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Inferring discourse relations in context
Citation for published version:
Lascarides, A, Asher, N & Oberlander, J 1992, 'Inferring discourse relations in context'. in Proceedings of
the 30th annual meeting on Association for Computational Linguistics. ACL '92, Association for
Computational Linguistics, Stroudsburg, PA, USA, pp. 1-8., 10.3115/981967.981968
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Download date: 06. Feb. 2015
Alex Lascarides
Nicholas Asher
Human Communication
Research Centre,
University of Edinburgh,
2 Buccleuch Place, Edinburgh
Center for Cognitive Science,
University of Texas,
Austin, Texas 78712
[email protected], utexas, edu
[email protected]£, ed. ac. uk
We investigate various contextual effects on text
interpretation, and account for them by providing
contextual constraints in a logical theory of text
interpretation. On the basis of the way these constraints interact with the other knowledge sources,
we draw some general conclusions about the role
of domain-specific information, top-down and bottom-up discourse information flow, and the usefulness of formalisation in discourse theory.
Introduction: Time Switching
and A m e l i o r a t i o n
Two essential parts of discourse interpretation involve (i) determining the rhetorical role each sentence plays in the text; and (ii) determining the
temporal relations between the events described.
Preceding discourse context has significant effects
on both of these aspects of interpretation. For
example, text (1) in vacuo may be a non-iconic
explanation; the pushing caused the falling and
so explains why Max fell. But the same pair
of sentences may receive an iconic, narrative interpretation in the discourse context provided by
(2): John takes advantage of Max's vulnerability
while he is lying the ground, to push him over the
edge of the cliff.
(1) Max fell. John pushed him.
(2) John and Max came to the cliff's edge. John
applied a sharp blow to the back of Max's
neck. Max fell. John pushed him. Max rolled
over the edge of the cliff.
aThe support of the Science and Engineering Research
Council through project number GR/G22077 is gratefully
acknowledged. HCRC is supported by the Economic and
SociM Research Council. We thank two anonymous reviewers for their helpful comments.
:Ion Oberlander
Human Communication
Research Centre,
University of Edinburgh,
2 Buccleuch Place, Edinburgh
j o n e c o g e c £ , ed. ac .uk
Moreover, the text in (3) in vacuo is incoherent,
but becomes coherent in (4)'s context.
?Max won the race in record time. He was
home with the cup.
Max got up early yesterday. He had a little bite to eat. He had a light workout. He
started the t o u r n a m e n t in good form. He
w o n t h e race in record time.
with the cup.
He was home
He celebrated until late into
the evening.
So we can see that discourse context can time
switch our interpretation of sentence pairs, (cf.
(1) and (2)); and it can ameliorate it, (cf. (4)'s
improvement of (3)). T h e purpose of this paper
is two-fold: we a t t e m p t to capture formally these
aspects of discourse context's impact on clausal
attachment; and in the process, we assess whether
the structure of the domain being described might
be sufficient alone to account for the phenomena.
Of course, the idea that discourse context constrains the discourse role assigned to the current
clause is by no means new. Reference resolution
is influenced by discourse structure (cf. Grosz
and Sidner 1986:188 for a very clear case); and
it in turn influences discourse structure. Now, on
the one hand, Polanyi and Scha (1984), Hobbs
(1985), and Thompson and Mann (1987) have
argued that 'genre' or 'rhetorical schemata' can
influence the relations used in discourse attachment. On the other hand, Sibun (1992) has recently argued that domain-specific information,
as opposed to domain-independent rhetorical information, plays the central role. Both ideas are
intriguing, but so far only the latter has been
specified in sufficient detail to assess how it works
in general, and neither has been applied to time
switching or amelioration in particular.
