  Where supervaluation theory allows a is not tall or a is not bald to be neither (super)true nor (super)false if a represents a borderline instance of tallness, subvaluation theory would characterize such an assessment as both (sub)true and (sub)false. But since tautologies are preserved if validity is defined in terms of truth and not subtruth (or supertruth)—an assumption that some might challenge—a is tall and a is not tall can never be true, even when a is tall and a is not tall are both true: