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<?xml version="1.0" standalone="yes"?> <Paper uid="J80-3003"> <Title>A Plan-Based Analysis of Indirect Speech Acts 1</Title> <Section position="3" start_page="0" end_page="0" type="metho"> <SectionTitle> 2. Introduction to Speech Acts 2.1. Basic Definitions </SectionTitle> <Paragraph position="0"> Prior to Austin \[1962\], logicians considered the meaning of a sentence to be determined only by its truth value. However, Austin noted that some sentences cannot be classified as true or false; the utterance of one of these sentences constitutes the performance of an action, and hence he named them performatives. To quote Austin: &quot;When I say, before the register or altar, etc., 'I do', I am not reporting on a marriage: I am indulging in it&quot;.</Paragraph> <Paragraph position="1"> Examples like this, and his inability to rigorously distinguish performative sentences from those which purportedly have truth value (which he called constatives) led Austin to the view that all utterances could be described as actions, or speech acts. He classified speech acts into three classes, the locutionary, illocutionary, and perlocutionary acts.</Paragraph> <Paragraph position="2"> A locutionary act is an act of saying something: it is the act of uttering sequences of words drawn from the vocabulary of a given language and conforming to its grammar.</Paragraph> <Paragraph position="3"> An illocutionary act is one performed in making an utterance; &quot;promise&quot;, &quot;warn&quot;, &quot;inform&quot; and &quot;request&quot; are names of illocutionary acts. In general, any verb that can complete the sentence &quot;I hereby <verb> you {that I to} ...&quot; names an illocutionary act. An utterance has illocutionary force F if the speaker intends to perform the illocutionary act F by making that utterance. Verbs that name types of illocutionary acts are called performative verbs. From now on, we take speech acts to mean the illocutionary acts.</Paragraph> <Paragraph position="4"> Perlocutionary acts are performed by making the utterance. For example, S may scare A by warning A, or convince A of something by informing A of it. The success of a perlocutionary act is typically beyond the control of the speaker. For example, S cannot convince A of something against A's will, S can only present A with sufficient evidence so that A will decide to believe it. Perlocutionary acts may or may not be intentional. For instance, S may or may not intend to scare A by warning A.</Paragraph> <Paragraph position="5"> Searle \[1969\] suggests that illocutionary acts can be defined by providing, for each act, necessary and sufficient conditions for the successful performance of the act. Certain syntactic and semantic devices, such as mood and explicit performative verbs, are used to indicate iUocutionary force.</Paragraph> <Paragraph position="6"> One of the conditions included in Searle's account is that the speaker performs an illocutionary act only if he intends that the hearer recognize his intention to perform the act, and thereby recognize the illocutionary force. This is important for it links Austin's work American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 169 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts on speech acts with the work of Grice on meaning, and is discussed in the next section.</Paragraph> <Paragraph position="7"> 2.2. Communication and the Recognition of Intention Many philosophers have noted the relationship between communication (or speaker meaning) and the recognition of intention (Grice \[1957, 1968\], Strawson \[1964\], Searle \[1969\], Schiffer \[1972\].) Grice presents informally his notion of a speaker meaning something as follows: &quot;'S meant something by x' is (roughly) equivalent to 'S intended the utterance of x to produce some effect in an audience by means of the recognition of this intention'&quot; In other words, in order for S to communicate M by uttering x to A, S must get A to recognize that S intended to communicate M by uttering x. To use and example of Grice's, if I throw a coin out the window expecting a greedy person in my presence to run out and pick it up, I am not necessarily communicating to him that I want him to leave. For me to have successfully communicated, he must at least have recognized that I intended him to leave. The same arguments hold when discussing illocutionary acts. For example, the only way S can request A to do ACT is to get A to recognize S's intention to request A to do ACT.</Paragraph> <Paragraph position="8"> 2.3. The Indirect Speech Act Problem The relation between speech acts and the devices used to indicate them is complicated by the fact that performative verbs are seldom present and the same device can be used to perform many illocutionary acts.</Paragraph> <Paragraph position="9"> The interrogative mood, for example, can be used to request: &quot;Can you pass the salt?&quot; question: &quot;Do you know the time?&quot; inform: &quot;Do you know that Sam got married?&quot; warn: &quot;Did you see the bear behind you?&quot; promise: &quot;Would I miss your party?&quot; As many authors have pointed out, an utterance conveys its indirect illocutionary force by virtue of its literal one (Searle \[1975\], Morgan \[1977\], Morgan \[1978\]). &quot;It's cold here&quot; can function as a request to, say, close the window, in part because it's an assertion that the temperature is low.