User Models And Discourse Models: United They Stand . . .

Opinions on the relationship between discourse models (DMs) and user models (UMs) are obviously influenced by preassumptions about their respective contents. As far as DMs are concerned, two divergent views have been expressed in the discussion published here: I. The DM contains only representations of the objects mentioned so far in the discourse (i.e., a mentioned-object memory--see Schuster, this issue). The term "object" will be used here in the broad sense of Schuster, thus also denoting events, properties, etc. 2. The DM contains in addition a. a representation of the purpose underlying the segments of the dialog (i.e. a dialog purpose--see Grosz Sidner 1986, Chin, this issue). b. an attentional structure, which is a subset of the representations mentioned in (1) containing the currently focused objects which are ordered in a focus stack (Cohen, this issue; Chin, this issue, who requires only that the user must be familiar with these objects). Less disagreement seems to exist about the components of a UM. Generally, it is regarded as containing explicit representations of the system's assumptions about all relevant aspects of the user, i.e., assumptions about his/ her "objective situation" (e.g., marital status, number of children), as well as about his/her prior knowledge, goals, plans and false beliefs with respect to the domain of discourse. In order to meet Wahlster's personneldatabase counterexample, it must be further required that the user model be separable by the system from the rest of the system's knowledge. To discuss the relationship between DMs and UMs, a general belief, goal, and plan maintenance system (BGP-MS) will be presented here, the purpose of which is to store and update the beliefs, goals, and plans of both the system and an arbitrary number of other agents, including the system's current user. Specific subcomponents and subfunctions of this system hopefully capture the general consensus on what constitutes a discourse model and a user model, respectively. However, we will see that these subcomponents are strongly interwoven and that--apart from a few rarely occurring exceptions--the DM is part of the UM at least at the level of content. The question arises then, of course, whether it makes sense to separate these notions conceptually. The belief, goal, and plan maintenance system outlined here is being implemented (in a somewhat simplified form) in XTRA, a natural language access system to expert systems (Allgayer et al. 1988). A previous implementation was VIE-DPM (Kobsa 1985a,b). In the knowledge base of BGP-MS, the representation of the various types of (nested) beliefs and goals (Kobsa 1988) is separated into a number of hierarchically ordered partitions (see Figure 1). If it is shared knowledge between S and U that U possesses certain beliefs (knowledge), then this knowledge or these beliefs are represented in MB(UB). 1 MB(UW) contains those goals and plans of the user, MB(SB) those beliefs of the system, and MB(SW) those goals of the system for which the same holds true. "Private" beliefs of the system about the domain of discourse / about the user's beliefs / about the user's beliefs about the system's goals are represented in SB, SBUB, and SBUBSW, respectively. MB contains the mutual beliefs (knowledge) with respect to the domain, and MW the mutual goals and plans of S and U. The arrows between the partitions denote inheritance relationships. In the partitions of BGP-MS, the content of the individual beliefs, goals, and plans can be expressed through arbitrary representational structures (e.g., a KL-ONElike representation as used in XTRA). Various markers for non-belief and uncertainty can be added: For instance, in SBUB it can be expressed, among other