<?xml version="1.0" standalone="yes"?>
<Paper uid="E89-1004">
  <Title>Dialog Control in a Natural Language System 1</Title>
  <Section position="3" start_page="0" end_page="0" type="metho">
    <SectionTitle>
REPRESENTING
PROPOSITIONAL ATTITUDES
</SectionTitle>
    <Paragraph position="0"> Knowledge about the state of the dialog is represented as a set of propositional attitudes. The following three types of propositional attitudes of an agent towards a proposition p form a basic repertoire : KNOW : The agent is sure that p is true. This does not imply that p is really true since the system has no means to find out the real state of the world. Assuming that the user of a dialog system obeys the sincerity condition (i.e., always telling the truth, c.f. \[Grice 75\]) an assertion uttered by the user implies that the user knows the content of that assertion.</Paragraph>
    <Paragraph position="1"> BELIEVE : The agent believes, but is not sure, that p is true, or he/she assumes p without sufficient evidence.</Paragraph>
    <Paragraph position="2"> WANT : The agent wants p to be true.</Paragraph>
    <Paragraph position="3"> Propositional attitudes are represented in our semantic representation language IRS, which is used by all system components involved in semantic-pragmatic processing. IRS is based on predicate calculus, and contains a rich collection of additional features required by NL processing (see \[Bergmann et al. 87\] for detailed information). A propositional attitude is written as (&lt;type&gt; &lt;agent&gt; &lt;prop&gt; &lt;time&gt;): * &lt;type&gt; is an element of the set: KNOW, BELIEVE, and WANT.</Paragraph>
    <Paragraph position="4"> * The two agents relevant in a dialog system are the USER and the SYSTEM.</Paragraph>
    <Paragraph position="5"> In addition, we use the notion 'mutual knowledge'. Informally, this means that both the user and the system know that &lt;prop&gt; is true, and that each knows that the other knows, recursively. We will use the notation (KNOW MUTUAL &lt; prop &gt; ...) to express that the proposition &lt; prop &gt; is mutually known by the user and the system.</Paragraph>
    <Paragraph position="6"> * &lt; prop &gt; is an IRS formula denoting the proposition the attitude is about.</Paragraph>
    <Paragraph position="7"> It may again be a propositional attitude, as in (WANT USER (KNOW USER x ...) ...) which means that the user wants to know x. The proposition may also contain the meta-predicates RELATED and AUGMENT: (RELATED x) means 'something which is related to the individual x', i.e., it must be possible to establish a chain of links connecting the individual and the proposition. In this general form RELATED iS only used to determine assumptions about the user's competence. For a more intensive application, however, further conditions must be put on the connecting links.</Paragraph>
    <Paragraph position="8"> (AUGMENT 0 means 'something more specific than the formula f', i.e., at least one of the variables must be quantified or categorized more precisely or additional propositions must be associated. These meta-predicates are used by the dialog control rules as a very compact way of expressing general properties of propositions. null * Propositional attitudes as any other states hold during a period of time.</Paragraph>
    <Paragraph position="9"> In WISBER we use Allen's time logic \[Allen 84\] to represent such temporal information \[Poesio 88\].</Paragraph>
    <Paragraph position="10"> &lt;time&gt; must be an individual of type TIME-INTERVAL. In this paper, however, for the sake of brevity we will use almost exclusively the special constants NOW, PAST and FUTURE, denoting time intervals which are asserted to be during, before or after the current time.</Paragraph>
  </Section>
  <Section position="4" start_page="0" end_page="0" type="metho">
    <SectionTitle>
INFERENCE RULES
</SectionTitle>
    <Paragraph position="0"> As new information is provided by the user and inferences are made by the system, the set of propositional attitudes to be represented in the system will evolve. While the semantic-pragmatic analysis of user utterances exploits linguistic features to derive the - 28 attitudes expressed by the utterances (c.f. \[Gerlach, Sprenger 88\]), the dialog control component interprets rules which embody knowledge about knowing and wanting as well as about the domain of discourse. These rules describe communicative as well as r/oncommunicative actions, and specify how new propositional attitudes can be derived. Rules about the domain of discourse express state changes including the involved action. The related states and the triggering action are associated with time-intervals so that the correct temporal sequence can be derived.</Paragraph>
