Flaws and inconsistencies are usually introduced with negation. A very neat theory for negation in first-order rule-based systems called "Well-Founded Semantics

Flaws and inconsistencies are usually introduced with negation. A very neat theory for negation in first-order rule-based systems called "Well-Founded Semantics" (https://en.wikipedia.org/wiki/Well-founded_semantics https://en.wikipedia.org/wiki/Well-founded_semantics) has the "side-effect" of assigning the "undefined" logical value to propositions that can't be proven true or false in a well-founded manner (think Russel and his barber). People have experimented with Prolog, the Well-Founded Semantics and rules used to infer information about mental disorders (http://www.psychiatry.org/practice/dsm http://www.psychiatry.org/practice/dsm) in exactly the context mentioned here. Maybe unsurprisingly, some of those rules were found to be...unfounded (pun intended) :)