INFERRING DISCOURSE RELATIONS IN CONTEXT* 
Alex Lascarides 
Human Communication 
Research Centre, 
University of Edinburgh, 
2 Buccleuch Place, Edinburgh 
alex@cogsc£, ed. ac. uk 
Nicholas Asher 
Center for Cognitive Science, 
University of Texas, 
Austin, Texas 78712 
asher@cgs, utexas, edu 
:Ion Oberlander 
Human Communication 
Research Centre, 
University of Edinburgh, 
2 Buccleuch Place, Edinburgh 
jonecogec£, ed. ac .uk 
Abstract 
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 con- 
straints interact with the other knowledge sources, 
we draw some general conclusions about the role 
of domain-specific information, top-down and bot- 
tom-up discourse information flow, and the use- 
fulness of formalisation in discourse theory. 
Introduction: Time Switching 
and Amelioration 
Two essential parts of discourse interpretation in- 
volve (i) determining the rhetorical role each sen- 
tence 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 in- 
terpretation 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. 
a The 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 re- 
viewers for their helpful comments. 
Moreover, the text in (3) in vacuo is incoherent, 
but becomes coherent in (4)'s context. 
(3) 
(4) 
?Max won the race in record time. He was 
home with the cup. 
Max got up early yesterday. He had a lit- 
tle 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. 
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)). The purpose of this paper 
is two-fold: we attempt 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 con- 
strains 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 attach- 
ment. On the other hand, Sibun (1992) has re- 
cently argued that domain-specific information, 
as opposed to domain-independent rhetorical in- 
formation, 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 in- 
troduce a domain-specific contextual constraint, 
following Sibun, and then place it in a formal the- 
ory of discourse attachment called DICE, devel- 
oped in Lascarides and Asher (1991a). We then 
show how the proposed domain-constraint is in- 
sufficient, and demonstrate how it can be aug- 
mented 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 that task structure, or more generally, do- 
main structure, is sufficient to account for many 
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. The constraint is: don't change 
trajectory. Sibun argues that in the temporal do- 
main, 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 ba- 
sic relation of causal proximity. So in the latter 
domain, Sibun's domain constraint precludes tex- 
tual 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) con- 
straint has two important attributes: first, it is 
domain-specific; secondly, it introduces into dis- 
course interpretation an element of top-down pro- 
cessing. To investigate these properties, and see 
how far they go towards explaining discourse time 
switch, and discourse amelioration, we now incor- 
porate 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. Kamp 1981), but goes on to as- 
sume with Grosz and Sidner (1986) that candi- 
date discourses possess hierarchical structure, with 
units linked by discourse relations modelled af- 
ter those proposed by IIobbs (1979, 1985) (cf. 
also Thompson and Mann 1987, Scha and Polanyi 
1988). 1 Lascarides and Asher (1991a) use Narra- 
tion, Explanation, Background, Result and Elab- 
oration. 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 Thompson 
and Mann (1987). 
DICE is a dynamic, logical theory for deter- 
mining the discourse relations between sentences 
in a text, and the temporal relations between 
the eventualities they describe. The logic used 
is the nonmonotonic logic Commonsense Entail- 
ment (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 Gricean- 
style pragmatic maxims and world knowledge. 
Discourse Structure and Implicature 
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 representa- 
XLascaxides 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. 
