What is insane paresis?

sometimes if they say they are then that doesn't really mean that there insane

If you are refering to The Cather in the Rye then yes, he is rather insane.

because everyone has to have insanity

Paresis is defined as slight or partial paralysis.

Depends on what type of insanity, I would think that the nutjobs locked up in straight jackets dont even know how to count to potato.