The semantic theory of truth states that “‘P’ is true if and only if P”. The theory was developed by logician and philosopher Alfred Tarski. He said that no language can contain its own truth predicate; that the expression is true can only apply to sentences in another language. What he meant was that natural languages that have their own truth predicates will have paradoxical sentences in them.
This is as philosophical as things can get, but to simplify it means that if you say ‘It is false’ and it is, indeed, false then the statement is true. But then again, if ‘It is false’ is true but says ‘It is false’ then ‘It is false’ is clearly false. Easy peasy…
