I’ve had a bit of experience dealing with people whose brains have been damaged by injury or illness. My maternal grandfather (from whom I inherited my irascibility) died of a brain tumor in the early 70s. A few years later, my paternal grandfather (from whom I inherited my looks) developed what was then called senile dementia. A few years later my mom started having strokes. Ten years ago this week, my then boss was in a car crash that gave her head injuries that prevented her from ever working again.
A few years ago my dad got a tumor that has really smashed up his brain.

I bring this up because all of those people had some degree of delusions as a result of their illness or injury. I’ve noticed a few things about delusions:

1. each person tends to develop a typical pattern to his or her delusion (happy, apprehensive, hurried, etc.);
2. most delusions I’ve seen involve the person going back to his or her past. For instance, my paternal grandfather worked as a night watchman for several years after leaving the farm life behind. Many years after he retired, his delusion pattern was to get ready for work and head out as if he were heading to work. My mother’s frequent delusions were that she was being called by her parents (who were by then long since dead).

I think it would be reasonable to require anyone who spouts PoMo ideas about there being no objective measure of reality should be required to spend a few days with a profoundly and stubbornly delusional person.