We limit our discussion to temporal aspects of
discourse interpretation; our strategy here is to
explore two possible contextual constraints; these
state how the discourse context filters the set of
discourse relations and temporal relations which
may be used to attach the current clause to the
representation of the text so far. We then frame
contextual constraints in a logical theory of text
interpretation, where their effects and interactions
can be precisely calculated. We therefore first introduce a domain-specific contextual constraint,
following Sibun, and then place it in a formal theory of discourse a t t a c h m e n t called DICE, developed in Lascarides and Asher (1991a). We then
show how the proposed domain-constraint is insufficient, and demonstrate how it can be augmented by adding a rhetorical, or presentational
constraint to the theory.
Constraints from the
Domain Context
In the field of NL generation, Sibun (1992) has
recently argued that coherent text must have a
structure closely related to the domain structure
of its subject matter; naturally, her remarks are
also relevant to NL interpretation. She pursues a
view t h a t task structure, or more generally, domain structure, is sufficient to account for m a n y
discourse phenomena (but cf. Grosz and Sidner
1986:182). She examines in detail the generation
of paragraph-length texts describing the layout
of a house. Houses have structure, following from
a basic relation of spatial proximity, and there
are also hierarchical levels to the structure (rooms
can be listed without describing what's in them,
or the objects within each room can be detailed).
Either way, one constraint on text structure is
defined in terms of the description's trajectory:
the spatial direction the description moved in the
domain, to get from the objects already described
to the current one. T h e constraint is: don't change
trajectory. Sibun argues that in the temporal domain, the basic relation is temporal proximity.
But Lascarides and Oberlander (1992a) urge that
the temporal coherence of text is characterised in
terms of, among other things, the stronger basic relation of causal proximity. So in the latter
domain, Sibun's domain constraint precludes textual descriptions which procede from a cause to
an effect to a further cause of that effect, or from
effect to cause to effect.
This Maintain Causal Trajectory (MCT) cons t r a i n t has two important attributes: first, it is
domain-specific; secondly, it introduces into discourse interpretation an element of top-down processing. To investigate these properties, and see
how far they go towards explaining discourse time
switch, and discourse amelioration, we now incorporate MCT into DICE's formal model of discourse
structure, where its interaction with other causal
information and strategies for interpretation can
be precisely calculated.
Discourse Interpretation and
Commonsense Entailment
DICE (Discourse and C_ommonsense Entailment)
starts with traditional discourse representation
structures (cf. K a m p 1981), but goes on to assume with Grosz and Sidner (1986) that candidate discourses possess hierarchical structure, with
units linked by discourse relations modelled after those proposed by IIobbs (1979, 1985) (cf.
also T h o m p s o n and Mann 1987, Scha and Polanyi
1988). 1 Lascarides and Asher (1991a) use Narration, Explanation, Background, Result and Elaboration. These are the discourse relations central
to temporal import and they are the only ones we
consider here. Full coverage of text would require
a larger set of relations, akin to that in T h o m p s o n
and Mann (1987).
DICE is a dynamic, logical theory for determining the discourse relations between sentences
in a text, and the temporal relations between
the eventualities they describe. T h e logic used
is the nonmonotonic logic Commonsense Entailment (CE) proposed by Asher and Morreau (1991).
Implicatures are calculated via default rules. The
rules introduced below are shown in Lascarides
and Asher (1991a) to be manifestations of Griceanstyle pragmatic maxims and world knowledge.
A formal notation makes clear both the logical
structure of these rules, and the problems involved
in calculating implicature. Let (% ~,fl) be the
update function, which means "the representaXLascaxides and Asher (1991a) introduces the general
framework and applies it to interpretation; Oberlander
and Lascaxides (1992) and Lascarides and Oberlander
(1992b) use the framework for generation.
t e m p o r a l order in interpretation. T h e Push Causal
Law is a mixture of linguistic knowledge and world
knowledge; given t h a t the clauses are discourserelated somehow, the events they describe m u s t
normally be connected in a causal, p a r t / w h o l e or
overlap relation; here, given the events in question, they m u s t normally stand in a causal relation. T h a t Causes Precede their Effects is indefeasible world knowledge.
tion r of the text so far (of which a is already
a part) is to be u p d a t e d with the representation
fl of the current clause via a discourse relation
with a " . Let a g /~ mean t h a t a is a topic
for fl; let e~ be a t e r m referring to the main
eventuality described by the clause a; and let
fall(m, e~) mean t h a t this event is a Max falling.
