Plans, Inference, and Indirect Speech Acts I 
James F. Allen 
Computer Science Department 
University of Rochester 
Rochester, NY Iq627 
C. Raymond Perrault 
Computer Science Department 
University of Toronto 
Toronto, Canada MSS IA7 
Introduction 
One of the central concerns of a theory of 
pra~atics is to explain what actions language users 
perform by making utterances. This concern is also 
relevant to the designers of conversational language 
understanding systems, especially those intended to 
cooperate with a user in the execution of some task 
(e.g., the Computer Consultant task discussed in Walker 
\[1978\]). 
All actions have effects on the world, and may have 
preconditions which must obtain for them to be 
successfully executed. For actions whose execution 
causes the generation of linguistic utterances (or 
s~eeqh acts), the preconditions may include the 
speaker/wrlter holding certain beliefs about the world, 
and having certain intentions as to how it should change 
(\[Austin, 1962\], \[Searle, 1969\]). 
In Cohen \[1978\] and Cohen and Perrault \[1979\] it is 
suggested that speech acts a• be defined in the context 
of a plannln~ s~stam (e.g., STRIPS of Fikes and Nllsson 
\[1971\]) i.e., as a class of parameterlzed procedures 
called operators, whose execution can modify the world. 
Each operator is labelled with formulas stating its 
preconditions and effects. 
The major problem of a theory of speech acts is 
relating the form of utterances to the acts which are 
performed by uttering them. Several syntactic devices 
can be used to indicate the speech act being performed: 
the most obvious are explicit performative verbs, mood, 
and intonation. But no combination of these provides a 
clear, single-valued function from form to illocutionary 
force. For example, (1.a)-(1.e) and even (1.f) can be 
requests to pass the salt. 
1.a) I want you to pass the salt. 
1.b) Do you have the salt? 
1.c) Is the salt near you? 
1.d) I want the salt. 
1.e) Can you pass the salt? 
1.f) John asked me to ask you to pass the salt. 
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 "Do you 
know when the train leaves?" expecting a yes/no answer 
as a confirmation. 
• This research was supported in part by the National 
Research Council of Canada under Operating Grant A9285. 
ee Unless otherwise indicated, we take "speech act" to 
be synon~nnous with "illocutionary act." 
The object of this paper is to discuss, at an 
intuitive level, an extension to the work in Cohen 
\[1978\] to account for indirect speech acts. Because of 
space constraints, we will need to depend explicitly on 
the intuitive meanings of various terms such as plan, 
action, believe, and goal. Those interested in a more 
rigorous presentation should see \[Allen, 1979\] or 
\[Perrault and Allen, forthcoming\]. The solution 
proposed here is based on the following slmple and 
independently motivated hypotheses: 
(2.a) Language users are rational agents and thus 
speech acts are purposeful. In particular, they 
are a means by which one agent can alter the 
beliefs and goals of another. 
(2.b) Rational agents are frequently capable of 
identifying actions being performed by others 
and goals being sought. An essential part of 
helpful behavior 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 "How many do you want?" to a customer who 
has asked "Where are the steaks? e, the clerk 
must have inferred that the customer wants 
steaks, and then he must have decided to get 
them himself. This might have occurred even if 
the clerk knew that the custamer had intended to 
get the steaks himself. Cooperative behavior 
must be accounted for independently of speech 
acts, for it often occurs without the use of 
language. 
(2.c) In order for a speaker to successfully perform a 
speech act, he must intend that the hearer 
recognize his intention to achieve certain 
(perlocutionary) effects, and must believe it is 
likely that the hearer will be able to do so. 
This is the foundation the account of 
illooutionary acts proposed by Strawson \[196q\] 
and Searle \[1969\], based on Grice \[1957\]. 
(2.d) Language users know that others are capable of 
achieving goals, of recognizing actions, and of 
cooperative behavior. Furthermore, they know 
that others know they know, etc. Thus, a 
speaker may intend not only that his actions be 
recognized but also that his goals be in/erred, 
and that the hearer be cooperative. 
(2.e) 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 
behavior by the hearer, intended by the speaker, 
the effects of A should be achieved. 