    <Paragraph position="1"> Both classes of rules are represented in a uniform formalism based on the schema precondition - action - effect: * The precondition consists of patterns of propositional attitudes or states in the domain of discourse. The patterns may contain temporal restrictions as well as the meta-predicates mentioned above. A precondition may also contain a rule description, e.g., to express that an agent knows a rule.</Paragraph>
    <Paragraph position="2"> * The action may be either on the level of communication (in the case of speech act triggering rules) or on the level of the domain (actions the dialog is about). However, there are also pure inference rules in the dialog control module; their action part is void.</Paragraph>
    <Paragraph position="3"> * The effect of a rule is a set of descriptions of states of the world and propositional attitudes which are instantiated when applying the rule yielding new entries in the system's knowledge base. We do not delete propositional attitudes or other pro-OSitions, i.e., the system will not rget them, but we can mark the time interval associated with an entry as being 'finished'. Thus we can express that the entry is no longer valid, and it will no longer match a pattern with the time of validity restricted to NOW.</Paragraph>
  </Section>
  <Section position="5" start_page="0" end_page="0" type="metho">
    <SectionTitle>
CONTROL STRUCTURE
</SectionTitle>
    <Paragraph position="0"> So far, we have only discussed how the actual state of the dialog (from the point of view of a certain agent) can be represented and how changes in this state can be described. We still need a method to determine and carry out the relevant changes, given a certain state of the dialog, after interpreting a user utterance (i.e., to decide which dialog rules may be tried and in which order).</Paragraph>
    <Paragraph position="1"> For reasons of simplicity we have divided the set of rules into three subsets each of them being responsible for accomplishing a specific subtask, namely: * gaining additional information inferable from the interrelation between recent information coming from the last user utterance and the actual dialog context. The combination of new and old information may, e. g., change the degree of certainty of some proposition, i. e., terminate an (uncertain) BELIEVE state and create a (certain) KNOW state with identical propositional content (the consistency maintenance rule package).</Paragraph>
    <Paragraph position="2"> * pursuing a global (cognitive or manipulative) goal; this may be done either by trying to satisfy this goal directly, or indirectly by substituting a more adequate goal for it and pursuing this new goal. In particular, a goal substitution is urgently needed in case the original goal is unsatisfiable (for the system), but a promising alternative is available (the goal pursuit rule package).</Paragraph>
    <Paragraph position="3"> * pursuing a communicative subgoal.</Paragraph>
    <Paragraph position="4"> If a goal can not (yet) be accomplished due to lack of information, this leads to the creation of a WANT concerning knowledge about the missing information. When a goal has been accomplished or a significant difference in the beliefs of the user and the system has been discovered, the system WANTS the user to be informed about that. All this is done in the phase concerned with cognitive goals. Once such a WANT is created, it can be associated with an appropriate speech act, provided the competent dialog partner (be it the user or an external expert) is determined (the speech act triggerin~ rule package).</Paragraph>
    <Paragraph position="5"> There is a certain linear dependency between these subtasks. Therefore the respective rule packages are applied in a suitable (sequential) order, whereas those rules belonging to the same pack- 29 age may be applied in any order (there exist no interrelations within a single rule package). This simple forward inferencing works correctly and with an acceptable performance for the actual coverage and degree of complexity of the system.</Paragraph>
    <Paragraph position="6"> A sequence consisting of these three subtasks forms a (cognitive) processing cycle of the system from receiving a user message to initiating an adequate reply. This procedure is repeated until there is evidence that the goal of the conversation has been accomplished (as indicated by knowledge and assumptions about the user's WANTS) or that the user wants to finish the dialog. In either case the system closes the dialog.</Paragraph>
  </Section>
  <Section position="6" start_page="0" end_page="0" type="metho">
    <SectionTitle>
APPLICATION IN A
CONSULTATION SYSTEM
</SectionTitle>
    <Paragraph position="0"> In this section we present the application of our method in the NL consulration system WISBER involving rather complex interaction with subdialogs, requests for explanation, recommendations, and adjustment of proposals.</Paragraph>