tion r of the text so far (of which a is already 
a part) is to be updated with the representation 
fl of the current clause via a discourse relation 
with a". Let a g /~ mean that a is a topic 
for fl; let e~ be a term referring to the main 
eventuality described by the clause a; and let 
fall(m, e~) mean that 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 ¢')and --* is the ma- 
terial conditional. The maxims for modelling im- 
plicature are then represented as schemas: 2 
• Narration: (r,a, fl) > Narration(a, fl) 
• Axiom on Narration: 
Narration(a, fl) ---* ea -q e# 
• Explanation: (r, ^ caus (  , > 
Ezplanation( a, fl) 
• Axiom on Explanation: 
Explanation(a, fl) ~ ~ea -~ e~ 
• Push Causal Law: 
(r, a, 1~) ^ fall(m, ca) ^ push(j, m, ca) > 
cause(ea, ec,) 
• Causes Precede Effects: 
cause(ei, el) ---, "-,st -~ e2 
• States Overlap: 
(r, a, fl) ^ state(e#) > overlap(ca, e#) 
• Background: (% a,fl) ^ overlap(e~, ca) > 
Background(a, fl) 
• Axiom on Background: 
Background(a, fl) ---. overlap(ca, c# ) 
The rules for Narration, Explanation and Back- 
ground constitute defeasible linguistic knowledge, 
and the axioms on them indefeasible linguistic 
knowledge. In particular, Narration and its ax- 
iom convey information about the pragmatic ef- 
fects of the descriptive order of events; unless 
there is information to the contrary, it is assumed 
that the descriptive order of events matches their 
2Discourse structure and c~ ~t/3 are given model theo- 
retical 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. 
3 
temporal order in interpretation. The Push Causal 
Law is a mixture of linguistic knowledge and world 
knowledge; given that the clauses are discourse- 
related somehow, the events they describe must 
normally be connected in a causal, part/whole or 
overlap relation; here, given the events in ques- 
tion, they must normally stand in a causal rela- 
tion. That Causes Precede their Effects is inde- 
feasible world knowledge. 
We also have laws relating the discourse struc- 
ture to the topic structure (Asher, in press): for 
example, A Common Topic for Narrative states 
that any clauses related by Narration must have 
a distinct, common (and perhaps implicit) topic: 
• A Common Topic for Narrative 
Narration(a, fl) -* 
^ ^ /3) ^ 
The hierarchical discourse structure is similar 
to that in Scha and Polanyi (1988): Elaboration 
and Explanation are subordinating relations and 
the others are coordinating ones. Equally, this 
structure defines similar constraints on attach- 
ment: the current clause must attach to the pre- 
vious 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 en- 
code the nucleus/satellite distinction used in RST 
(Thompson and Mann, 1987). 
Interpretation by Deduction 
cE and the defeasible rules are used to infer the 
discourse and temporal---structures of candidate 
texts, cE represents nonmonotonic validity as 
~. Three patterns of nonmonotonic inference are 
particularly relevant: 
• Defeasible Modus Ponens: ~ > ~b,~b ~ ¢ 
e.g. Birds normally fly, Tweety is a bird; so 
Tweety flies 
• The Penguin Principle: 
e.g. Penguins are birds, birds normally fly, 
penguins normally don't fly, Tweety is a 
penguin; so Tweety doesn't fly. 
• Nixon Diamond: 
Not: ¢ > X,¢ > -~X,¢,¢ ~ X (or -~X) 
e.g. Not: Quakers are pacifists, Republi- 
cans are not, Nixon is both a quaker and 
republican 
Nixon is a pacifist/Nixon is a non-pacifist. 
Iconic and Non-lconic text: 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). 
(5) Max stood up. John greeted him. 
(1) Max fell. John pushed him. 
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 an- 
tecedents to two of our defeasible laws: Narration 
and the Push Causal Law. The consequents of 
these default laws cannot both hold in a consis- 
tent KS. By the Penguin Principle, the law with 
the more specific antecedent wins: the Causal 
Law, because its antecedent logically entails Nar- 
ration's. Hence (1) is interpreted as: the push- 
ing caused the falling. In turn, this entails that 
the antecedent to Explanation is verified; and 
whilst conflicting with Narration, it's more spe- 
cific, and hence its consequent--Explanation-- 
follows by the Penguin Principle. 3 Notice that 
deductions about event structure and discourse 
structure are interleaved. 
Incoherence and popping: Consider the in- 
coherent text (3). 
(3) ?Max won the race in record time. He was 
home with the cup. 
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 Law: 
win(max, race, ex) A athome(max, e2) > 
-~overlap(e x, e2) 
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 
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 that for the 
particular case considered here, GE validates it. 