</Paragraph> <Paragraph position="10"> Most of the literature on the treatment of indirect speech acts within the theory of grammar stems from the work of Gordon and Lakoff \[1975\] (hereafter GL). They claim that direct and indirect instances of the same speech act have different &quot;meanings&quot;, i.e. different logical forms, and they propose a set of &quot;conversational postulates&quot; by which literal forms &quot;entail&quot; indirect ones. The postulates for requests correspond to conditions that must obtain for a request to be sincere. For A to sincerely request B to do ACT, the following sincerity conditions must hold: (1) A wants ACT.</Paragraph> <Paragraph position="11"> (2) B can do ACT.</Paragraph> <Paragraph position="12"> (3) B is willing to do ACT.</Paragraph> <Paragraph position="13"> (4) B will not do ACT in the absence of the request.</Paragraph> <Paragraph position="14"> They then propose that one can convey a request by asserting a speaker-based sincerity condition (condition 1), or querying a hearer-based sincerity condition (conditions 2-4).</Paragraph> <Paragraph position="15"> The postulates for indirect requests given in GL do not account for the readings of 2.3a and 2.3b as requests, and although more rules could be added (and some should be weakened) we believe this solution to be misguided.</Paragraph> <Paragraph position="16"> (2.3a) Is the salt near you? (2.3b) John asked me to ask you to pass the salt.</Paragraph> <Paragraph position="17"> GL's postulates directly relate the literal form of one speech act to the indirect form of another. Thus they do not predict why certain acts allow certain indirect forms. For example, the postulates do not account for why 2.3c-d can be requests while 2.3e-f cannot. But 2.3e is infelicitous as a (literal) question since there is no context where one can acquire information by querying one's own mental state. Utterance 2.3f is a reasonable question but even if the speaker found out the answer, it would not get him any closer to acquiring the salt (by having the hearer pass it). A theory of indirect speech acts should capture these facts; GL's does not (although they agree it should).</Paragraph> <Paragraph position="18"> (2.3c) I want the salt.</Paragraph> <Paragraph position="19"> (2.3d) Do you want to pass the salt? (2.3e) Do I want the salt? (2.3f) Does he want to pass the salt? Similarly, GL's postulates fail to explain the relation between indirect forms of different speech acts. For example, 2.3g can be an assertion that P and 2.3h cannot, for the same reasons that 2.3i can be a request to do A and 2.3j cannot.</Paragraph> <Paragraph position="20"> (2.3g) I want you to know that P.</Paragraph> <Paragraph position="21"> (2.3h) Do I want you to know that P? (2.3i) I want you to A.</Paragraph> <Paragraph position="22"> (2.3j) Do I want you to A? The hearer's knowing that P obtains is an intended perlocutionary effect of an informing act, just as the hearer's doing an act A is an intended effect of a request. A speaker can indirectly inform or request by informing the hearer that the speaker desires the perlocutionary effect of that act, and intending that the hearer recognize the speaker's intention that the perlocutionary effect should be achieved.</Paragraph> <Paragraph position="23"> This paper shows that what GL achieve with their postulates can be derived from the five hypotheses given in the Introduction. Our proposal here is a de170 American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts velopment of Searle \[1975\]. It requires separating the surface form conditions completely from the definitions of the illocutionary acts and introducing an intermediary level, the surface acts.</Paragraph> <Paragraph position="24"> Our theory of indirection will however share with GL some problems brought up by Sadock \[1970\], Green \[1975\], and Brown \[1980\]. These are discussed further in section 4.5.</Paragraph> <Paragraph position="25"> 3. Plans, Plan Construction, and Plan Inference.</Paragraph> <Paragraph position="26"> Our analysis of indirect REQUESTs and INFORMs relies on the inference by the hearer of some of the goals of the speaker and of some of the actions which the speaker is taking to achieve those goals. Section 3.1 outlines the form of the models of the world which language users are assumed to have, in particular their beliefs about the world (and about other agents), and their goals. In section 3.2 we define actions and how they affect the belief model. The rules for plan construction and inference are considered in sections 3.3 and 3.4. Because of space limitations, this section is very sketchy. More detail, motivation, and problems, We assume that every agent S has a set of beliefs about the world, which may include beliefs about other agents' beliefs. Agents can hold false beliefs. As Quine \[1956\] pointed out, belief creates a context where substitution of coreferential expressions need not preserve truth-value.</Paragraph> <Paragraph position="27"> We add to a first-order language with equality the operator B, and B(A,P) (usually written BA(P)) is to be read &quot;A believes that P&quot;, for any formula P. The B operator is assumed to satisfy the following axiom schemas (inspired by Hintikka \[1962\]), where P and Q are schema variables ranging over propositions, and A ranges over agents: (B.0) all theorems of First Order Predicate Calculus</Paragraph> <Paragraph position="29"> The rules of inference are Modus Ponens and: If T is a theorem, then BA(T ) is a theorem, for every agent A.</Paragraph> <Paragraph position="30"> i.e. every agent believes every valid consequence of the logical axioms.