One of the central issues to be addressed in basing a theory of speech acts on in-dependently motivated accounts of prop-ositional attitudes (belief, knowledge, intentions, . . . ) and action is the spec-ification of the effects of communicative acts. The very fact that speech acts are largely conventional means that specifying, for example, the effects of the utterance of a declarative sentence, or the perform-ance of an assertion, requires taking into consideration many possible exceptions to the conventional use of the utterances (e.g., the speaker may be lying, the hearer may
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not believe him, etc.). Previous approaches to the problem have paid insufficient attention to the dependence of the par-ticipants' mental state before the utter-ance on their mental state following it. We present a limited solution to the re-vision of beliefs within Reiter's non-monotonic Default Logic and show how to formulate the consequences of many uses of declarative sentences. Default rules are used to embody a simple theories of belief adoption, of action observation, and of the relation between the form of a sentences and the attitudes it is used to convey.
