Doubt truth to be a liar.

There’s that old riddle about the two brothers at a crossroads, the one in which one always lies and the other always tells the truth but you don’t know which; you can only ask one question to figure out which way you should go. If you don’t know this riddle and want to work it out on your own, read no further and do that. If you’ve heard this riddle as many times as I have, you may recall that the solution is (SPOILER ALERT) to ask what his brother would say. In lying, the lying brother will reveal the truth and the truth teller will illuminate his brother’s lie.
The trouble with this thought experiment is how absolute it is. There was never yet a human who was so reliable a liar that you could tell the truth by his lies – nor was there ever a truth-teller so rock solid that you couldn’t find some reason to doubt him. Even the most die-hard radical honesty advocate might slip off the rock of truth, if only by accident. But apart from that, a person you could set your Truth Watch by, ceases to be human somehow – you certainly wouldn’t want to chat with him at a party. Maybe this riddle would work better with robots.

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