4 
therefore form a pattern out of which a Nixon 
Diamond crystallises: no temporal or discourse 
relation can be inferred. We stipulate that it is in- 
coherent 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. 
DICE exploits this account of incoherence in its 
approach to discourse popping. When a Nixon 
Diamond occurs in attempting to attach the cur- 
rent 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). 
Trajectory in DICE 
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 ame- 
lioration (cf. (3) vs (4)). Incorporating some 
form of contextual constraint may be one way to 
deal with such cases. Because DICE makes essen- 
tial use of nonmonotonic inference, adding con- 
textual constraints will alter the inferences with- 
out requiring modification of the existing knowl- 
edge representation. We now investigate the con- 
sequences of adding MCT. 
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 be- 
low 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: 
• Maintain Causal Trajectory: (r, fl,7)A 
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 inter- 
preter is permitted to 'remember' which predic- 
tion they made last time, and use this to constrain 
the kind of relation that can be inferred for at- 
taching 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 
MCT says how the event structures predicted for 
preceding context can affect the temporal rela- 
tions predicted for the current clause. But how 
does it interact with other causal knowledge in 
DICE? Does it account for time switching? Since 
MCT is a contextual constraint, it will only inter- 
act 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. 
Ot 
b. John applied a sharp blow to the back 
of Max's neck. fl 
c. Max fell. 7 
d. John pushed him. 6 
e. Max rolled over the edge of the cliff. 
Suppose that the logical forms of the clauses (2a- 
e) are respectively o~ to e, and suppose that the 
discourse structure up to and including 3" has 
been constructed in agreement with intuitions: 
Narration Narration 
(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 de- 
feasible laws are verified: the Push Causal Law 
and Narration just as before, and also MCT. The 
consequents of the Push Causal Law and MCT 
conflict; moreover, their antecedents aren't logi- 
cally 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 switch- 
ing (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, nei- 
ther logic nor intuition can say which default is 
preferred. This suggests that using domain struc- 
ture alone to constrain interpretation will be in- 
sufficient. It seems likely that presentational is- 
sues will be significant in cases such as these; 
where domain-specific knowledge sources are in 
irresolvable conflict, aspects of the existing dis- 
course structure may help determine current clause 
attachment. Since MCT has some motivation, it 
would be preferrable to let presentational infor- 
mation 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 dis- 
course as narrative unless linguistic markers in- 
dicate otherwise; similarly for non-narrative con- 
texts (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 ac- 
cording to the constraint this clause would be 
attached with Narration in agreement with in- 
tuitions. 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 
context. 
Patterns in DICE 
Can we use presentational constraints without ac- 
cidentally blocking discourse popping and inter- 
ruptions? The problem is to represent in formal 
terms exactly when an interpreter should try to 
preserve the pattern of rhetorical structure estab- 
lished in the context. Because DICE provides a 
formal account of how discourse popping occurs-- 
the Nixon Diamond is the key--we are in a good 
position to attempt this. 
Discourse Pattern and Inertia 
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 pre- 
ceding context, filters out those discourse rela- 
tions 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 problem- 
solution or event-outcome. 
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 output would be those same 
discourse relations. At the other extreme, the 
whole discourse structure may be input; DP would 
have to establish the regularity in the configu- 
ration 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 re- 
sult whatever it takes as input--Narration. 
Using DP, we can represent the discourse pat- 
tern constraint. The intuition it captures is the 
following. If the sentence currently being pro- 
cessed can't attach to any of the open nodes be- 
cause there's a Nixon Diamond of irresolvable con- 
flict, 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 Di- 
amond 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 an- 
tecedents of default rules that lead to a conflict 
that can't be resolved'. 