Let el -~ e2 mean the eventuality et precedes e~,
and cause(el,ei) mean el causes ei. Finally, we
represent the defeasible connective as in Asher
and Morreau (1991) as a conditional > (so ¢ > ¢
means 'if ¢, then normally ¢ ' ) a n d --* is the material conditional. T h e m a x i m s for modelling implicature are then represented as schemas: 2
We also have laws relating the discourse structure to the topic s t r u c t u r e (Asher, in press): for
example, A C o m m o n Topic for Narrative states
t h a t any clauses related b y Narration m u s t have
a distinct, common (and perhaps implicit) topic:
• Narration: ( r , a , fl) > Narration(a, fl)
• A Common Topic for N a r r a t i v e
Narration(a, fl) -*
• Axiom on Narration:
Narration(a, fl) ---* ea -q e#
• Explanation: (r,
^ caus ( ,
on Explanation:
Explanation(a, fl) ~ ~ea -~ e~
• Push C a u s a l
(r, a, 1~) ^ fall(m, ca) ^ push(j, m, ca) >
cause(ea, ec,)
• Causes P r e c e d e E f f e c t s :
cause(ei, el) ---, "-,st -~ e2
• States
(r, a, fl) ^ state(e#) > overlap(ca, e#)
by Deduction
cE and the defeasible rules are used to infer the
discourse and temporal---structures of candidate
texts, cE represents nonmonotonic validity as
~ . T h r e e p a t t e r n s of nonmonotonic inference are
particularly relevant:
• Background: (% a , f l ) ^ overlap(e~, ca) >
Background(a, fl)
• Axiom
/3) ^
T h e hierarchical discourse s t r u c t u r e is similar
to t h a t in Scha and Polanyi (1988): Elaboration
and Explanation are subordinating relations and
the others are coordinating ones. Equally, this
structure defines similar constraints on attachment: the current clause m u s t attach to the previous clause or else to the clauses it elaborates
or explains. In other words, the open clauses are
those on the right frontier. We do not directly encode the nucleus/satellite distinction used in RST
( T h o m p s o n and Mann, 1987).
Ezplanation( a, fl)
• Axiom
on Background:
Background(a, fl) ---. overlap(ca, c# )
The rules for Narration, Explanation and Background constitute defeasible linguistic knowledge,
and the axioms on t h e m indefeasible linguistic
knowledge. In particular, Narration and its axiom convey information about the pragmatic effects of the descriptive order of events; unless
there is information to the contrary, it is assumed
that the descriptive order of events matches their
• D e f e a s i b l e Modus Ponens: ~ > ~b,~b ~ ¢
e.g. Birds normally fly, T w e e t y is a bird; so
Tweety flies
2Discourse structure and c~ ~t/3 are given model theoretical interpretations in Asher (in press); e(~ abbreviates
me(c~), which is formally defined in Lascarides and Asher
(1991b) in an intuitively correct way. For simplicity, we
have here ignored the modal nature of the indefeasible
knowledge; in fact, an indefeasible rule is embedded within
the necessity operator 1:3.
• Nixon Diamond:
Not: ¢ > X , ¢ > - ~ X , ¢ , ¢ ~ X (or -~X)
e.g. Not: Quakers are pacifists, Republicans are not, Nixon is b o t h a quaker and
Nixon is a pacifist/Nixon is a non-pacifist.
• The P e n g u i n Principle:
e.g. Penguins are birds, birds normally fly,
penguins normally d o n ' t fly, T w e e t y is a
penguin; so Tweety doesn't fly.