</Paragraph> <Paragraph position="31"> The partial deduction system used in the implementation of Allen \[1979\] is based on Cohen \[1978\]. The foundations for a more elaborate system can be found in Moore \[1979\].</Paragraph> <Paragraph position="32"> 3.1.2. Knowing The word &quot;know&quot; is used in at least three different senses in English. One may know that a proposition P is true, know whether a proposition P is true or know what the referent of a description is.</Paragraph> <Paragraph position="33"> We define &quot;A knows that P&quot;, written KNOW(A,P), as P ^ BA(P). This is weaker than some definitions of &quot;know&quot; in the philosophical literature, where, among other things, &quot;A knows that P&quot; entails that A believes P for the &quot;right reasons&quot;; i.e. knowledge is true and justified belief (Ayer \[1956\], but see also Gettier \[1963\]). If S believes that A knows that P, S is committed to believing that P is true.</Paragraph> <Paragraph position="34"> Unfortunately, the meaning of &quot;A does not know that P&quot; is not captured by ~(P a BA(P)), but by the</Paragraph> <Paragraph position="36"> In other words, if S believes A does not know P, then S must believe that P is true in addition to believing that A does not believe P is true. This problem is analogous to the wide/narrow scope distinction that Russell found in his account of definite descriptions (Russell \[1919\]). One solution to this problem is to consider KNOW as a &quot;macro&quot; whose expansion is sensitive to negation. Details may be found in Allen \[1979\].</Paragraph> <Paragraph position="37"> A knows whether a proposition P is true if A KNOWs that P or A KNOWs that ~P.</Paragraph> <Paragraph position="39"> Knowing what the referent of a description is requires quantification into belief. One of its arguments is a formula with exactly one free variable.</Paragraph> <Paragraph position="41"> A KNOWREF the departure time of TRAIN1 if TRAIN1 has a unique departure time y, and if A believes that y is TRAINI's unique departure time.</Paragraph> <Paragraph position="42"> 3.1.3. Wanting We let W(A,P) (usually written WA(P)) mean &quot;agent A wants P to be true&quot;. P can be either a state or the execution of some action. In the latter case, if ACT is the name of an action, WA(ACT(b)) means &quot;A wants b to do ACT&quot;.</Paragraph> <Paragraph position="43"> The logic of want is even more difficult than that of belief. It is necessary for us to accept the following: null American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 171 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts</Paragraph> <Paragraph position="45"> The most interesting interactions between the belief and want operators come from the models that agents have of each other's abilities to act and to recognize the actions of others. This will be further discussed in the following section.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.2. Actions and Plans </SectionTitle> <Paragraph position="0"> Actions model ways of changing the world. As with the operators in STRIPS (Fikes and Nilsson \[1971\]), the actions can be grouped into families represented by action schemas, which can be viewed as parameterized procedure definitions. An action schema consists of a name, a set of parameters with constraints and a set of labelled formulas in the following classes: Effects: Conditions that become true after the execution of the procedure.</Paragraph> <Paragraph position="1"> Body: a set of partially ordered goal states that must be achieved in the course of executing the procedure. In the examples given here, there will never be more than one goal state in a body.</Paragraph> <Paragraph position="2"> Preconditions: Conditions necessary to the successful execution of the procedure. We distinguish for voluntary actions a want precondition: the agent must want to perform the action, i.e. he must want the other preconditions to obtain, and the effects to become true through the achievement of the body.</Paragraph> <Paragraph position="3"> The constraints on the parameters consist of type specifications, and necessary parameter interdependenties. Each action has at least one parameter, namely, the agent or instigator of the action. In the blocks world, for example, the action of putting one block on top of another could be defined as:</Paragraph> <Paragraph position="5"> effect: ON(bl,b2) The preconditions, effects and body provide information to the plan construction and inference processes so that they can reason about the applicability and effect of performing the action in a given context.</Paragraph> <Paragraph position="6"> Finally, the body of the action specifies what steps must be achieved in the course of the execution of the action. Primitive actions have no bodies; their execution is specified by a non-examinable procedure.</Paragraph> <Paragraph position="7"> All agents are assumed to believe that actions achieve their effects and require their preconditions.</Paragraph> <Paragraph position="8"> We need the following axioms: For all agents a and b, and for all actions ACT, if PRE is the precondition of ACT and</Paragraph> <Paragraph position="10"> Every predicate and modal operator in these axioms, and throughout the paper, should be indexed by a state or time. The resulting logic would be, accordingly, more complex. The issue is raised again in sect. 6.</Paragraph> </Section> <Section position="2" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.3. Plan Construction </SectionTitle> <Paragraph position="0"> A plan to transform a world W\[0\] (represented by a formula) into a world W\[n\] is a sequence of actions A1 ..... An such that the preconditions of Ai are true in W\[i-1\], and Ai transforms world W\[i-1\] into W\[i\].