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Preventing False Inferences 1
Aravind Joshi and Bonnie Webher
Department of Computer and Information Science
Moore School/D2
University of Pennsylvania
Philadelphia PA 19104
Ralph M. Weischedel 2
Department of Computer & Information Sciences
University of Delaware
Newark DE 19716
ABSTRACT
I
Introduction
In cooperative man-machine interaction, it is taken as
necessary that a system truthfully and informatively
respond to a user's question. It is not, however,
sufficient. In particular, if the system has reason to
believe that its planned response nfight lead the user to
draw an inference that it knows to be false, then it
must block it by nmdifying or adding to its response.
The problem is that a system neither can nor should
explore all eonchtsions a user might possibly draw:
its
reasoning must be constrained in some systematic and
well-motivated way.
Such cooperative behavior was investigated in [5], in
which a modification of Griee's
Maxim of Quality
is
proposed:
Grice's
Maxim of Quality-
Do not say what you believe to be false or for which
you lack adequate evidence.
Joshi's
Revised Maxim of Quality -
If you, the speaker, plan to say anything which may
imply for the hearer something that you believe to be
false, then provide further information to block it.
This behavior was studied in the context of interpreting
certain definite noun phrases. In this paper, we
investigate this revised principle as applied to question
answering. In particular the goals of the research
described here are to:
I. characterize tractable cases in which the
system as respondent (R) can anticipate the
possibility of the user/questioner (Q)
drawing false conclusions from its response
and can hence alter or expand its response
so as to prevent it happening;
2. develop a formal method for computing the
projected inferences that Q may draw from
a particular response, identifying those
1This work is partially supported by NSF Grants MCS
81-07290, MCS 8.3-05221, and [ST 83-11,100.
2At present visiting the Department of
Computer
and
Information Science, University of Pennsylvania, Philadelphia, PA
19104.
factors whose presence or absence catalyzes
the inferences;
3. enable the system to generate modifications
of its response that can defuse possible false
inferences and that [nay provide additional
useful information as well.
Before we begin, it is important to see how this work
differs from our related work on responding when the
system notices a discrepancy between its beliefs and
those of its user [7, 8, 9, 18]. For example, if a user asks
• How many French students failed CSEI21 last term?',
he shows that he .believes inter alia that the set of
French students is non-empty, that there is a course
CSEI21, and that it, was given last term. If the system
simply answers "None', he will assume the system
concurs w'ith these b~diefs since the answer is consistent
with them. Furthermore, he may conclude that French
students do r;'d.her well in a difficult course. But this
may be a false conclusion if the system doesn't hold to
all of those beliefs (e.g., it doesn't know of any French
students). Thus while the system's assertion "No
French students failed CSEI21 last term" is true, it has
misled the user (1) inlo believing it concurs with the
user's beliefs and (2) into drawing additional false
conclusions from its response. 3 The differences between
this related work and the current enterprise are that:
1. It is no_~t assumed in the current enterprise
that there is any overt indication that the
domain beliefs of the user are in any way at
odds with those of the system.
2. In our related work, the user draws a false
conclusion from what is said because the
presuppositions of the response are not in
accord with the system's beliefs {following a
nice analysis in
[lO]).
In the current
enterpri.~e,
the
us~,r draws a false conclusion
from what is said because the system's
response behavior is not in accord with the
user's expectations. It. may or may not
also
31t is a feature of Kaplan's CO-OP system [7] that it point~ out
the discrepancy by saying "| don't know of any French students °
134
involve false domain beliefs that the system
attributes to the user.
In this paper, we describe two kinds of false
conclusions we are attempting to block by modifying
otherwise true response:
• false conclusions drawn by standard default
reasoning - i.e., by the user/listener
concluding (incorrectly) that there is nothing
special about this case
• false conclusions drawn in a task-oriented
context on the basis of the user's
expectations about the way a cooperative
expert will respond.
