  All forms of circumscription involve restricting attention to models in which certain sets are minimized; for this reason, circumscription can be grouped with the preferred models approaches to nonmonotonicity: see  Section 3.4,  below. McCarthy’s formalism is fairly conservative; though it raises interesting logical issues in higher-order logic and complexity, it uses familiar logical frameworks. And much of the focus is on the development of formalization techniques. The other varieties of nonmonotonic logic, including default logic and the modal nonmonotonic logics, raise issues of the sort that are familiar to philosophical logicians, having to do with the design of new logics, the systematic investigation of questions concerning validity, and managing the proliferation of alternative logics.