</Paragraph> <Paragraph position="1"> An agent can achieve a goal by constructing and then executing a plan which transforms the current state of the world into one in which the goal obtains.</Paragraph> <Paragraph position="2"> This can be done by finding an operator which, if executed in some world, would achieve the goal. If its preconditions are satisfied in the initial world, the plan is complete. Otherwise, the planning process attempts to achieve the preconditions. This simple view of plan construction as a &quot;backward chaining&quot; process can be refined by assuming different levels of &quot;detail&quot; in the representation of the world and of the operators. This view (as developed in Sacerdoti \[1973, 1975\], for example) allows plans constructed at one level of detail to be expanded to a lower level through the bodies of their constituent acts.</Paragraph> <Paragraph position="3"> As noted earlier, the agent of an action must believe that its precondition is true to believe that his executing the action will succeed. For agent A to plan that agent S should perform action ACT, A must achieve that S should believe that the precondition of ACT holds, and S's beliefs should not be inconsistent with A's, i.e. it must be true that BA(KNOW(S,P)), where P is the precondition of ACT.</Paragraph> <Paragraph position="4"> We assume that an agent cannot do an action without wanting to do that action. Thus a precondition of every action ACT by an agent A is that WA(ACT(A)).</Paragraph> <Paragraph position="5"> We are concerned with the model that agents have of each other's plan construction and inference process, and consider these two processes as consisting of chains of plausible inferences operating on goals and observed actions. The processes are specified in two parts: first as schemas of rules which conjecture that certain states or actions can be added to a plan being constructed. The plausibility of the plans containing the result of the inferences is then evaluated by rating heuristics. Thus the plan construction and inference rules are not to be interpreted as valid logical rules of inference.</Paragraph> <Paragraph position="6"> 172 American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts The first three plan construction (PC) rules are: 2 (PC.EA) \[Effect-action rule\] For any agent A, if Y is an effect of action X, then if A wants Y to hold, it is plausible that A will want action X to be done.</Paragraph> <Paragraph position="7"> (PC.AP) \[Action-precondition rule\] For any agent A, if X is a precondition of action Y, and if A wants Y to be done, then it is plausible that S will want X to hold. P$, (PC.AB) \[Action-body rule\] For any agent A, if A wants an action Y to be done, and if X is a part of the body of Y then it is plausible that S will want X to be done.</Paragraph> <Paragraph position="8"> If X and Y are systematically replaced by one of the pairs in Figure 1, then rules PC.EA, PC.AP, and</Paragraph> <Paragraph position="10"> with =c=> indicating that the rule is a construction rule.</Paragraph> <Paragraph position="11"> We also need a rule based on KNOWIF: (PC.KI) \[KNOWlF rule\] For any agent A, if A wants P to be true, then it is plausible that A should want to know whether P is true.</Paragraph> <Paragraph position="12"> action-precondition, and action-body rules of the previous section: (PI.AE) \[Action-effect rule\] For all agents S and A, if Y is an effect of action X and if S believes that A wants X to be done, then it is plausible that S believes that A wants Y to obtain.</Paragraph> <Paragraph position="13"> (PI.PA) \[Precondition-action rule\] For all agents S and A, if X is a precondition of action Y and if S believes A wants X to obtain, then it is plausible that S believes that A wants Y to be done. (PI.BA) \[Body-action rule\] For all agents S and A, if X is part of the body of Y and if S believes that A wants X done, then it is plausible that S believes that A wants Y done.</Paragraph> <Paragraph position="14"> There are two inverses to the KNOWIF rule: if A wants to know whether P is true, then A may want P to be true, or A may want P to be false.</Paragraph> <Paragraph position="15"> 2 Throughout the rest of the paper agent A will usually denote the constructor/executor of plans, and S (or System) the recognizer of plans (usually constructed by A). rule where the precondition is the want precondition: (PI.W) \[Want rule\] For all agents S, A, and C and for all actions ACT whose agent is C, it is plausible that</Paragraph> <Paragraph position="17"> The plan inference rules generate formulas which the recognizing agent believes are possible. A separate mechanism is used to evaluate their plausibility. An agent S attempting to infer the plans of another agent A starts with an observed action of A and a (possibly empty) set of goals or expectations which S believes A may be trying to achieve. S attempts to construct a plan involving the action and preferably also including some of the expectations.</Paragraph> <Paragraph position="18"> Plan inference is a search through a space of partial plans each consisting of two parts. One part is constructed using the plan inference rules from the observed action (and called the alternative); the other is constructed using the plan construction rules from an expected goal (and called the expectation).</Paragraph> <Paragraph position="19"> The partial plans are manipulated by a set of tasks which decide what rules are to be applied, what &quot;merges&quot; between alternatives and expectations should be attempted, and when the process terminates.