In Section II, we discuss examples of the first type,
where the respondent (R) can reason that the questioner
{Q) may inappropriately apply a default rule to the
(true) information conveyed in R's response and hence
draw a false conclusion. We characterize appropriate
information for R to include in his response to block it.
In Section HI, we describe examples of the second type.
Finally, in Section IV, we discuss our claim regarding
the primary constraint posed here on limiting R's
responsibilities with respect to anticipating false
conclusions that Q may draw from its response: that is,
it is only that part of R's knowledge base that is
already in focus (given the interaction up to that point,
including R's formulating a direct answer to Q's query)
that will be involved in anticipating the conclusions
that Q may draw from R's response.
H Blocking Potential Misapplication of Default
Rules
Default reasoning is usually studied in the context of a
logical system in its own right or an agent who reasons
about the world from partial information and hence
may draw conclusions unsupported by traditional logic.
However, one can also look at it in the context of
interacting agents. An agent's reasoning depends not
only on his perceptions of the world but also on the
information he receives in interacting with other agents.
This information is partial, in that another agent
neither will nor can make everything explicit. Knowing
this, the first agent (Q) will seek to derive information
implicit in the interaction, in part by contrasting what
the other agent (R) has made explicit with what Q
assumes would have been made explicit, were something
else the case. Because of this, R must be careful to
forestall inappropriate derivations that Q might draw.
The question is on what basis R should rea.~on that Q
may ~sume some piece of infotmati(>n (P) would have
been made explicit in the interaction, were it the ease.
One basis, we contend, is the likelihood that Q will
apply some
staudard default rule
of the type discussed
by Reiter [15] if R doesn't make it explicite that the
rule is not applicable. Reiter introduced the idea of
default rules in the stand-alone context of an agent or
logical system filling in its own partial information.
Most standard default rules embody the sense that
"given no reason to suspect otherwise,
there's nothing
special about the current case'.
For example, for a bird
what would be special is that it can't fly - i.e., •Most
birds fly•. Knowing only that Tweety is a bird and no
reason to suspect otherwise, an agent may conclude by
default that there's nothing special about Tweety and
so he can fly.
This kind of default reasoning can lead to false
conclusions in a stand-along situation, but also in an
interaction. That is, in a question-answer interaction, if
the respondent (l{) has reason for knowing or suspecting
that the situation goes counter to the standard default,
it seems to be common practice to convey this
information to the questioner (Q), to block his
pote, tially a.ssuming the default. To see this, consider
the following two examples. (The first is very much like
the "Tweety" case above, while the second seems more
general.)
A. Example 1
Suppose it's the case that most associate professors are
tenured and most of them have Ph.Ds. Consider the
following interchange
Q: Is Sam an ~sociate professor?
R: Yes, but he doesn't have tenure.
There are two thi, gs to account for here: (1) Given the
information w&s not requested, why did R include the
"but" clause, and (2) why this clause and not another
one? We claim that the answer to the second question
has to do with that part of R's knowledge base that is
currently in focus. This we discuss more in Section IV.
In the meantime, we will just refer to this subset as
• RBc ".
Assume RBc contains at least the following
information:
(a) Sam is an associate professor.
(b) Most associate professors are tenured.
(c) Sam is not tenured.
(b) may be in RBc because the question of tenure may
be in context. Based on RBc, R's direct response is
clearly "Yes'. This direct response however eouJd lead
Q to conclude falsely, by default reasoning, that Sam is
tenured. That is, R can reason that, given just (b) and
his planned response "Yes" (i.e., if (c) is not in Q's
knowledge base}, Q could infer by default reasoning
that
Sam is tenured,
which R knows with respect to
!RBc is false. Hence, R will modify that planned
response to block this false inference, as in the response
above.
In general, we can represent R's reasoning about Q's
reaction to a simple direct response •Yes, B(a)', given
Q believes "Most Bs F=, in terms of the following
default schema, using the notation introduced in [15 I.
135
told{ILQ,l~(c)) k (Most x)[B(x) = F(x)]
&-~h:,ld(R,Q,-~Flc)): M(F[c})
..__" ............................................
