An old idea, but a good one: being crazy (or psychotic) is based on looking at a point of normalcy outside yourself, reflecting upon an internal point, and discerning how well you align.
The difficulty is, when you are a sane person in a sick world, you must perform sickness to appear sane.

And to go further: "You aren’t thinking or really existing unless you’re willing to risk even your own sanity in the judgment of your existence.” Frank Herbert, Children of Dune