Studying properties of classes of default logics

The study of different variants of default logic reveals not only differences but also properties they share. For example, there seems to be a close relationship between semi-monotonicity and the guaranteed existence of extensions. Likewise, formula-manipulating default logics tend to violate the property of cumulativity. The problem is that currently such properties must be established separately for each approach. This paper describes some steps towards the study of properties of classes of default logics by giving a rather general definition of what a default logic is. Essentially our approach is operational and restricts attention to purely formula-manipulating logics. We motivate our definition and demonstrate that it includes a variety of well-known default logics. Furthermore, we derive general results regarding the concepts of semi-monotonicity and cumulativity. As a benefit of the discussion we uncover that some design decisions of concrete default logics were not accidental as they may seem, but rather they were due to objective necessities.