I c o n i c a n d N o n - l c o n i c t e x t : In interpreting
text (5) we attempt to attach the second clause
to the first (so (a, c~, fl) holds, where a and fl
are respectively the logical forms of the first and
second clauses).
therefore form a pattern out of which a Nixon
Diamond crystallises: no temporal or discourse
relation can be inferred. We stipulate that it is incoherent to assume that (% a,/3) if one can't infer
which discourse relation holds between a and ft.
So the assumption that the clauses are connected
must be dropped, and hence no representation of
(3) is constructed.
(5) Max stood up. John greeted him.
(1) Max fell. John pushed him.
DICE exploits this account of incoherence in its
approach to discourse popping. When a Nixon
Diamond occurs in attempting to attach the current clause to the previous one, they don't form
a coherent text segment. So the current clause
must attach to one of the other open clauses,
resulting in discourse popping (Lascarides and
Asher, 1991a).
In the absence of further information, the only
rule whose antecedent is satisfied is Narration.
So we infer via Defeasible Modus Ponens that
the Narration relation holds between its clauses.
This then yields, assuming logical omniscience,
an iconic interpretation; the standing up precedes
the greeting. In contrast, text (1) verifies the antecedents to two of our defeasible laws: Narration
and the Push Causal Law. The consequents of
these default laws cannot both hold in a consistent KS. By the Penguin Principle, the law with
the more specific antecedent wins: the Causal
Law, because its antecedent logically entails Narration's. Hence (1) is interpreted as: the pushing caused the falling. In turn, this entails that
the antecedent to Explanation is verified; and
whilst conflicting with Narration, it's more specific, and hence its consequent--Explanation-follows by the Penguin Principle. 3 Notice that
deductions about event structure and discourse
structure are interleaved.
Trajectory in DICE
I n c o h e r e n c e a n d p o p p i n g : Consider the incoherent text (3).
It should be clear DICE's devices, while formal,
are also quite powerful. However, the maxims
introduced so far cannot actually explain either
discourse time switching (cf. (1) vs (2)) or amelioration (cf. (3) vs (4)). Incorporating some
form of contextual constraint may be one way to
deal with such cases. Because DICE makes essential use of nonmonotonic inference, adding contextual constraints will alter the inferences without requiring modification of the existing knowledge representation. We now investigate the consequences of adding MCT.
(3) ?Max won the race in record time. He was
home with the cup.
Maintain Causal Trajectory
Suppose R(a, ~) holds for some discourse relation
R; then a appears in the text before/3, and we
use this fact to define MCT. The default law below states that if the existing discourse context
is one where a cause/effect relation was described
in that order, then the current clause should not
describe a further cause of the effect:
The Win Law captures the intuition that if Max
wins the race and he is at home, then these events
normally don't temporally overlap--regardless of
whether they're connected or not.
• Win
win(max, race, ex) A athome(max, e2) >
-~overlap(e x, e2)
• Maintain Causal Trajectory: (r,fl,7)A
The appropriate knowledge base in the analysis
of (3) satisfies States Overlap, the Win Law and
Narration. The first two of these conflict, but
their antecedents aren't logically related. They
In using this rule, an interpreter brings to bear
'top-down' information, in the following sense.
Up to now, discourse and temporal relations have
been determined by using the input discourse as
data, and predicting the relations using general
linguistic and world knowledge. Now, the interpreter is permitted to 'remember' which predic-
3The formal details of how the logic CB models these
interpretations are given in Lascarides and Asher (1991b).
Although the double application of the Penguin Principle,
as in (1), is not valid in general, they show t h a t for the
particular case considered here, GE validates it.
tion they made last time, and use this to constrain
the kind of relation that can be inferred for attaching the current clause; this new prediction
needs no data to drive it. Of course, incoming
data can prevent the prediction from being made;
MCT is just a default, and (6) is an exception.
(6) Max switched off the light. The room went
pitch dark, since he had drawn the blinds too.