85 
Th__~e Speech Act Model 
In the spirit of Searle \[1975\]; Gordon and Lakoff 
\[1975\], and Horgan \[1978\]. we propose an account of 
speech acts with the following constituents: 
(\].a) For each language user S. a model of the beliefs 
and plans of other language users A with which 
s/he is coenunicating. Including a model of A's 
model of S's beliefs and plans, etc, 
(3.b) Two sets of operators for speech acts: a set of 
surface level operators which are realized by 
utterances having specific syntactic and 
semantic features (e.g.. mood), and a set of 
lllocutionary level operators whlch are 
performed by perfoming surface level ones. The 
tllocutionary acts model the intent of the 
speaker Independent of the form of the 
utterance. 
(3.c) A set of plausible Inference rules with which 
language users construct and reco~nlze plans. 
It Is convenient to view the rules as either 
simple or augmented: A couple of examples of 
simple plan recognition rules are: 
fAction-Effect Znference\] 
"If agent S believes that agent A wants to 
do action ACT then it is plausible that 3 
believes that A wants to achieve the 
effects of ACT." 
\[Know-Positive Znferenoe\] 
"Zf S believes A wants to know whether a 
proposition P is true. then it is plausible 
that S believes that A wants to achieve P." 
Of course, given the conditions in the second 
inference above. S might also infer that A ham a 
goal of achieving not P. This is another 
possible inference. Which applies in a given 
setting is detemlned by the rating heuristics 
(see 3.d below). 
Simple rules can be augmented by adding the 
condition that the recognizer believes that the 
other agent intended him to perfom the 
inference. An example of an augmented 
recognition rule is: 
"If S believes that A wants S to re.=ognize 
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." 
Notice that the augmented rule is obtained 
by intrc~uclng "S believes A wants" In the 
antecedent and consequent of the simple rule. 
and by interpreting "S recognizes A's intention" 
as "S comes to believe that A wants." Theme 
rules can be constructed from the simple ones by 
assuming that language users share a model of 
the construction and recognition processes. 
(3.d) A set of heuristics to guide plan recognition by 
rating the plausibility of the outcomes. One of 
the heuristics iS: "Decrease the plausibility 
of an outcome in which an agent Is believed to 
be executing an action whose effects he already 
believes to be true." Soripl~-derived 
expectations also provide s~e of the control of 
the recognition process. 
(3.e) A set of heuristics to identify the obstacles in 
the recognized plan. These are the goals that 
the speaker cannot easily achieve without 
assistance. If we assume that the hearer is 
cooperating with the speaker, the hearer will 
usually attempt to help achieve these goals in 
his response. 
With these constituents, we have a model of helpful 
behavior: an agent S hears an utterance from some other 
agent A. and then Identifies the surface speech act. 
From this. S applies the inference rules to reconstruct 
A's plan that produced the utterance. S can then 
examine this plan for obstanles and give s helpful 
response based on them. However, some of the inference 
rules may have been augmented by the recognition of 
intention condition. Thus. some obstacles may have been 
intended to be communicated by the speaker. These 
specify whet tllooutionary act the speaker performed. 
an Example 
This may become clearer if we consider an example. 
Consider the plan that must be deduced In order to 
answer (4.e) with (..b): 
(~.a) A: Do you know when the Windsor train leaves? 
(4.b) S: Yes, at 3:15. 
The seal deduced from the literal Interpretation is that 
(4.o) A wants to know whether S knows the departure 
time. 
From this goal. 3 may infer that A in fact wants (4.d) 
by the Know-Positive Znference: 
(..d) A wants S to know the departure time 
from which S may infer that 
(q.e) A wants $ to inform Aot the departure time 
by the precondition-action Inference (not shown). S can 
then infer, using the action-effect inference, that 
(4.f) A wants to know the departure time. 
S'S response (~.b) indicates that ha believed that both 
(~.c) and (4.f) were obstacles that S could overcome In 
this response. 
However. a sentence such as (4.a) could often be 
uttered in a context where the literal goal is not an 
obstacle. For instance. A might already know that $ 
knows the departure time. Met still utter (4.a). Xn 
such cases. A's goals are the same as If ha had uttered 
the request 
(4.g) When does the Windsor train leave? 
Hence (~.a) is often referred to as an indirect request. 
Thus we have described two different 
interpretations of (q.a): 
a) A said (q.a) merely expecting a yes/no answer, 
but $ answered wlth the extra information in 
order to be helpful; 
b) A said (4.a) Intending that S deduce his plan 
and realize that A really wants to ~now the 
departure time. 
