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<Paper uid="J80-3003">
  <Title>A Plan-Based Analysis of Indirect Speech Acts 1</Title>
  <Section position="2" start_page="0" end_page="0" type="abstr">
    <SectionTitle>
1. Introduction
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    <Paragraph position="0"> Austin \[1962\] was one of the first to stress the distinction between the action(s) which a speaker performs by uttering a sentence (such as informing, requesting, or convincing) and the truth conditions of propositions contained in the sentence. Actions have effects on the world, and may have preconditions which must obtain for them to be felicitously performed. For actions whose execution involves the use of language (or speech acts), the preconditions may include the speaker holding certain beliefs about the world, and having certain intentions or wants as to how it should change.</Paragraph>
    <Paragraph position="1"> As well as being important to the study of natural language semantics, speech acts are important to the designer of conversational natural language understanding systems. Such systems should be able to recognize what actions the user is performing. Conversely, if such a system is to acquire information or request assistance from its user, it should know how and when to ask questions and make requests. (See Bruce \[1975\] for an early attempt.) Cohen and Perrault \[1979\] (hereafter referred to as CP) argue for the distinction between a competence t This research was supported in part by the National Research Council of Canada under Operating Grant A9285. Thanks to Phil Cohen, Michael McCord, Corot Reason, and John Searle for their comments. We assume the usual responsibility for remaining inaccuracies, misunderstandings, and downright errors.</Paragraph>
    <Paragraph position="2"> theory of speech acts, which characterizes what utterances an ideal speaker can make in performing what speech acts, and a performance theory which also accounts for how a particular utterance is chosen in given circumstances, or how it is recognized. We are only concerned here with a competence theory.</Paragraph>
    <Paragraph position="3"> In Perrault, Allen, and Cohen \[1978\] we suggested that it is useful to consider speech acts in the context of a planning system. A planning system consists of a class of parameterized procedures called operators, whose execution can modify the world. Each operator is labelled with formulas stating its preconditions and effects. A plan construction algorithm is a procedure which, given a description of some initial state of the world and a goal state to be achieved, constructs a plan, or sequence of operators, to achieve it.</Paragraph>
    <Paragraph position="4"> It is assumed there, and in all our subsequent work, that language users maintain a model of the world (their beliefs) and a set of goals (their wants). One person S's beliefs may include beliefs about another person A's beliefs and wants, including A's beliefs about S, etc. We do not concern ourselves with obligations, feelings, etc., which clearly can also be affected by speech acts.</Paragraph>
    <Paragraph position="5"> CP discuss criteria for judging the correctness of the preconditions and effects of the operators corresponding to speech acts, and specifically those of the acts INFORM and REQUEST. However, the conditions on INFORM and REQUEST given in CP are at best necessary and certainly not sufficient. In particu-Copyright 1980 by the Association for Computational Linguistics. Permission to copy without fee all or part of this material is granted provided that the copies are not made for direct commercial advantage and the Journal reference and this copyright notice are included on the first page. To copy otherwise, or to republish, requires a fee and/or specific permission. 0362-613X/80/030167-16501.00 American Journal of Computational Linguistics, Volume 6, Number 3-4, July-December 1980 167 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts lar they say nothing about the form of utterances used to perform the speech acts. Several syntactic devices can be used to indicate the speech act being performed: the most obvious are explicit performative verbs such as &amp;quot;I hereby request you to ...&amp;quot;, and mood (indicative for assertions, imperative for requests to do, interrogative for requests to inform). But the mood of an utterance is well known to not completely specify its illocutionary force: 1.a-b can be requests to close the door, 1.c-e can be requests to tell the answer, and 1.f can be an assertion.</Paragraph>
    <Paragraph position="6">  (1.a) I want you to close the door.</Paragraph>
    <Paragraph position="7"> (1.b) Will you close the door? (1.c) Tell me the answer.</Paragraph>
    <Paragraph position="8"> (1.d) I want you to tell me the answer.</Paragraph>
    <Paragraph position="9"> (1.e) Do you know what the answer is? (1.f) Do you know that Jack is in town?  Furthermore, all these utterances can also be intended literally in some contexts. For example, a parent leaving a child at the train station may ask 1.g expecting a yes/no answer as a confirmation.</Paragraph>
    <Paragraph position="10"> (1.g) Do you know when the train leaves? The object of this paper is to extend the work in CP to account for indirect use of mood, loosely called indirect speech acts. The solution proposed here is based on the following intuitively simple and independently motivated hypotheses:  (1) Language users are rational agents engaged in goal seeking behaviour. Among these goals are the modification of the beliefs and goals of other agents.</Paragraph>
    <Paragraph position="11"> (2) Rational agents are frequently capable of  identifying actions being performed by others and goals being sought. An essential part of helpful or cooperative behaviour is the adoption by one agent of a goal of another, followed by an attempt to achieve it. For example, for a store clerk to reply &amp;quot;How many do you want?&amp;quot; to a customer who has asked &amp;quot;Where are the steaks?&amp;quot;, the clerk must have inferred that the customer wants steaks, then he must have decided to get them himself.</Paragraph>
    <Paragraph position="12"> This might have occurred even if the customer had intended to get the steaks him or herself.</Paragraph>
    <Paragraph position="13"> Cooperative behaviour must be accounted for independently of speech acts, for it often occurs without the use of language.</Paragraph>