    <Paragraph position="1"> However, it is possible to introduce some simplifications typical for consulration dialogs. These are urgently needed in order to reduce the otherwise excessive amount of complexity. In particular, we assume that the user does not lie and take his/her assertions about real world events as true (the sincerity condition). Moreover, we take it for granted that the user is highly interested in a consultation dialog and, therefore, will pay attention to the conversation on the screen so that it can be reasonably assumed that he/she is fully aware of all utterances occurring in the course of the dialog.</Paragraph>
    <Paragraph position="2"> Based on these (implicit) expectations,  They express that the user knows something that 'has to do' (expressed by the meta-predicate RELATED) with states (STATE Y) concerning him/herself and that he/she wants to achieve a state (STATE X). In assumption 1, (STATE X) is in fact specialized for a consultation system as a real world state (instead of a mental state which is the general assumption in any dialog system). This state can still be made more concrete when the domain of application is taken into account: In WISBER, we assume that the user wants his/her money 'to be invested.' The second assumption expresses (a part of) the competence of the user. This is not of particular importance for many other types of dialog systems. In a consultation system, however, this is the basis for addressing the user in order to ask him\]her to make his/her intentions more precise. In the course of the dialog these assumptions are supposed to be confirmed and, moreover, their content is expected to become more precise.</Paragraph>
    <Paragraph position="3"> In the subsequent paragraphs we outline the processing behavior of the system by explaining the application and the effect of some of the most important dialog rules (at least one of each of the three packages introduced in the previous section), thus giving an impression of the system's coverage. In the rules presented below, variables are suitably quantified as they appear for the first time in the precondition. In subsequent appearences they are referred to like constants. The interpretation of the special constants denoting time-intervals depends on whether they occur on the left or on the right side of a rule: in the precondition the associated state/event must hold/occur during PAST, FUTURE or overlaps NOW; in the effect the state/ event is associated with a time-interval that starts at the reference time-interval. null In a consultation dialog, the user's wants may not always express a direct request for information, but rather refer to events and states in the real world. From such user wants the system must derive requests for knowledge useful when attempting to satisfy  guous consequences (pursuing a global goal) ring communicative goals is of central importance for the functionality of the system.</Paragraph>
    <Paragraph position="4"> There is, however, a fundamental distinction whether the content of a want refers to a state or to an event (to be more precise, to an action, mostly). In the latter case some important inferences can be drawn depending on the domain knowledge about the envisioned action and the degree of precision expressed in its specificatiqn. If, according to the system's domain model, the effect of the specified action is unambiguous, the user can be expected to be familiar with this relation, so he/she can be assumed to envision the resulting state and, possibly, the pre-condition as well, if it is not yet fulfilled. Thus, in principle, a plan consisting of a sequence of actions could be created by application of skillful rule chaining.</Paragraph>
    <Paragraph position="5"> This is exactly what Rule 1 asserts: Given the mutual knowledge that the user wants a certain action to occur, and the system's knowledge (in form of a unique rule) about the associated pre-condition and effect, the system concludes that the user envisions the resulting state and he/she is familiar with the connecting causal relation. If the uniqueness of the rule cannot be 2 Unlike other systems, e.g., UC \[Wilensky et al. 84\], which can directly perform some kinds of actions required by the user, WISBER is unable to affect any part of the real world in the domain of application.</Paragraph>
    <Paragraph position="6">  established, sufficient evidence derived from the partner model might be an alternative basis to obtain a sufficient categorization of the desired event so that a unique rule is found. Otherwise the user has to be asked to precise his/her intention.</Paragraph>
    <Paragraph position="7"> Let us suppose, to give an example, that the user has expressed a want to invest his/her money. According to WISBER's domain model, there is only one matching domain rule expressing that the user has to possess the money before but not after investing his/her money, and obtains, in exchange, an asset of an equivalent value. Hence Rule 1 fires.</Paragraph>
