"This proposition is true" which is ambiguous
Any system that does not give this proposition the value of 'true' is wrong, for all definitions of true and wrong that are useful, coherent, or reasonable.
Mind explaining why? I don't see any reason it's any more true than it is false.
Graham Priest discusses The Liar's Paradox for a NY Times blog. It seems that one way of solving the Liar's Paradox is defining dialethei, a true contradiction. Less Wrong, can you do what modern philosophers have failed to do and solve or successfully dissolve the Liar's Paradox? This doesn't seem nearly as hard as solving free will.
This post is a practice problem for what may become a sequence on unsolved problems in philosophy.