Maybe you already have heard of the Liar Paradoxon. It is about a Cretan (no offense, thats just he original though) who says "All Cretans are Liars".
As you can imagine, that from this Sentence, a Paradoxon rises: if he tells the truth, then - in conclusion - he would be lying - but in conclusion - saying the truth again.

MY SOLUTION:


1. A Lie is a statement that - in any given Way - is not fitting the actual Stand of things.
2. The Opposite of a Lie is not necessarily the Truth (if "a Tree is yellow" is a lie, then "a Tree is Magenta" is not necessarily the truth)
3. If ze Cretan thus is lying, then he isn't necessarily saying that all Cretans tell the truth, but maybe only that SOME Cretans lie and AOME Cretans tell the truth. Thus, the Cretan is Lying, as he is saying that ALL Cretans lie, which is not true. But - in conclusion of point one and two - This does not mea; that all Cretans necessarily always tell the truth.

4. CONCLUSION:
The Cretan has told us a Lie [​which interestingly hasn't been denied with proof for over Two Thusand years]