</Paragraph> <Paragraph position="20"> The partial plans and their associated tasks are rated by a set of heuristics, and the most highly rated task is executed first.</Paragraph> <Paragraph position="21"> The rating of a partial plan reflects how likely it is to be part of the &quot;correct&quot; plan, i.e. the plan the speaker is executing. If several incompatible inferences can be made from one point in the alternative, then its rating is divided among them. The heuristics described in this section are based on domain independent relations between actions, their bodies, preconditions, and effects. The need for more domain dependent measures is discussed later.</Paragraph> <Paragraph position="22"> American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 173 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts The heuristics are described here only in terms of increasing or decreasing ratings of partial plans.</Paragraph> <Paragraph position="23"> Decrease the rating of a partial plan in which the preconditions of executing actions are currently false.</Paragraph> <Paragraph position="24"> Decrease the rating of a partial plan containing a pending action ACT by an agent A if A is not able to do ACT. 3 Decrease the rating of a partial plan in which the effects of a pending act already obtain or are not wanted by the planner. 4 Other heuristics depending on how well the utterance fits with the expectations are not immediately relevant to understanding indirect speech acts and will not be discussed here. One further heuristic is added in section 4.3.</Paragraph> <Paragraph position="25"> In general several rating heuristics are applicable to an partial plan. Their effects on the rating of the partial plan are cumulative.</Paragraph> <Paragraph position="26"> 3.4.3. Extending the Inference Rules A hearer S identifies the illocutionary force of an utterance by recognizing that the speaker A has certain intentions, namely that S should recognize some intention P of A's. This can be represented by a formula of the form BsWA(BsWA(P)). To do the recognition, the simple plan construction and inference rules of sections 3.3 and 3.4 must be extended so that they can operate on these nested formulas. This can be done by assuming that every agent is aware that other agents construct and infer plans in the same way he can. In fact, both the simple inference and construction rules are necessary to derive the extended inferenee rules.</Paragraph> <Paragraph position="27"> The extended rules are specified by &quot;meta-rules&quot; which show how to construct new PC/PI rules from old ones. The first extended construction rule (EC. 1) is: A can achieve that S recognizes that A wants the effect of ACT by achieving that S recognizes that A wants ACT to be done, assuming that S would infer that the effects of ACT are also desired. The same rule applies if we replace &quot;wants the effect of ACT&quot; and &quot;wants ACT to be done&quot; by any pair of Y and X, as given in Figure 1. We assume all these sul~stitu3 This definition is the same as Cohen's CANDO relation. Being able to do an action means that the action's preconditions are either presently true, achieved within the existing plan, or can be achieved by a &quot;relatively simple plan&quot;, which we take to be a single action whose preconditions are presently true or achieved in the existing plan.</Paragraph> <Paragraph position="28"> 4 We have avoided the problem here of planning to do a task that requires one to deny a subgoal temporarily so that some action can execute, and then needing to reaehieve that (presently true) goal.</Paragraph> <Paragraph position="29"> tions are possible in rules EC.1 - EC.3 and EI.1 -</Paragraph> <Paragraph position="31"> rule.</Paragraph> <Paragraph position="32"> EI. 1 allows prefixing BsW A to plan inference rules. Plan construction rules can also be embedded: if A wants S to want to do ACT, then A should be able to achieve this by achieving that S wants the effect of ACT, and by relying on S to plan ACT. In other words:</Paragraph> <Paragraph position="34"> Finally, any agent A can plan for S to recognize A's intention that S plan, and for S to be able to recognize this intention in A. For example, A can plan for S to recognize A's intention that S want to close the door by planning for S to recognize A's intention that S watlt the door closed. These rules are obtained by using EI.2 as the PI rule which is &quot;extended&quot; by EC. 1 and El. 1.</Paragraph> <Paragraph position="36"> Our &quot;toolkit&quot; is now sufficiently full to allow us to consider some speech acts and their recognition.</Paragraph> <Paragraph position="37"> 4. Plan Inference and Indirect Speech Acts</Paragraph> </Section> <Section position="3" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 4.1. Speech Acts </SectionTitle> <Paragraph position="0"> The definitions of the speech acts REQUEST and INFORM used in this paper are slightly different from the ones in Cohen and Perrault \[1979\] in that they rely on the existence of speech act bodies to account for indirect forms. Plans including speech acts are now thought of as having two levels, the illocutionary level and the surface level. Acts at the illocutionary level model the intentions motivating an utterance independently of the syntactic forms used to indicate those intentions. Acts at the surface level are realized by utterances having specific illocutionary force indicators. null 174 American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts 'The first illocutionary level act is one by which a speaker informs a hearer that some proposition is true.