 Guarina, Carrara, and Giaretta (1994) develop an account of sortals within knowledge representation, a subfield of artificial intelligence intended to bridge the gap between abstract formal logics and natural languages. Their definition of a sortal is that it is a predicate that provides countability and is temporally stable. A predicate provides countability if it does not apply to any proper part of what it applies to. As they explain this, “student” provides countability because no proper part of a student is a student; but “red” does not provide countability because parts of red things are also red. Temporal stability means that if the predicate applies to something at one time then it must at some other times as well. This is approximately the concept Strawson had in mind. They go on to distinguish a subset of these, the substantial sortals. A substantial sortal is one that, if it applies at all, necessarily applies all times. This is in close agreement with Wiggins’ use of the term. So while “student” provides countability and is temporally stable, it is not ontologically rigid, so it is a sortal but not a substantial sortal. On the other hand, a cat is always necessarily a cat.