Time Switching
says how the event structures predicted for
preceding context can affect the temporal relations predicted for the current clause. But how
does it interact with other causal knowledge in
DICE? Does it account for time switching? Since
M C T is a contextual constraint, it will only interact with causal knowledge in a discourse context.
So consider how it affects the attachment of (2c)
and (2d).
(2) a.
John and Max came to the cliff's edge.
John applied a sharp blow to the back
of Max's neck.
Max fell.
John pushed him.
Max rolled over the edge of the cliff.
Suppose that the logical forms of the clauses (2ae) are respectively o~ to e, and suppose that the
discourse structure up to and including 3" has
been constructed in agreement with intuitions:
(29 ~
' ~
" "r
Furthermore, assume, in line with intuitions, that
the interpreter has inferred that e# caused e 7.
Consider how 6 is to be attached to the above
discourse structure. 3' is the only open clause; so
(% 3', 6) must hold. The antecedents to three defeasible laws are verified: the Push Causal Law
and Narration just as before, and also M C T . The
consequents of the Push Causal Law and M C T
conflict; moreover, their antecedents aren't logically related. So by the Nixon Diamond, we can't
infer which event--or discourse--relation holds.
Accordingly, the discourse is actually incoherent.
Yet intuitively, a relation can be inferred: the
push happened after the fall, and the clauses 3"
and 6 must be related by Narration.
On its own, MCT cannot account for time switching (or, indeed, amelioration). In one sense this
isn't surprising. Causal knowledge and MCT were
in conflict in (2), and since both laws relate to
the domain, but in incommensurable ways, neither logic nor intuition can say which default is
preferred. This suggests t h a t using domain structure alone to constrain interpretation will be insufficient. It seems likely that presentational issues will be significant in cases such as these;
where domain-specific knowledge sources are in
irresolvable conflict, aspects of the existing discourse structure may help determine current clause
attachment. Since M C T has some motivation, it
would be preferrable to let presentational information interact with it, rather than replace it.
Constraints from the
Presentational Context
To what degree does existing rhetorical structure
determine clause attachment? It's plausible to
suggest that a speaker-writer should not switch
genre without syntactically marking the switch.
Thus, if the preceding context is narrative, then
a hearer-reader will continue to interpret the discourse as narrative unless linguistic markers indicate otherwise; similarly for non-narrative contexts (cf. Caenepeel 1991, Polanyi and Scha 1984).
This constraint relies on the continuation of a
characteristic pattern of discourse relations, rather
than on maintaining trajectory on some domain
relation. Let's call this a presentational constraint;
it may be able to get the right analyses of (2) and
(4). In (2), for example, the context to which
John pushed him is attached is narrative, so according to the constraint this clause would be
attached with Narration in agreement with intuitions. But clearly, this constraint must be a
soft one, since discourse pops can occur without
syntactic markers, as can interruptions (Polanyi
1985:306). Both of these cause a change in the
discourse 'pattern' established in the preceding
P a t t e r n s in D I C E
Can we use presentational constraints without accidentally blocking discourse popping and interruptions? The problem is to represent in formal
terms exactly when an interpreter should try to
preserve the pattern of rhetorical structure established in the context. Because DICE provides a
formal account of how discourse popping o c c u r s - the Nixon Diamond is the k e y - - w e are in a good
position to a t t e m p t this.
are candidates for attachment) form a Nixon Diamond. Let Info(a) be glossed 'the information
Info is true of the clause a'. It is an abbreviation
for statements such as fall(max, ea), cause(e~, ep),
and so on. If a Nixon Diamond occurs when attempting to attach a t o / 3 on the basis of information other than DP, the following holds:
Discourse P a t t e r n and Inertia
• In fo( ) A ln fo(/3) A
^ Zn/oO))&(7.,
First, we define the discourse pattern established
by the context in terms of a function DP. This
takes as input the discourse structure for the preceding context, filters out those discourse relations which would break the pattern, and outputs
the remaining set of relations. This is similar to
Hobbs' (1985:25-26) notion of genre, where, for
example (in his terms) a story genre requires that
the type of occasion relation can be only problemsolution or event-outcome.
