Circumscription with homomorphisms

One important facet of common-sense reasoning is the ability to draw default conclusions about the state of the world, so that one can, for example, assume that a given bird flies in the absence of information to the contrary. A deficiency in the circumscriptive approach to common-sense reasoning has been its difficulties in producing default that Tweety ≠ Blutto using ordinary circumscription, or conclude by default that a particular bird flies, if some birds are known not to fly. In this paper, we introduce a new form of circumscription, based on homomorphisms between models, that remedies these two problems and still retains the major desirable properties of traditional forms of circumscription.