</Paragraph> <Paragraph position="1"> INFORM(speaker, hearer, P) that P (the preconditions), and must intend to get S to know that P is true (the effect), which is done by constructing a plan that will achieve S's recognition of this intention (i.e. that Bs(WA(KNOW(S,P))~). A then must depend on S to bring about the effect: S must decide to believe what A said. This is made explicit by introducing an admittedly simplistic DECIDE TO In many cases, agents reason about INFORM acts to be performed (by others or by themselves) where the information for the propositional content is not known at the time of plan construction. For example, A may plan for S to inform A whether P is true. A cannot plan for S to perform INFORM(S,A,P) since this assumes the truth of P. We get around this diffi- null Similarly, it must be possible for A to plan for S to tell A the referent of a description, without A knowing the referent. This is the role of the INFORMREF act.</Paragraph> <Paragraph position="2"> The intention of a request is to get the hearer to want to do the action, and this is accomplished by getting the hearer to believe that the speaker wants the hearer to do the action and then depending on the hearer to decide to do it. 5 To explicitly represent this decision process, a CAUSE TO WANT act defined along the lines of the DECIDE TO BELIEVE act above is necessary.</Paragraph> <Paragraph position="3"> CAUSE TO WANT(agent, other, P) prec: B(other,B(agent,W(agent,P))) effect: W(other,P) As examples of the use of speech acts, &quot;Tell me whether the train is here&quot; and &quot;Is the train here?&quot;, intended literally, are both REQUESTs by A that S INFORMIF the train is here. &quot;When does the train arrive?&quot;, intended literally, is a REQUEST by A that H INFORMREF of the departure time of the train.</Paragraph> <Paragraph position="4"> Finally we define the two surface level acts: S.INFORM produces indicative mood utterances, and S.REQUEST produces imperative utterances, or interrogative utterances, if the requested act is an IN-FORM. These acts have no preconditions, and serve solely to signal the immediate intention of the speaker, the starting point for all the hearer's inferencing.</Paragraph> <Paragraph position="5"> S.INFORM(speaker, hearer, P) effect: B(hearer,W(speaker,KNOW(hearer,P))) S.REQUEST(speaker, hearer, action) effect: B (hearer,W(speaker,action(hearer))) The effects of S.INFORM match the body of the INFORM act, reflecting the fact that it is a standard way of executing an INFORM. It is important, however, that S.INFORM is only one way of executing an IN-FORM. The same relationship holds between the S.REQUEST and REQUEST actions.</Paragraph> <Paragraph position="6"> 4.2. Recognizing IIIocutionary Force Given the speech act definitions of section 4.1, we say that A performed an illocutionary act IA by uttering x to S if A intends that S should recognize (and be able to recognize) that (1) x is an instance of a surface act SA, and (2) A intended S to infer (using the PI rules and associated heuristics) from A having performed SA that A wants to achieve the effects of IA.</Paragraph> <Paragraph position="7"> This definition allows more than one illoeutionary act to be performed by a single surface act. In this section we show how the hearer of an utterance can recognize the speaker's intention(s) indicated by a speech act, especially when these intentions are communicated indirectly.</Paragraph> <Paragraph position="8"> 5 See Cohen and Perrault \[1979\] for a discussion of why Searle's preparatory conditions &quot;Speaker believes Hearer can do the action&quot; need not bc part of the preconditions on REQUEST. American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 175 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts All inferencing by S of A's plans starts from S's recognition that A intended to perform one of the surface acts, and that A in fact wanted to do the act. All inference chains will be shown as starting from a formula of the form BsWA(A do the surface act). The object of the inferencing is to find what illocutionary level act(s) A intended to perform. The action-effect rule applied to the starting formula yields one of the form BsWA(BsWA(P)), i.e. S believes that A wants S to recognize A's intention that P. The inferencing process searches for plausible formulas of the form BsWA(IA(A)) where IA is an illocutionary level act.</Paragraph> <Paragraph position="10"> Example 1 shows a direct request to pass the salt, where the surface request maps directly into the intended request interpretation. 6 The actions relevant to the examples given here are: PASS(agent, beneficiary, object) prec: HAVE(agent, object) effect: HAVE(beneficiary, object) REACH(agent, object) prec: NEAR(agent, object) effect: HAVE(agent, object) Let us also assume that S presently has the salt, i.e. HAVE(S,SALT) is true, and mutually believed by S and A.</Paragraph> <Paragraph position="11"> The rating heuristics for the complex rules El. 1 to EI.3 are the same as for the PI rules but each heuristic may be applicable several times at different levels. For example, consider the frequently recurring inference chain:</Paragraph> <Paragraph position="13"> It shows the line of inference from the point where S recognizes that A requested S to do ACT (at step (2)) to the point where the effects of the requested action are inferred as part of A's plan. Of interest here is the evaluation of the plausibility of step (3). Two heuristics are applicable. The proposition &quot;Ws(ACT(S))&quot; is 6 To improve readability of inference chains in the examples, we drop the prefix BsW A from all propositions. The formula on line (n) follows from the one on line (n-l) by the rule at the beginning of line (n). Applications of EI.1 will be labelled &quot;rule&quot;/EI.l, where &quot;rule&quot; is a PI rule embedded by EI.1. Similarly, applications of EI.2 and EI.3 will be labelled &quot;rule&quot;/EI.2 and &quot;rule&quot;/EI.3, where &quot;rule&quot; is a PC rule name.