 On the other hand, the criterion of identity for sets given by the Axiom of Extensionality (sets are the same iff they have the same members), unlike the criterion of identity for numbers given by Hume’s Principle, and Davidson’s criterion of event identity (events are the same iff they have the same causes and effects (“The Individuation of Events” in his 1980)) are one-level: the objects for which the criterion of identity is stated are the same as those between which the criterial relation obtains. In general, a one-level criterion for objects of sort F takes the form: