The immediate goal of such an utterance is to change the mental state (especially beliefs, desires and intentions) of the hearer(s), and speech act theory concerns the way in which particular types of speech acts effect such changes, directly or indirectly (Austin 1962; Grice 1968; Searle 1969). To choose speech acts sensibly, each participant also needs to take account of the mental state of the other(s); in particular, each needs to recognize the other's beliefs, desires and intentions. Discourse conventions in cooperative conversation are adapted to facilitate this process: The speakers employ locutions that reveal their intended effects, and their acknowledgments and turn-taking consolidate mutual understanding. In this way mixed-initiative dialogue and potentially, cooperative domain action are achieved.