    <Paragraph position="8"> The want expressed by the second part of the conclusion can be immediately satisfied as a consequence of the user utterance 'I have inherited 40 000 DM' by applying Rule 5 (which will be explained later). The remainder part of the conclusion matches almost completely the precondition of Rule 2.</Paragraph>
    <Paragraph position="9"> This rule states: If the user wants to achieve a goal state (G) and is informed about the way this can be done (he/she knows the specific RULE R and is capable of performing the relevant action), the system is right to assume that the user is lacking some information which inhibits him/her from actually doing it.</Paragraph>
    <Paragraph position="10"> Therefore, a want of the user indicating the intention to know more about this transaction is created (expressed by the meta-predicate AUGMENT). If the necessary capability cannot be attributed to the user a consultation is impossible.</Paragraph>
    <Paragraph position="11"> If, to discuss another example, the user has expressed a want aiming at a cer- null quaintance with the associated causal relation (pursuing a global goal) rain state (e.g., 'I want to have my money back'), the application of another rule almost identical to Rule 1 is attempted. When its successful application yields the association of a unique event, the required causal relation is established. Moreover, the user's familiarity with this relation must be derivable in order to follow the path indicated by Rule 2. Otherwise, a want of the user would be created whose content is to find out about suitable means to achieve the desired state (as expressed by Rule 3, leading to a system reaction like, e.g., 'you must dissolve your savings account').</Paragraph>
    <Paragraph position="12"> It is very frequently the case that the satisfaction of a want cannot immediately be achieved because the precision of its specification is insufficient. When the domain-specific problem solving component indicates a clue about what information would be helpful in this respect this triggers the creation of a system want to get acquainted with it.</Paragraph>
    <Paragraph position="13"> Whenever the user's uninformedness in a particular case is not yet proved, and this information falls into his/her competence area, control is passed to the generation component to address a suitable question to the user (as expressed in Rule 4).</Paragraph>
    <Paragraph position="14"> Provided with new information hopefully obtained by the user's reply the system tries again to satisfy the (more precisely specified) user want. This process is repeated until an adequate degree of specification is achieved at some stage.</Paragraph>
    <Paragraph position="15">  want in this area (triggering a speech act) In the course of the dialog each utterance effects parts of the system's current model of the user (concerning assumptions or temporarily established knowledge). Therefore, these effects are checked in order to keep the data base consistent. Consider, for instance, a user want aiming at investing some money which, after a phase of parameter assembling, has led to the system proposal 'I recommend you to buy bonds' apparently accomplishing the (substitued) goal of obtaining enough information to perform the envisioned action. Consequently, the state of the associated user want is subject to change which is expressed by Rule 5.</Paragraph>
    <Paragraph position="16"> Therefore, the mutual knowledge about the user want is modified (by closing the associated time-interval) and the the user's want is marked as being 'finished' and added to the (new) mutual knowledge.</Paragraph>
    <Paragraph position="17"> However, this simplified treatment of the satisfaction of a want includes the restrictive assumptions that the acceptance of the proposal is (implicitly) anticipated, and that modifications of a want or of a proposal are not manageable. In a more elaborated version, the goal accomplishment has to be marked as provisory. If the user expresses his/her acceptance either explicitly or changes the topic (thus implicitly agreeing to the proposal), the application of Rule 5 is fully justified.</Paragraph>
    <Paragraph position="18"> Apart from the problem of the increasing complexity and the amount of necessary additional rules, the preliminary status of our solution has much to do with problems of interpreting the AUGMENT-predicate which appears in the representation of a communicative goal according to the derivation by Rule 2: The system is satisfied by finding any additional information augmenting the user's knowledge, but it is not aware of the requirement that the information must be a suitable supplement (which is recognizable by the user's confirmation only).</Paragraph>
  </Section>
  <Section position="7" start_page="0" end_page="0" type="metho">
    <SectionTitle>
(KNOW MUTUAL
(WANT USER (MEETS TI NOW)
</SectionTitle>
    <Paragraph position="0"/>
  </Section>
class="xml-element"></Paper>