</Paragraph> <Paragraph position="14"> evaluated with respect to what S believes A believes.</Paragraph> <Paragraph position="15"> (Remember that BsW A should appear as a prefix to all propositions in inference chains.) If BsBAWs(ACT(S)) is true, the request interpretation is considered unlikely, by the effect-based heuristic. In addition, the pre-conditions of ACT(S) are considered with respect to what S believes A believes S believes. This step will only be reasonable if S can do the action, by a precondition-based heuristic.</Paragraph> <Paragraph position="16"> To make more explicit the distinction between inferences in BsW A and inferences in BsWABsWA, let us consider two inference chains that demonstrate two interpretations of the utterance &quot;Do you know the secret?&quot;. Lines 1-3 of Example 2 show the chain which leads S to believe that A asked a (literal) yes/no question; lines 1-6 of Example 3 show the interpretation as a request to S to inform A of the secret. Notice that in both interpretations S may be led to believe that A wants to know the secret. In the literal case, S infers A's goal from the literal interpretation, and may tell the secret simply by being helpful (lines 4-9). In the indirect case, S recognizes A's intention that S inform A of the secret (lines 1-6).</Paragraph> <Paragraph position="17"> Telling the secret is then conforming to A's intentions (lines 7-9).</Paragraph> <Paragraph position="18"> There is in fact a third interpretation of this sentence. If A and S both know that A already knows the secret, then the utterance could be intended as &quot;If you don't know the secret, I will tell it to you.&quot; This requires recognizing a conditional action and is beyond our present abilities.</Paragraph> <Paragraph position="19"> 4.3. The Level of Embedding Heuristic Two sets of PI rules are applicable to formulas of the form BsWABsWA(P): the simple rules PI.1 to PI.6 operating &quot;within&quot; the prefix BsW A, and the rules generated by EI.1 and EI.3 which allow the simple rules to apply within the prefix BsWABsW A. To reflect the underlying assumption in our model that intention will always be attributed if possible, the inferences at the most deeply nested level should be preferred. null Of course, if the inferences at the nested level lead to unlikely plans, the inferences at the &quot;shallow&quot; levels may be applied. In particular, if there are multiple mutually exclusive inferences at the nested level, then the &quot;shallow&quot; inferences will be preferred. This reflects the fact that the nested inferences model what the speaker intends the hearer to infer. If there are many inferences possible at the nested level, the speaker would not be able to ensure that the hearer would perform the correct (i.e., the intended) one.</Paragraph> <Paragraph position="20"> 176 American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts Example 4 shows the interpretation of &quot;I want you to pass the salt&quot; as a request. Taking the utterance literally, S infers that A wants him to know that A wants him to pass the salt. This yields proposition (2) which leads through the next three inferences to the intention that would be recognized from a request act, i.e. that A wants S to pass the salt (5). Notice that an application of the body-action rule to step (2) yields: INFORM(A, S, WA(PASS(S, A, SALT))), for, in fact, the speaker may be performing both speech acts. The level of inferencing heuristic favours the indirect form.</Paragraph> <Paragraph position="21"> The key step in Example 5 is the application of the know-positive rule from line (3) to line (4). Since, given the context, S assumes that A knows whether S has the salt, the literal interpretation (from (2)) would not produce a reasonable goal for A. This supports the nested know-positive inference, and attributes further intention to the speaker (4). Once this is done, it is easy to infer that A wants S to pass him the salt (5), hence the request interpretation.</Paragraph> <Paragraph position="22"> &quot;Can you pass the salt?&quot; and &quot;Do you want to pass the salt?&quot; are treated similarly, for they inquire about the preconditions on PASS(S, A, SALT).</Paragraph> <Paragraph position="23"> Example 6 begins like Example 5, leading to the inference that A wants S to be able to reach the salt (4). 7 Since being able to reach the salt is a precondition to reaching the salt (5), which then enables passing the salt (6), S can infer that he is being requested to pass the salt. &quot;Is the salt near you?&quot; can be treated in the same way, as being near the salt is a precondition on reaching the salt.</Paragraph> <Paragraph position="24"> 7 Let CANDO(S,ACT) be true if S believes the preconditions of ACT are true.</Paragraph> <Paragraph position="25"> application, through EI.3, of the effect-action rule. A informs S of A's goal of having the salt (2) and then depends on S's planning on that goal to infer the PASS action. Because the action is the &quot;obvious&quot; way of achieving the goal, S believes that A intended him to infer it.