intent, it becomes a mechanism by which the problem of the riddle is avoided instead of solved, a sort of intellectual procrastination if you will.

So for the purpose of this riddle, the man who always lies does so not by choice but by definition. There is a truth and there is a lie and they are independent of observation. When asked, the man who always lies does so, not because he intends to deceive, but merely by virtue of definition. Likewise, the other is honest by definition, not moral virtue. This keeps the riddle on point, just like asserting the inevitability of death rather than attempting to describe it.

Or, if you prefer, maybe it was just a poorly constructed riddle.