We conclude by stating that set theory can be useful in commonsense reasoning. The methodology may change, of course. A universal commonsense set theory can be developed by means of proposing new axioms or modifying existing ones. Alternatively, different set-theoretic concepts may be examined and modified based on existing set theories. No matter what proposal is followed, we believe that further research in this field should be promising and may even lead to a ``mathematical metaepistemology analogous to metamathematics,'' as pointed out by McCarthy (1988).