> ±)
We will use ND(a,/3) as a gloss for the above
schema, and open(7.,a) means a is an open clause
in the discourse structure 7-; assume that DP(7.)
returns some discourse relation R. So the presentational constraint for preserving discourse pattern is defined as follows:4
(Vot)(open(7.,a) A ND(a,/3)) >
(3a')(open(r, a ' ) A DP(7.)(a',/3))
• Inertia:
How much of the preceding discourse context
does DP take as input? At one extreme, it could
be just the discourse relations used to attach the
previous clause; the o u t p u t would be those same
discourse relations. At the other extreme, the
whole discourse structure m a y be input; DP would
have to establish the regularity in the configuration of discourse relations, and evaluate which
discourse relation would preserve it when the new
clause is added. We leave this question open; for
the examples of time switching and amelioration
we consider here, DP would produce the same result whatever it takes as input--Narration.
The antecedent to Inertia is verified only when all
the information availablc cxcept for the preceding discourse p a t t e r n - - y i e l d s a Nixon Diamond
in attempting the attachment o f / 3 at all open
nodes. Inertia thus won't prevent discourse popping, because there a Nixon Diamond is averted
at a higher-level open node. T h e model of text
processing proposed here restricts the kind of information that's relevant during text processing:
the discourse pattern is relevant only when all
other information is insufficient. Like MCT, Inertia is top-down, in the sense that it relies on earlier predictions about other discourse relations,
rather than on incoming data; but unlike MCT,
the 'theory-laden' predictions are only resorted
to if the data seems recalcitrant.
Using DP, we can represent the discourse pattern constraint. T h e intuition it captures is the
following. If the sentence currently being processed can't attach to any of the open nodes because there's a Nixon Diamond of irresolvable conflict, then assume that the discourse relation to be
used is defined by DP. In other words, discourse
pattern preservation applies only when all other
information prevents attachment at all available
open nodes. To express this formally, we need
a representation of a state in which a Nixon Diamond has formed. In cE, we use the formula
± (meaning contradiction) and the connective &,
whose semantics is defined only in the context
of default laws (of. Asher and Morreau 1991b).
Intuitively, (A&B) > _1_ means 'A and B are antecedents of default rules that lead to a conflict
that can't be resolved'.
We now look at text (2) in detail. Suppose as
before that the discourse structure 7- for the first
three clauses in (2) is (2'), and the task now is
to attach 6 (i.e. John pushed him). The only
open clause is 7, because the previous discourse
relations are all Narration. Moreover, DP(v) is
Narration. As before, a Nixon Diamond forms
between MCT and the Push Causal Law in attempting to attach 6 to 3'- Where Area is the
antecedent to MCT, and Apcl the antecedent to
the Push Causal Law substituted with 7 and 6:
We use this to represent cases where the information provided by the clauses ~ and /3 (which
4Inertia features an embedded default connective. Only
t w o nonmonotonic logics can express this: Circumscrip-
tion and Or..
• Area A Apa A ((Apct&Ama) >
where no discourse relation was inferred, leading
to incoherence.
So ND(7,8) is verified, and with it, the antecedent
to Inertia; substituting in the Inertia schema the
value of DP(r), the Nixon Diamond, and the open
clauses yields the following:
• I n e r t i a for
Inertia enables discourse context to establish
coherence between sentence pairs that, in isolation, are incoherent. It would be worrying if Inertia were so powerful that it could ameliorate any
text. But incoherence is still possible: consider
replacing (4f) with (4if):
(Area A Apa A ((Apet&Ama) > .L)) >
Narration(7 , 6)
The antecedent to Inertia entails that of Maintain
Trajectory (Area) and that of Push Causal Law
(Apcz). In cE the most specific law wins. So the
discourse context in this case determines the relation between the fall and the push: it is Narralion. Hence even though WK yields a causal preference for the pushing causing the falling, given
the discourse context in which the pushing and
falling are described in (2), Narration is inferred
after all, and so the falling precedes the push.