</Paragraph> <Paragraph position="26"> Since questions are treated as requests to inform, most of them are handled in a similar manner to the requests above. 4.4a-h can all be understood as questions about the departure time of some train.</Paragraph> <Paragraph position="27"> (4.4a) When does the train leave? (4.4b) I want you to tell me when ...</Paragraph> <Paragraph position="28"> (4.4c) I want to know when ...</Paragraph> <Paragraph position="29"> (4.4d) Tell me when ...</Paragraph> <Paragraph position="30"> (4.4e) Can you tell me when ...</Paragraph> <Paragraph position="31"> (4.4f) Do you know when ...</Paragraph> <Paragraph position="32"> (4.4g) Do you want to tell me when ...</Paragraph> <Paragraph position="33"> (4.4h) Will you tell me when ...</Paragraph> <Paragraph position="34"> 4.5. An Example of an Indirect INFORM An interesting example of an indirect INFORM is 4.5a for it is very similar to 4.5b-c which both seem to only be requests. The interpretation of 4.5a as an indirect INFORM follows from the fact that inference chains which would make it a REQUEST are all inhibited by the heuristics.</Paragraph> <Paragraph position="35"> (4.5a) Do you know that the RAPIDO is late? (4.5b) Do you believe that the RAPIDO is late? (4.5c) Do you know whether the RAPIDO is late? In Example 8, the possible body-action inference from (2) to REQUEST(A,S,INFORMIF(S,A,KNOW(S,P))) is downgraded because the embedded inference to (3) is possible. The interesting case is the embedded know-negative inference which is also possible from (3). It implies that BsWA(~KNOW(S,P)), or equiva- null But such a goal is highly unlikely. A is attempting to achieve the goal ~Bs(P) by having S recognize that A wants P to be true! As a result, no speech act interpretation is possible from this step. For instance, the bodies of the acts INFORM(A, S, P) and INFORM(A, S, ~P) are BsWA(P A Bs(P)) , and BsWA(~P A Bs(~P)), respectively. Both of these are contradicted by part of 4.5d. Thus the know-negative possibility can be eliminated. This allows the know-positive inference to be recognized as intended, and hence leads to the indirect interpretation as an INFORM(A, S, P).</Paragraph> <Paragraph position="36"> 4.5b has only a literal interpretation since both the know-positive and know-negative rules are applicable at the nested level; without a reason to favour either, the literal REQUEST(A,S,INFORMIF(S,A,Bs(P))) is preferred. The interpretations of 4.5c are similar to those of Examples 2 and 3.</Paragraph> </Section> <Section position="4" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 4.6. Using Knowledge of Deduction </SectionTitle> <Paragraph position="0"> All the examples of indirect speech acts so far have been explained in terms of rules PI.1-PI.6, and complex inference rules derived from them. In this section, we give one more example relying on somewhat more specific rules. A full investigation of how many such specific rules are necessary to account for common forms of indirect REQUESTs and INFORMs remains to be done.</Paragraph> <Paragraph position="1"> This example shows how a completely non-standard form can be intended indirectly. Suppose that A tells S (4.6a) &quot;John asked me to ask you to leave&quot; This has at least three possible interpretations: (4.6b) A is asking S to leave, and giving a reason. (4.6c) A wants to simply report the fact to S that John did the action of asking S to leave.</Paragraph> <Paragraph position="2"> (4.6d) A wants to inform S that John wants him to leave.</Paragraph> <Paragraph position="3"> Interpretations c and d can hold even if S decides that A actually does want him to leave. However, in these cases, he would not say that A intended to communicate the intent that he leave, i.e. he would not say the utterance was a REQUEST.</Paragraph> <Paragraph position="4"> Both interpretations rely on axioms ACT.1 and ACT.2 (of section 3.2) which state that if some agent A believes that agent S executed some action ACT, then A may believe that the preconditions of ACT obtained before, and the effects of ACT obtained after, the execution of ACT.</Paragraph> <Paragraph position="5"> They also require a new PC/PI rule: if A wants S to believe some proposition P, then A may get S to believe some proposition Q, as long as A believes that S believes that Q implies P.</Paragraph> <Paragraph position="6"> (PC.I) WA(Bs(P)) =c=> WA(Bs(Q)) , if BABs(Q ~ P).</Paragraph> <Paragraph position="7"> (PI.I) BsWA(Bs(Q)) =i=> BsWA(Bs(P)) , if BsBABs(Q => P).</Paragraph> <Paragraph position="8"> In Example 9, S recognizes that A asked him to leave. The interpretation depends on S concluding that John performed his REQUEST successfully (through PI.I and ACT.2), and hence that A wants to request S to leave. It is then an easy step to infer that A wants S to leave, which leads to the request interpretation. Interpretation (c), a simple report of some previous action, follows from (2) by PI.BA.</Paragraph> <Paragraph position="9"> In Example 10, S recognizes that A intended to tell him that John wants him to leave. This depends on the fact that S concludes that John wanted to perform the REQUEST that A reported. Most of the needed inferences call for the use of EI.1 to embed simple inference rules twice. Note that an INFORM act could have been inferred at each of the four previous steps; for example, from (5) the body inference would produce INFORM(A, S, Wj(REQUEST(A, S, LEAVE(S))).</Paragraph> <Paragraph position="10"> But the inferences at the &quot;BsWABsWj&quot; level were so direct that they were continued.</Paragraph> </Section> </Section> class="xml-element"></Paper>