We use this to represent cases where the infor- 
mation provided by the clauses ~ and /3 (which 
are candidates for attachment) form a Nixon Di- 
amond. 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 at- 
tempting to attach a to/3 on the basis of infor- 
mation other than DP, the following holds: 
• In fo( ) A ln fo(/3) A 
^ Zn/oO))&(7., > ±) 
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 presen- 
tational constraint for preserving discourse pat- 
tern is defined as follows: 4 
• Inertia: (Vot)(open(7., a) A ND(a,/3)) > 
(3a')(open(r, a') A DP(7.)(a',/3)) 
The antecedent to Inertia is verified only when all 
the information availablc cxcept for the preced- 
ing discourse pattern--yields a Nixon Diamond 
in attempting the attachment of/3 at all open 
nodes. Inertia thus won't prevent discourse pop- 
ping, because there a Nixon Diamond is averted 
at a higher-level open node. The model of text 
processing proposed here restricts the kind of in- 
formation that's relevant during text processing: 
the discourse pattern is relevant only when all 
other information is insufficient. Like MCT, Iner- 
tia is top-down, in the sense that it relies on ear- 
lier 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. 
6 
Time Switching 
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 at- 
tempting 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: 
4Inertia features an embedded default connective. Only 
two nonmonotonic logics can express this: Circumscrip- 
tion and Or.. 
• Area A Apa A ((Apct&Ama) > I) 
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: 
• Inertia for (2): 
(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 re- 
lation between the fall and the push: it is Narra- 
lion. Hence even though WK yields a causal pref- 
erence 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 
Amelioration 
Now consider texts (3) and (4). A Nixon Dia- 
mond 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). 
(4) a. 
b. 
¢. 
d. 
e. 
f. 
g. 
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. 
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 Iner- 
tia is verified. DP delivers Narration, since the 
discourse context is narrative. So (4e-f) is in- 
terpreted 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. 
where no discourse relation was inferred, leading 
to incoherence. 
Inertia enables discourse context to establish 
coherence between sentence pairs that, in isola- 
tion, are incoherent. It would be worrying if Iner- 
tia were so powerful that it could ameliorate any 
text. But incoherence is still possible: consider 
replacing (4f) with (4if): 
f. ?Mary's hair was black. 
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 de- 
feasible 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 time- 
switching and amelioration, direct comparison here 
is difficult. However, the following points are rel- 
evant. 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 ef- 
fects of context hard to handle, since 'the abduc- 
tion scheme attempts to make global judgements 
on the basis of strictly local information' \[p48\]. 
7 
Conclusion 
We examined instances of two types of contextual 
constraint on current clause attachment. These 
were Maintain Causal Trajectory, a domain con- 
straint; and Inertia, a presentational constraint. 
We argued that domain constraints seemed insuf- 
ficient, but that presentational constraints could 
constructively interact with them. This interac- 
tion then explains the two discourse interpreta- 
tion phenomena we started out with. Context can 
switch round the order of events; and it can ame- 
liorate an otherwise incoherent interpretation. 
Both of the constraints allow predictions about 
new discourse relations to be driven from previ- 
ous predictions. But MCT simply adds its predic- 
tion 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. 
This formalisation has also raised a number of 
questions for future investigation. For example, 
the discourse pattern (or Hobbsian 'genre') func- 
tion is important; but how much of the preceding 
discourse structure should the DP function take 
as input? How do we establish--and improve-- 
the linguistic coverage? What is the relation be- 
tween communicative intentions and contextual 
constraints? How do we actually implement con- 
textual constraints in a working system? 
The idea of contextual constraints is a famil- 
iar and comfortable one. In this respect, we have 
merely provided one way of formally pinning it 
down. Naturally, this requires a background log- 
ical theory of discourse structure, and we have 
used DICE, which has its own particular set of dis- 
course relations and implicature patterns. How- 
ever, the process of logically specifying the con- 
straints has two important and general benefits, 
independent of the particular formalisation we 
have offered. First, it demands precision and uni- 
formity in the statement both of the new con- 
straints, and of the other knowledge sources used 
in interpretation. Secondly, it permits a program- 
independent assessment of the consequences of 
the general idea of contextual constraints. 

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