Explicit preferences on assumptions as used in prioritized circumscription (McCarthy, 1986; Lifschitz, 1985; Grosof, 1991) and preferred subtheories (Brewka, 1989) provide a clear and declarative method for defining preferred models. In this paper, we shwo how to embed preferences in the logical theory itself. this gives a high freedom for expressing statements about preferences. Preferences can now depend on other assumptions and are thus dynamic. We elaborate a preferential semantics based on Lehmann's cumulative models, as well as a corresponding constructive characterization, which specifies how to correctly treat dynamic preferences in the default reasoning system Ex-CEPT (Junker, 1992).
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