Role change in database domains

In data modelling the universe of discourse (UoD) is divided up into classes having a taxonomic structure which is intended to express some of the structure inherent in the UoD. Some of these classes. for example the class of persons or departments, may be called "natural kinds," in that they are a fixed set of possible objects, existing in some possible state of the UoD, and all of which have a similar structure and behavior. Others have a more dynamic nature, such as the class of stu­ dents. Whereas an object is created as as person and, when it ceases to be a person, ceases to exist, an object may come to be a student and cease to be one without com­ ing into existence or passing away. A class like persons is a natural kind, and a class like students will be called roles in this report. This report studies the formal defini­ tion of roles and the resulting taxonomy of natural kinds and roles.