F(c)
As in Reiter's discussion, "M(P)" means it is consistent
to assume that P. In the associate professor example, B
corresponds to the predicate "is an associate professor',
F, to the predicate "has tenure', and c, to Sam. Using
such an inslantiated rule schema, R will recognize that
Q is likely to conclude F(c) - "Sam has tenure" - which.
is false with rvspe(.t to RBc {and hence, with respect to
all of R's knowledge base). Thus R will modify his
direct response so as to block this false conclusion.
B. Example 2
Consider a user one of the mail systems on the
DEC-20. To exit from this system, a user who has
finished reading all the messages he earlier specified can
just type a carriage return. To exit under other
circumstances, the user must type QUIT. Consider the
following interchange between a new user who has
finished reading all his messages and either a mail
system expert or the mail system itself.
Q: How (In I get out of mail?
R~ Since you h:tve read all your specified messages,
you can just type a carriage return. In all cases,
you (':ill got ()lit by typing QHT.
Here tile prohh,m is to account for all that part of R's
response beyond the simple truthful statement "You
can type a carriage return."
A general statement of this probh,m is a.s follows:
Agent Q is in one situation (Sl) and wants to be in
another ($2). There is a general procedure P for
achieving $2 from any of several situations including Sl.
There is a special prodecure P* (i.e., shorter, faster,
simpler, etc.) for achieving $2 frolu Sl. Q doesn't know
how to achieve $2, but R does (including proced,res P
and P*). Q asks R how to achieve $2.
If R knows.i~lat Q is in situation SI and truthfully
responds to Q's request by simply telling him P*, Q
may falsely conclude that P* is a general procedure for
achieving $2. That is, as in the Tweety and Sam
examples, if Q has no reason to suspect anything special
about SI (such that P* only applies to it), then there is
nothing special about it. Therefore P* is adequate for
achieving $2, whatever situation Q is in. 4 Later when Q
tries to apply P* in a different situation to achieve $2,
he may find that it doesn't work. As a particular
examl)le of this, consider the mail case again. In this
ca.se~
SI = Q has read all his messages
$2 = Q is out of the mail system
P ~--- typing QUIT
P* -- typing a carriage return
~Lssume RBc contains at least the following
informa.tion:
(a) Sl
(b) want(Q,S2)
(c) ¥s6S. P(s) = S2
(d)
P*(Sl) =
s2
(e) Sl6r
(f) simpler(P*,P)
(g) VsE,~. "-{s = SI) =* -~(P*ls) = $21
where 17 is some set of states which includes SI and P(s)
indicates action P applied to state S.
Based on RBc, R's direct response would be "You can
exit the mail system by typing carriage return'. (It is
&ssumed that an expert will always respond with the
"best" procedure according to some metric, unle..~ he
explicitly indicates otherwise - of. Section lIl, case 2}.
However, this could lead Q to conclude falsely,-by
default, something along tile lines of Vs . P*(s) ---- $2. 5
Thus R will modify his planned response to call
attention to SI {in particular, how to recognize it) and
the limited applicability of P* to SI alone. The other
modification to R's response ('In all cages, you can get
out by typing QUIT'), we would ascribe simply to R's
adhering to Grice's
Alaxim of Quantity -
"Make your
contribution ,~s informative as is required for tile
current purposes of tile exchange" given R's
assumption of what is required of him in his role as
expert/teacher.
HI Blocking False Conclusions in Expert
Interactions
Tile situations we are concerned with here are ones in
which the system is explicitly tasked with providing
help and expertise to the user. In such circumstances,
the user has a strong expectation that the system has
both the experience and motivation to provide the most
appropriate help towards achieving the user's goals. The
user does not expect behavior like:
Q:
How can
I
get to Camden?
R: You can't.
As many studies have shown Ill, what an advice seeker
(Q)
expects is that an expert
(R)
will attempt to
recognize what plan Q is attempting to follow in pursuit
of what goal and respond to Q's question accordingly.
