ok , let me explain - say (a <= 3) and (a>3) what is the value of a ?........ ofcourse a belongs to real number .
taking the same analogy if a tm ( here P ) accepts a particular type of language and other tm (p") rejects the same type of language then what is the union of the above languages , offcourse complete language and a complete language is regular. now if you could remember then Regular language is decidable (i.e there exist an algorithm or TM that decides whether a language is regular or not )
coming to second point that whether there exist a TM that accepts its own encoding is undecidable