Paradox
A paradox is a sentence in logic that cannot be true but also cannot be false. Many famous problems of this kind exist.
One of most famous paradoxes is called the liar's paradox. It is the simple sentence "This sentence is a lie."
If the sentence is true, then it is a lie, as it says. But if it is a lie, how can it be true? A lie cannot also be the truth. So the sentence being true makes it a lie.
If the sentence is a lie, then it is not as it says, it is true. But that is just what the sentence says. So that makes it true. So the sentence being a lie makes it true.
This paradox is not just true in English but in any language powerful enough for a sentence to make a claim about itself. This is true of mathematics as well. Paradox can never be removed from any symbol system that makes claims about itself.
Another example is the statement that there is no cabal. Only a cabal can know if there is no cabal, so this is either a guess, or, it is a cabal trying to pretend it doesn't exist.
Other famous examples:
A paradox can also arise in ethics. Taking power over others is often required to protect them, but also, one of the things being protected is their ability to do as they please, which this power interferes with. There is another article on ethical dilemma which means "a paradox arising in ethics".
Because a paradox forces us to think "out of the box", about possibilities other than true or false in logic, right or wrong in morality, it is considered very important in education. People who do not see a paradox where others do, are likely to be too certain they are right.