    <Paragraph position="14"> (3) In order for a speaker to successfully perform a speech act, he must intend that the hearer recognize his intention to achieve the effects of the speech act, and must believe it is likely that the hearer will be able to do so. This is the foundation for the philosophical account of speech acts.</Paragraph>
    <Paragraph position="15"> (4) Language users know that others are capable of achieving goals, of recognizing actions, and of cooperative behaviour. Furthermore, they know that others know they know, etc. A speaker may intend not only that his actions be recognized but also that his goals be inferred. null (5) Thus a speaker can perform one speech act A by performing another speech act B if he intends that the hearer recognize not only that B was performed but also that through cooperative behaviour by the hearer, intended by the speaker, the effects of A should be achieved. The speaker must also believe that it is likely that the hearer can recognize this intention.</Paragraph>
    <Paragraph position="16"> The process by which one agent can infer the plans of another is central to our account of speech acts.</Paragraph>
    <Paragraph position="17"> Schmidt et al \[1978\] and Genesereth \[1978\] present algorithms by which one agent can infer the goals of another, but assuming no interaction between the two.</Paragraph>
    <Paragraph position="18"> We describe the process in terms of a set of plausible plan inference rules directly related to the rules by which plans can be constructed. Let A and S be two agents and ACT an action. One example of a simple plan inference rule is: &amp;quot;If S believes that A wants to do ACT then it is plausible that S believes that A wants to achieve the effects of ACT.&amp;quot; From simple rules like this can be derived more complex plan inference rules such as: &amp;quot;If S believes that A wants S to recognize A's intention to do ACT, then it is plausible that S believes that A wants S to recognize A's intention to achieve the effects of ACT.&amp;quot; Notice that the complex rule is obtained by introducing &amp;quot;S believes A wants&amp;quot; in the antecedent and consequent of the simple rule, and by interpreting &amp;quot;S recognizes A's intention&amp;quot; as &amp;quot;S comes to believe that A wants&amp;quot;. Throughout the paper we identify &amp;quot;want&amp;quot; and &amp;quot;intend&amp;quot;.</Paragraph>
    <Paragraph position="19"> We show that rules of the second type can account for S's recognition of many indirect speech acts by A, i.e. those in which S recognizes A's intention that S perform cooperative acts.</Paragraph>
    <Paragraph position="20"> To distinguish the use of, say, the indicative mood, in an assertion from its use in, say, an indirect request, the speech act operators REQUEST and INFORM of CP are reformulated and two further acts S.REQUEST and S.INFORM are added. These surface level acts are realized literally as indicative and imperative utterances. An S.REQUEST to INFORM is realized as a question. The surface level acts can be recognized immediately as parts of the higher level (or illocutionary level) acts, to which the simple plan construction 168 American Journal of Computational Linguistics, Volume 6, Number 3deg4, July-December 1980 C. Raymond Perrault and James F. Allen A Plan-Based Analysis of Indirect Speech Acts and inference rules can apply. Alternatively, the complex rules can be applied to the effects of the surface acts, and the intended performance of one of the illocutionary acts inferred later.</Paragraph>
    <Paragraph position="21"> For example, there are two ways an agent S could be led to tell A the secret after hearing A tell him &amp;quot;Can you tell me the secret?&amp;quot;. Both start with S's recognition that A asked a yes/no question. In the first case, S assumes that A simply wanted to know whether S could tell the secret, then infers that A in fact wants to know the secret and, helpfully, decides to tell it. In the second case S recognizes that A intends S to infer that A wants to know the secret and that A intends S to tell A the secret, and thus that A has requested S to tell the secret.</Paragraph>
    <Paragraph position="22"> In general, several of the plan inference rules could apply at any time, and none of them guarantees a valid consequence. The application of the rules is controlled by a set of heuristics which rate the plausibility of the outcomes.</Paragraph>
    <Paragraph position="23"> Following a review of the relevant aspects of speech act theory in section 2, section 3 outlines our assumptions about beliefs, goals, actions, plans, and the plan inference process. Section 4 shows how the speech act definitions and the plan inference process can be used to relate literal to indirect meanings for REQUESTs and INFORMs. We show how utterances such as 1.h-l, and even 1.m can be used as requests to pass the salt, and what the origin of the several interpretations of 1.m is.</Paragraph>
    <Paragraph position="24">  (1.h) I want you to pass the salt.</Paragraph>
    <Paragraph position="25"> (1.i) Do you have the salt? (1.j) Is the salt near you? (1.k) I want the salt.</Paragraph>
    <Paragraph position="26"> (1.1) Can you pass the salt? (1.m) John asked me to ask you to pass the salt.</Paragraph>
    <Paragraph position="27">  Similarly we show how 1.n can be used to inform while 1.o cannot. Section 5 relates this work to the literature, while section 6 suggests further problems and draws some conclusions.</Paragraph>
    <Paragraph position="28">  (1,n) Do you know that the train is late? (1.o) Do you believe that the train is late?  The speech act recognition process described here has been implemented as a computer program and tested by having it simulate an information clerk at a railway station. This domain is real, but sufficiently circumscribed so that interchanges between clerk and patrons are relatively short and are directed towards a limited set of goals. The program accepts as input simple English sentences, parses them using an ATN parser, and produces as output the speech act(s) it recognized and their associated propositional contents. It can handle all the examples discussed here. Details of the implementation can be found in Allen \[1979\].</Paragraph>
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