In this way, we can represent the presentational,
and domain-specific, information that must be
brought to bear to create a time switch.5
If world knowledge is coded as intuitions would
suggest, then no common topic can be constructed
for (4e) and (4g); and this is necessary if they are
to be attached with Narration or Background-the only discourse relations available given the defeasible laws that are verified. Moreover, Inertia
won't improve the coherence in this case because
it predicts Narration, which because of Common
Topic for Narration cannot be used to attach (4t*)
to (4 0 . So the text is incoherent.
Hobbs et al (1990) also explore the effects of
linguistic and causal knowledge on interpretation,
using abduction rather than deduction. Now,
Konolige (1991) has shown that abduction and
nonmonotonic deduction are closely related; but
since Hobbs et al don't attempt to treat timeswitching and amelioration, direct comparison here
is difficult. However, the following points are relevant. First, weighted abduction, as a system of
inference, isn't embeddable in CE, and vice versa.
Secondly, the weights which guide abduction are
assigned to predicates in a context-free fashion.
Hobbs et al observe that this may make the effects of context hard to handle, since 'the abduction scheme attempts to make global judgements
on the basis of strictly local information' [p48].
Now consider texts (3) and (4). A Nixon Diamond formed between Narration, States Overlap
and the Win Law in the analysis of (3) above,
leading to incoherence. Now consider attaching
the same clauses (4e) and (4f).
Max got up early yesterday.
He had a little bite to eat.
He had a light workout.
He started the tournament in good form.
He won the race in record time.
He was home with the cup.
He celebrated until late into the evening.
?Mary's hair was black.
We examined instances of two types of contextual
constraint on current clause attachment. These
were Maintain Causal Trajectory, a domain constraint; and Inertia, a presentational constraint.
We argued that domain constraints seemed insufficient, but that presentational constraints could
constructively interact with them. This interaction then explains the two discourse interpretation phenomena we started out with. Context can
switch round the order of events; and it can ameliorate an otherwise incoherent interpretation.
Given the discourse (4a-e), (4e) is the only open
clause to which (4f) can attach. Moreover, as
in (3), attempting to attach (4f) to (4e) results
in a Nixon Diamond. So the antecedent to Inertia is verified. DP delivers Narration, since the
discourse context is narrative. So (4e-f) is interpreted as a narrative. Compare this with (3),
5If a speaker-writer wanted to avoid this contextual
inference pattern, and sustain the non-iconic reading, then
they could switch to the pluperfect, for example.
Both of the constraints allow predictions about
new discourse relations to be driven from previous predictions. But MCT simply adds its prediction to the data-driven set from which the logic
chooses, whereas discourse pattern and Inertia
are only relevant to interpretation when the logic
can otherwise find no discourse relation.
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This formalisation has also raised a number of
questions for future investigation. For example,
the discourse pattern (or Hobbsian 'genre') function is important; but how much of the preceding
discourse structure should the D P function take
as input? How do we establish--and i m p r o v e - the linguistic coverage? W h a t is the relation between communicative intentions and contextual
constraints? How do we actually implement contextual constraints in a working system?
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The idea of contextual constraints is a familiar and comfortable one. In this respect, we have
merely provided one way of formally pinning it
down. Naturally, this requires a background logical theory of discourse structure, and we have
used DICE, which has its own particular set of discourse relations and implicature patterns. However, the process of logically specifying the constraints has two i m p o r t a n t and general benefits,
independent of the particular formalisation we
have offered. First, it demands precision and uniformity in the s t a t e m e n t both of the new constraints, and of the other knowledge sources used
in interpretation. Secondly, it permits a programindependent assessment of the consequences of
the general idea of contextual constraints.
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