86 
Theoretically, these are very different: (a) describes 
a yes/no question, while (b) describes an (indirect) 
request for the departure time. But the distinction is 
also IMportant for practical reasons. For instance, 
assume S is not able to tell A the departure time for 
some reason. With interpretation (a), S can simply 
answer the question, whereas with interpretation (b), S 
is obliged to glve a reason for not answering with the 
departure time. 
The distinction between these two cases is simply 
that in the latter, S believes that A intended S to make 
the inferences above and deduce the goal (q,f). Thus 
the inferences applied above were actually augmented 
inferences as described previously. In the former 
interpretation, S does not believe A intended S to make 
the inferences, but did anyway in order to be helpful. 
Concludln~ Remarks 
This speech act model was implemented as part of a 
program which plays the role of a clerk at a train 
station information booth \[Allen, 1979\]. The main 
results are the following: 
(5.a) 
(5.b) 
It accounts for a wide class of indirect forms 
of requests, assertions, and questions, 
including the examples in (I). This includes 
idiomatic forms such as (1.a) and non-idlomatlc 
ones such as (1.f). It does so using only a few 
independently necessary mechanisms. 
It maintains a distinction between tllocuttonary 
and perlocutionary acts. In particular, it 
accounts for how a given response by one 
participant B to an utterance by A may be the 
result of different chains of inferences made by 
B: either B believed the response given was 
intended by A, or 8 believed that the response 
was helpful (i.e., non-intended). It also shows 
some ways in which the conversational context 
can favor some interpretations over others. 
The main objective of our work is to simplify the 
syntactic and semantic components as much as possible by 
restricting their domain to literal meanings. The 
indirect meanings are then handled at the plan level. 
There remain several open problems In a theory of 
speech acts which we believe to be largely independent 
of the issue of indirection, notably identifying the 
features of a text which determine literal tllocutlonary 
force, as well as constructing representations adequate 
to express the relation between several lllocutionary 
force indicators which may be present in one sentence 
(see \[Lakoff, 197q\] and \[Morgan, 1973\]). 

Bibliography 
Allen, J.F. A Plan-Based Approach to Speech Ac_tt 
Recognition. Ph.D. thesis, Computer Science 
Department, University of Toronto, 1979. 
Austin, J.L. How To Do Thln~s With Words. New York, 
Oxford University Press, 1962. 
Brown, G.P. An Approach to Processing Task-Oriented 
Dialogue, unpublished ms, MIT, 1978. 
Cohen, P.R. On Znowin 6 What to Say: Plannin~ Speech 
Acts, TR 118, Computer Science Department, 
University of Toronto, January 1978. 
Cohen, P.R. and Perrault, C.N. Elements of a Plan 
Based Theory of Speech Acts, forthcoming. 
Cole, P. and Morgan, J.L. Syntax and Semantics, Vol 3: 
Speech Acts. New York, Academic Press, 1975. 
Flkes, R.E. and Nllsson, N.J. STRIPS: A New Approach 
to the Application of Theorem Proving to Problem 
Solving. Artificial Intelli~ence 2, 189-205, 1971. 
Gordon, D. and Lakoff, G. Conversational Postulates, 
in Cole and Morgan (ads), 1975. 
Grice, H.H. Meaning. Phil. Rev. 66, 377-388, 1957. 
Lakoff, O. Syntactic Amalgams. CL__SS 10, 321-3qU, 197q. 
Morgan, J.L. Sentence Fra~ents and the Notion 
'Sentence,' in B.B. Kachru et al. (ads), Issues 
in Lln~uistics. Urbana, University of Illinois 
Press, 1973. 
Morgan, J.L. Towards a Rational Model of Discourse 
Comprehension, in Proceedin~s __2nd Conf. 
Theoretical Issues in Natural Language Procesain 6, 
Champaign-Urbana, 1978. 
Perrault, C.R. and Allen, J.F. A'Plan-Based Analysis 
of Indirect Speech Acts, in preparation. 
Searle, J.R. Speech Acts. New York, Cambridge 
University Press, 1969. 
Searle, J.R. Indirect Speech Acts, in Cole and Morgan 
(eda), 1975. 
Strawson, P.F. Intention and Convention in Speech Acts. 
Phil. Rev. 73, q, q39-~60, 196~. 
Walker, D.E. Understandin~ Spoken Language. New York, 
North Holland, 1978. 