Further studies [11, 12, 13] show that Q may also
expect that R will respond in terms of a better plan if
the recognized one is either sub-optimal or unsuitable
for attaining Q's perceived goal. Thus because of this
principle of "expert cooperative behavior', Q may
expect a response to a more general question than the
one he has actually asked. That is, in asking an expert
• flow do 1 do X?" or "Can I do X?', Q is anticipating a
response to "How can I achieve my goal?"
4Moreover if Q (falsely) believes that R doesn't know Q is in SI,
Q will certainly assume that P* is a general procedure. However,
this isn't necessary to the default reasoning behavior we are
investigating.
5Clearly , this is only for some subset of states, ones
corresponding to being in the mail system.
136
Con',id,.r a slud,.ut ((,~) :+skhig th,' foll,+,+i.g que+thm, near the
end of the
term.
Q'.
Can
I
dr~q, C1~,-,77?
Since it is already too late to drop
a
course, ti~e o~.!y dire,'t answer
the ,x~*~rt (R) can give is "No'. Of course, part of :,:, expert's
knowledge concerns the typical states users get into and the
possible actions that permit transitions between them. Moreover it
is al~o part of this expertise to infer such states from the current
state of the
inlrerac(.ion,
Q's query, some shared knowledge of Q's
goals and Pxpectali,ns and the shared assmnption that an expert is
expected to attend to these higher goals. How the system should
go about in"erring these states is a difficult task that others are
exami,iug [2, 12, 13]. We assume that such an inference has been
made. We al,~o assume for simplicity that the states are uniquely
det.ermined. For example, we assume that the system has inferred
that Q i.,: in state Sb (student is doing badly in the course} and
wants to be in a state Sg {student is in a position to do better in
this course or another one later), and that the a~tion a (diopping
the course) will take him f:om Sb to Sg.
Given this, the response in (2) may lead Q to draw some
conclusiuns that I/. knows to be false. For example, R can reason
that since a principle of cooperative behavior for an expert is to
tell Q the best way to go from Sb to Sg, Q is likely to conclude
from R's response that there is no way to go from Sb to Sg. This
con+:lusion however would be false if R knows some other ways of
going from Sb to Sg. To avoid potenlially misleading Q, R must
provide additional information, such as
R: No, bul you can take an incomplete and ask for
more time to finish the work.
As we noted earlier, an important question is how much
reasoning R should do to block fals~ conclusions on Q's part.
Again. we assume that R should only concern itself with those false
conclusions that Q is likely to draw that involve that part of R's
knowledge base currently in focus (RBc}, including of course that
subset it nc~ds in order to answer the query in the first place.
We will make this a little more precise by considering several
cases corresponding to the different states of R's knowledge base
with r~peet to Sb, Sg. and tran~iti,m~ between them. For
convenie,,.e, ~,: ~ill give an appropriate re~p~mse in terms of Sb,
Sg and the actions. Clearly, it should be given in terms of
descriptions of ~lat,.s and actions understandable to Q. (Moreover,
by making further assumptions about Q's beliefs, R may be able to
validly trim some of its respond.)
1. Suppose that it is possible to go from Sb to Sg by
dropping the course aml that. this is the only action
that will take one from Sb to Sg.
Sb Sg
In this ca.se, the respon~ is
R: Yes. ct is t h~ only action that will take
you fr,,m Sb to St.
2. Suppose that in addition to going from Sb to Sg by
dropping the cour~,~o there is a better way, say ~, of
doing so.e
• .j
Sb :
Sg
In this ca~e, the
response is
6"Betteruess" is yet another urea for future research.
H: Yes, but there is a better action ,9 that
will take you from Sb to Sg.
3. Suppose that dropping the course does not take you
from Sb to St, but another action ~ will. This is the
situation we considered in our earlier discussion.
Sb Sg
In this case the response is
H: No, but there is an action ~ that will
take you from Sb to St.
4. Suppose that there is no action that will take one from
Sb to Sg.
Sb Sg ,
/
In this the rcspon~ is
R: No. There is no action that will take you
from Sb to Sg.
Of course, other situations are possible. The point, however, is
that the additional information that R provides to prevent Q from
drawing fal~ conclusions is limited to just that part of R's
knowledge hase that R is focussed on in answering Q's query.
IV
Constraining the Renpondent's Obligations
As many people have observed - from studies across a range of
linguistic phenomena, including co-referring expressions [3, 4, 16],
left dislocations [14], epitomizatkm [17], etc. - a speaker (R)
normally focuses on n particular part of its knowledge base. What
he focuses on dcpends in part oil (1) eoutext,
(2}
R's partial
knf~wledge of Q's overall goals, as well as what Q knows already as
a result of the interaction up to that point, and (3} Q's particular
query, etc. The precise nature of how these various factors affect
focusing is complex and is receiving much attention [3, 4, 16].
However, no matter how these various factors contribute to
focusing, we can certainly assume that H comes to focus on a
subset of its knowledge base in order to provide a direr answer to
Q's query (at some level of inl,.rpretalion). Let us call this subset
RBc for "R's current belief.~ ~. Our claim is tlmt one important
constraint on cooperative behavior is that it is determined b.v RBc
only. Clearly the i;ib~rmal.ion needed for a direct response is
contained in RBc, a.~ is the information needed for many types of
helpful responses. In other words, RBc
--
that part of R's
knowledge base that R deeide~ to focus on in order to glve-a direct.
response to Q's quer~ - also has the information needed to
generate several classes of h~Ipful responses. The simplest ease is
presupposition failure [7], as in (he following
Q: llow many A's were given in (',IS 500 ?
where Q presumes that CIS 500 was offered. In trying to
formulate a direct response, R will have to ascertain that CIS 500
was offered. If it was (Q's presumption is true}, then R can go
ahead and give a direct response. If not, then R can indicate that
CIS 500 was not offered and thereby avoid misleading Q. All of
this is straightforward. The point here is that the information
needed to provide this extra response is already there in that part
of R's knowledge base which R had to look up anyway in order to
try to give the direct, response.
In the above example, it is clear how the response can be
localized to RP, c. We would like to claim that this approach has a
wider applicability: that RBc alone is the basis for responses that
anticipate and attempt to block interactional defaults as well.
Since RBc contains the information for a direct response, R can
plan one (r}. From r, R can reason whether it is possible for Q to
infer some conclusion (g) which R knows to be false because -~g is
in RBe. If so, then R should modify r so as to eliminate this
possibility. The point is that the only false inferences that R will
attempt to block are those whose falsity can be checked in RBc.
137
There may be other false inferences that Q may draw, whose
falsity cannot be deterntined solely with respect to RBc (although
it might be possible with respect to R's entire knowledge base).
While intuitively this may not seen enough of a constraint on the
amount of anticipatory reasoning that Joshi's revised maxim
imposes on R, it does constrain things a lot by only considering a
(relatively small) subset of knowledge base. Factors such as
context may further delimit S's responses, but they will all be
relative to RBc.
V
Conclusion
There are many gaps in the current work and several aspects not
discussed here. In particular,
1. We are developing a formMism for accommodating the
system's reasoning based on a type of HOLDS
predicate whose two arguments are a proposition and a
state; see [6].
2. We are working on more examples, especially more
problematic cases in which, for example, a direct
answer to Q's query would be myes m [or the requested
procedure} BUT a response to Q's higher goals would
be "no t or "no" plus a warning
- e.g.,
Q: Can I buy a 50K savings bond?
S: Yes, but you could get the same security
on other investments with higher returns.
3. We need to be more precise in specifying RBc, if we are
to assume that all the information needed to account
for R's cooperative behevior is contained there. This
may in turn reflect on how the user's knowledge base
must be structured.
4. We need to be more precise in specifying how default
rules play a role in causing R to modify his direct
response, in recognition of Q's likelihood of drawing
what seems like a generalized "script" default - if there
is no reason to assume that there is anything special
about the current case, don't.
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138
