 Belnap and Müller develop an alternative account of sortals within a non-standard formal logic for tense and modality. Unlike the usual background of possible worlds for modality, they use ideas from Bressan (1972) and adopt cases as the fundamental background notion They define a sortal as a predicate which is modally constant and provides separability. A predicate is modally constant iff it applies necessarily to something whenever it possibly applies. The idea is that while someone might possibly be a student without necessarily being a student, if they are human they are necessarily human. Thus “student” is not a sortal but “human” is. A predicate is modally separated just in case it is necessarily true that if any two things having the property are possibly identical, then they are in fact necessarily identical. Using their technical terminology of cases, if two things which have a modally separated property coincide in any case, then they coincide in every case. Belnap and Müller are explicit that they are providing a formal framework for classifying predicates, but that applications will require other kinds of thought: