DISCOURSE RELATIONS AND DEFEASIBLE KNOWLEDGE* 
Alex Lascarides t 
Human Communication Research Centre 
University of Edinburgh 
2 Buccleuch Place, Edinburgh, EH8 9LW 
Scotland 
alex@uk, ac. ed. cogsc£ 
Nicholas Asher 
Center for Cognitive Science 
University of Texas 
Austin, Texas 78712 
USA 
asher@sygmund, cgs. utexas, edu 
Abstract 
This paper presents a formal account of the 
temporal interpretation of text. The distinct nat- 
ural interpretations of texts with similar syntax 
are explained in terms of defeasible rules charac- 
terising causal laws and Gricean-style pragmatic 
maxims. Intuitively compelling patterns of defea,- 
sible entailment that are supported by the logic 
in which the theory is expressed are shown to un- 
derly temporal interpretation. 
The Problem 
The temporal interpretation of text involves an 
account of how the events described are related 
to each other. These relations follow from the 
discourse relations that are central to temporal 
import. 1 Some of these are listed below, where 
the clause a appears in the text before fl: 
Narration(a,fl): The event described in fl is 
a consequence of (but not necessarily caused by) 
tile event described in a: 
(1) Max stood up. John greeted him. 
Elaboration(a,~): The event described in /? 
contributes to the occurrence of the culmination 
*This paper is greatly influenced by work reported in 
(Lascarides & Oberlander, 1991). We would llke to thank 
Hans Kamp, Michael Morreau and .Ion Oberlander for 
their significant contributions to the content of this pa- 
per. All mistakes are solely our responsibility. 
t The support of the Science and Engineering Research 
Council through project number GR/G22077 is gratefully 
acknowledged. HCRC is supported by the Economic and 
Social Research Council. 
1 Extensive classifications of discourse relations are of- 
fered in (Polanyi, 1985), (Scha & Polanyi, 1988) and 
(Thompson & Mann, 1987). 
of the event described in a, i.e. fl's event is part 
of the preparatory phase of a's: 2 
(2) The council built the bridge. The architect 
drew up the plans. 
Explanation(a, fl): For example the event de- 
scribed in clause fl caused the event described in 
clause a: 
(3) Max fell. John pushed him. 
Background(a, fl): For example the state de- 
scribed in fl is the 'backdrop' or circumstances 
under which the event in a occurred (so the event 
and state temporally overlap): 
(4) Max opened the door. The room was pitch 
dark. 
Result(a, fl): The event described in a caused 
the event or state described in fl: 
(5) Max switched off the light. The room was 
pitch dark. 
We assume that more than one discourse re- 
lation can hold between two sentences; the sick- 
ness in (6) describes the circumstances when Max 
took the aspirin (hence the sentences are related 
by Background) and also explains why he took 
the aspirin (hence the sentences are related by 
Explanation as well). 
(6) Max took an aspirin. He was sick. 
The sentences in texts (1) and (3) and in (4) 
and (5) have similar syntax, and therefore similar 
2We assume Moens and Steedman's (1988) tripartite 
structure of events, where an event consists of a prepara- 
tory phase, a culmination and a consequent phase. 
55 
logical forms. They indicate, therefore, that the 
constraints on the use of the above discourse re- 
lations cannot rely solely on the logical forms of 
the sentences concerned. 
No theory at present is able to explain the dis- 
tinct temporal structures of all the above texts. 
Webber (1988) observes that Kamp & Rohrer 
(1983), Partee (1984), Hinrichs (1986) and Dowty 
(1986) don't account for the backwards movement 
of time in (2) and (3). Webber (1988) can account 
for the backwards movement of time in (2), but 
her theory is unable to predict that mismatching 
the descriptive order of events and their temporal 
order is allowed in some cases (e.g. (2) and (3)) 
but not in others (e.g. (1), which would be mis- 
leading if the situation being described were one 
where the greeting happened before Max stood 
up). 
Our aim is to characterise the circumstances 
under which each of the above discourse relations 
hold, and to explain why texts can invoke dif- 
ferent temporal relations in spite of their similar 
syntax. 
Dahlgren (1988) represents the difference be- 
tween (1) and (3) in terms of probabilistic laws 
describing world knowledge (WK) and linguistic 
knowledge (LK). Our approach to the problem 
is generally sympathetic to hers. But Dahlgren's 
account lacks an underlying theory of inference. 
Furthermore, it's not clear how a logical conse- 
quence relation could be defined upon Dahlgren's 
representation scheme because the probabilistic 
laws that need to interact in certain specific ways 
are not logically related. Unlike Dahlgren (1988), 
we will supply an inference regime that drives the 
interpretation of text. 
The properties required of an inference mech- 
anism for inferring the causal structure underly- 
ing text is discussed in (Lascarides & Oberlander, 
1991). The work presented here builds on this in 
two ways; first by supplying the required notion of 
inference, and second by accounting for discourse 
structure as well as temporal structure. 
Temporal Relations and 
Defeasible Reasoning 
Let us consider texts (1) and (3) on an intu- 
itive level. There is a difference in the relation 
that typically holds between the events being de- 
scribed. Intuitively, world knowledge (WK) in- 
eludes a causal 'law' gained from perception and 
experience that relates falling and pushing: 3 
• Causal Law 3 
Connected events el where x falls and e2 
where y pushes z are normally such that e2 
causes el. 
There is no similar law for standing up and greet- 
ing. The above law is a de feasible law. Our claim 
is that it forms the basis for the distinction be- 
tween (1) and (3), and that defeasible reasoning 
underlies the temporal interpretation of text. 
First consider text (1). Intuitively, if there 
is no temporM information at all gained from 
WK or syntactic markers (apart from the simple 
past tense which is the only temporal 'expres- 
sion' we consider here), then the descriptive order 
of events provides the only vital clue as to their 
temporal order, and one assumes that descriptive 
order matches temporal order. This principle is 
a re-statement of Grice's (1975) maxim of Man- 
ner, where it is suggested that text should be or- 
derly, and it is also motivated by the fact that 
the author typically describes events in the or- 
der in which the protagonist perceives them (cf. 
Dowty (1986)). This maxim of interpretation can 
be captured by the following two laws: 
• Narration 
Unless there's information to the contrary, 
clauses a and j3 that are discourse-related 
are such that Narration(a, ~) holds. 
• Axiom for Narration 
If Narration(a, fl) holds, and a and fi de- 
scribe the events el and e2 respectively, then 
el occurs before e2. 
Narration is defensible and the Axiom for Narra- 
tion is indefeasible. The idea that Gricean-style 
pragmatic maxims should be represented as de- 
feasible rules is suggested in (Joshi, Webber & 
Weischedel (1984)). 
The above rules can be defined in MASH--a 
logic for defensible reasoning described in (Asher 
& Morrean, 1991). We will demonstrate shortly 
that an intuitively compelling pattern of defensi- 
ble inference can then underly the interpretation 
of (1). 
MASH supplies a modal semantics for a lan- 
guage with a default or generic quantifier, and a 
3The causal law's index corresponds to the index of the 
text for which it is relevant. 
56 
dynamic partial semantics of belief states is built 
on top of this modal semantics to c~pture intu- 
itively compelling patterns of non-monotonic tea- 
soning. We use a propositional version of MASH 
here. Defaults are represented as ¢ > ¢ (read 
as "¢ then ¢, unless there is information to the 
contrary"). The monotonic component of the the- 
ory defines a notion of validity ~ that supports 
axioms such as ~ \[:3(¢ --* ¢) ~ ((X > ¢) --~ 
(X > ¢)). The dynamic belief theory supplies the 
nonmonotonic component, and the corresponding 
nonmonotonic validity, ~, describes what reason- 
able entailments follow from the agent's beliefs. 
supports (at least) the following patterns of 
common sense reasoning: 
Defensible Modus Ponens 
¢>¢,¢ ~ ¢ 
but not ¢>¢,¢,-~¢ ~ ¢ 
e.g. Birds fly, Tweety is a bird ~ Tweety flies, 
but not: Birds fly, Tweety is a bird that doesn't 
fly ~ Tweety flies. 
Penguin Principle 
¢ >¢,¢>C¢>-~,¢ ~-~i 
but not: ¢ > ¢,¢ :> (,¢ > -,(,¢ ~ ( 
e.g. Penguins are birds, Birds fly, Penguins don't 
fly, Tweety is a Penguin ~ Tweety doesn't fly, 
and does not ~ Tweety flies. 
Nixon Diamond 
not (¢ > ¢,I > "¢,¢,( ~ ¢ (or --¢)) 
e.g. There is irresolvable conflict in the follow- 
ing: Quakers are pacifists, Republicans are non- 
pacifists, Nixon is a Quaker and Republican. 
We assume a dynamic theory of discourse struc- 
ture construction in which a discourse structure 
is built up through the processing of successive 
clauses in a text. To simplify our exposition, 
we will assume that the basic constructs of these 
structures are clauses. 4 Let (4,13) mean that the 
clause ~ is to be attached to the clause a with a 
discourse relation, where a is part of the already 
built up discourse structure. Let me(a) be a term 
that refers to the main eventuality described by 
a (e.g. me(Max stood up) is the event of Max 
standing up). 5 Then Narration and the axiom 
on Narration are represented in MASH as follows 
(cl -~ e.~ means "el wholly occurs before e2"): 
4The theory should extend naturally to an account 
where the basic constructs are segments of text; the 
approach adopted here is explored extensively in Asher 
(forthcoming). 
5me(c~) is formally defined in Lascarides & Asher 
(1991) in a way that agrees with intuitions. 
• Narration 
(or, ~) > Narration(c~,~3) 
• Axiom on Narration 
r~ (Na,','atio,~(~, ~) --, me(~) ~ me(Z)) 
We assume that in interpreting text the reader 
believes all LK and WK (and therefore believes 
Narration and its axiom), the laws of logic, and 
the sentences in the text. The sentences in (1) are 
represented in a DnT-type framework as follows: 6 
(7) \[e1,~1\]\[~1 <now, hold(el,Q),s~andup(rn, el)\] 
(8) \[~, t~\]\[t2 < now, hold(~2, t2),gr~t(j, m, ~2)\] 
In words, (7) invokes two discourse referents el 
and ~1 (which behave like deictic expressions), 
where el is an event of Max standing up, tl is 
a point of time earlier than now and et occurs at 
it. (8) is similar save that the event e2 describes 
John greeting Max. (7) and (8) place no condi- 
tions on the relative temporal order between et 
and e2. These are derived at a higher level of anal- 
ysis than sentential semantics by using defensible 
reasoning. 
Suppose that the reader also believes that the 
clauses in text (1) are related by some discourse 
relation, as they must be for the text to be coher- 
ent. Then the reader's beliefs also include (7, 8). 
The natural interpretation of (1) is derived by 
calculating the common sense entailments from 
the reader's belief state. Given the assumptions 
on this state that we have just described, the an- 
tecedent to Narration is verified, and so by Defen- 
sible Modus Ponens, Narration(7, 8) is inferred. 
Since the belief states in MASH support modal clo- 
sure, this result and the Axiom on Narration en- 
tail that the reader believes the main eventuality 
of (7), namely el, precedes the main eventuality 
of (8), namely e2. So the intuitive discourse struc- 
ture and temporal interpretation of (1) is derived 
by exploiting defeasible knowledge that expresses 
a Gricean-style pragmatic maxim. 
But the analysis of (1) is satisfactory only if 
the same technique of exploiting defeasible rules 
can be used to obtain the appropriate natural in- 
terpretation of (3), which is different from (1) in 
spite of their similar syntax. 
eFor the sake of simplicity we ignore the problem of 
resolving the NP anaphora in (8). The truth definitions 
of (7) and (8) are llke those given in DRT save that they 
are evaluated with respect to a possible world index since 
MASH is modal. 
67 
(3) a. Max fell. 
b. John pushed him. 
As we mentioned before, Causal Law 3 will pro- 
vide the basis for the distinct interpretations of 
(1) and (3). The clauses in (3) must be related 
by a discourse relation for the text to be coherent, 
and therefore given the meanings of the discourse 
relations, the events described must be connected 
somehow. Therefore when considering the do- 
main of interpreting text, one can re-state the 
above causal law as follows: 7 
Causal Law 3 
Clauses a and/3 that are discourse-related 
where a describes an event el of x falling 
and/3 describes an event e~ of y pushing x 
are normally such that e2 causes el. 
The representation of this in MASH is: 
Causal Law 3 
(a,/3)^f.n(x, me(a))^push(y, x, me(/3)) > 
ca~se(m~(~), me(a)) 
This represents a mixture of WK and linguistic 
knowledge (LK), for it asserts that given the sen- 
tences are discourse-related somehow, and given 
the kinds of events that are described by these 
sentences, the second event described caused the 
first, if things are normal. 
The logical forms for (3a) and (3b) are the 
same as (7) and (8), save that standup and greet 
are replaced respectively with fall and push. 
Upon interpreting (3), the reader believes all de- 
feasible wK and LK together with (3a), (3b) and 
(3a, 3b). Hence the antecedents to two defeasible 
laws are satisfied: Narration and Causal Law 3. 
Moreover, the antecedent of Law 3 entails that 
of Narration, and the laws conflict because of the 
axiom on Narration and the axiom that causes 
precede effects: 
• Causes Precede Effects 
\[\] (Vele2)(cause(el, e2) ~ ~e2 -~ el) 
The result is a 'Complex' Penguin Principle: it 
is complex because the consequents of the two 
defeasible laws are not ~ and -~ff, but instead the 
laws conflict in virtue of the above axioms. MASH 
supports the more complex Penguin Principle: 
;'This law may seem very 'specific'. It could potentially 
be generalised, perhaps by re-stating el as x moving and 
e2 as y applying a force to x. For the sake of brevity we 
ignore this generalisation. 
• Complex Penguin Principle 
o(¢ ¢),¢ > x,¢ > ¢, o(x 0), 
\[\] (¢ ¢ 
but not: \[\] (¢ --* ¢), ¢ > X, ¢ > (, 
o (x 0), n (¢ -. ¢ x 
Therefore there is a defeasible inference that the 
pushing caused the falling from the premises, as 
required. 
The use of the discourse relation Explanation 
is characterised by the following rule: 
• Explanation 
(a, A > 
Explanation(a, jr) 
In words, if a and f~ are discourse-related and the 
event described in/3 caused the event described in 
a, then Explanation(a, ~) normally holds. Fur- 
thermore, Explanation imposes a certain tempo- 
ral structure on the events described so that if 
is a causal explanation of a then fPs event doesn't 
precede a's: 
• Axiom on Explanation 
\[\] (Explanation(a,/3) -~ -~me(a ) -~ rne(/3 ) ) 
The antecedent to Narration is verified by the 
reader's beliefs, and given the results of the Com- 
plex Penguin Principle above, the antecedent to 
Explanation is also verified. Moreover, the an- 
tecedent to Explanation entails that of Narration, 
and these laws conflict because of the above ax- 
ioms. So there is another complex Penguin Prin- 
ciple, from which Explanation(3a, 3b) is inferred. 
The second application of the Penguin Prin- 
ciple in the above used the results of the first, 
but in nonmonotonic reasoning one must be wary 
of dividing theories into 'subtheories' in this way 
because adding premises to nonmonotonic deduc- 
tions does not always preserve conclusions, mak- 
ing it necessary to look at the theory as a whole. 
(Lascarides & Asher, 1991) shows that the pred- 
icates involved in the above deduction are suffi- 
ciently independent that in MASH one can indeed 
divide the above into two applications of the Pen- 
guin Principle to yield inferences from the theory 
as a whole. Thus our intuitions about the kind of 
reasoning used in analysing (3) are supported in 
the logic. We call this double application of the 
Penguin Principle where the second application 
uses the results of the first the Cascaded Penguin 
Principle. s 
8On a general level, MASH is designed so that the con- 
58 
Distinct Discourse Structures 
Certain constraints are imposed on discourse 
structure: Let R be Explanation or Elaboration; 
then the current sentence can be discourse re- 
lated only to the previous sentence a, to a sen- 
tence fl such that R(fl, a), or to a sentence 7 such 
that R(7, fl) and R(~, a). This is a simpler ver- 
sion of the definition for possible attachment sites 
in Asher (forthcoming). Pictorially, the possi- 
ble sites for discourse attachment in the example 
structure below are those marked open: 
Open 
Explana~ lanati°n 
Closed Open 
Narration Explanation/// ~xplanation 
Closed ~ Open Narration 
There are structural similarities between our 
notion of openness and Polanyi's (1985). The 
above constraints on attachment explain the awk- 
wardness of text (9a-f) because (9c) is not avail- 
able to (gf) for discourse attachment. 
(9) a. Guy experienced a lovely evening last 
night. 
b. He had a fantastic meal. 
c. He ate salmon. 
d. He devoured lots of cheese. 
e. He won a dancing competition. 
f. ?He boned the salmon with great ex- 
pertise. 
According to the constraint on attachment, the 
only available sentence for attachment if one were 
to add a sentence to (1) is John greeted him, 
whereas in (3), both sentences are available. Thus 
although the sentences in (1) and (3) were as- 
signed similar structural semantics, they have very 
different discourse structures. The events they 
flict between defeasible laws whose antecedents axe such 
that one of them entails the other is resolvable. Thus un- 
wanted irresolvable conflicts can be avoided. 
describe also have different causal structures. These 
distinctions have been characterised in terms of 
defeasible rules representing causal laws and prag- 
matic maxims. We now use this strategy to anal- 
yse the other texts we mentioned above. 
Elaboration 
Consider text (2). 
(2) a. The council built the bridge. 
b. The architect drew up the plans. 
We conclude Elaboration(2a, 2b) in a very sim- 
ilar way to example (3), save that we replace 
cause(me(~), me(a)) in the appropriate defensi- 
ble rules with prep(me(~), me(a)), which means 
that rne(~) is part of the preparatory phase of 
me(a). In Law 2 below, Info(a,~) is a gloss for 
"the event described in a is the council build- 
ing the bridge, and the event described in fl is 
the architect drawing up the plans", and the law 
represents the knowledge that drawing plans and 
building the bridge, if connected, are normally 
such that the former is in the preparatory phase 
of the latter: 
• Elaboration 
(a, ^ prep( e( ), me(a)) > 
Elaboration(a, fl ) 
• Axiom on Elaboratio~ 
n (Elaboration(a, -*   ne(a) 
• Law 2 
(a,/3) ^ Info(a, > prep(me(Z), ) 
The inference pattern is a Cascaded Penguin Prin- 
ciple again. The two resolvable conflicts are Law 
2 and Narration and Elaboration and Narration. 
Background 
Intuitively, the clauses in (4) are related by Back- 
ground. 
(4) Max opened the door. The room was pitch 
dark. 
The appropriate reader's belief state verifies the 
antecedent of Narration. In addition, we claim 
that the following laws hold: 
59 
• States Overlap 
(a, A state(me( )) > 
overlap(me(a), me( ) ) 
• Background 
(a, Z> ^ overlap(me(a), me(Z)) > 
Background(a, fl ) 
• Axiom on Background 
\[\] (Background(a, 
overlap(me(a), me(~) ) ) 
States Overlap ensures that when attached clauses 
describe an event and state and we have no knowl- 
edge about how the event and state are connected, 
gained from WK or syntactic markers like because 
and therefore, we assume that they temporally 
overlap. This law can be seen as a manifesta- 
tion of Grice's Maxim of Relevance as suggested 
in (Lascarides, 1990): if the start of the state is 
not indicated by stating what caused it or by in- 
troducing an appropriate syntactic marker, then 
by Grice's Maxim of Relevance the starting point, 
and is irrelevant to the situation being described. 
So the start of the state must have occurred be- 
fore the situation that the text is concerned with 
occurs. As before, we assume that unless there is 
information to the contrary, the descriptive order 
of eventualities marks the order of their discovery. 
This together with the above assumption about 
where the state starts entail that unless there's 
information to the contrary, the state temporally 
overlaps events or states that were described pre- 
viously, as asserted in States Overlap. 
We assume that the logical form of the sec- 
ond clause in (4) entails state(me(~)) by the 
classification of the predicate dark as stative. 
So Background is derived from the Cascaded 
Penguin Principle: the two resolvable conflicts 
are States Overlap and Narration and Back- 
ground and Narration. States Overlap and Nar- 
ration conflict because of the inconsistency of 
overlap(el,e~) and el -~ e~; Background and 
Narration conflict because of the axioms for Back- 
ground and Narration. 
Result 
(5) has similar syntax to (4), and yet unlike (4) 
the event causes the state and the discourse rela- 
tion is Result. 
(5) a. Max switched off the light. 
b. The room was pitch dark. 
Let Info(a,fl) be a gloss for "me(a) is Max 
switching off the light and me(fl) is the room be- 
ing dark". So by the stative classification of dark, 
Info(a, fl) entails state(me(~)). Then Law 5 re- 
flects the knowledge that the room being dark and 
switching off the light, if connected, are normally 
such that the event causes the state: 9 
• Causal Law 5 
(a,/7) A Info(a,~) > cause(me(a), me(/7)) 
The use of the discourse relation of Result is char- 
acterised by the following: 
• Result 
(a,  )^eause(me( ), > 
• Axiom on Result 
D(Result(a,~) --. me(a) ~ me(fl)) 
The reader's beliefs in analysing (5) verify the an- 
tecedents of Narration, States Overlap and Law 
5. Narration conflicts with States Overlap, which 
in turn conflicts with Law 5. Moreover, the an- 
tecedent of Law 5 entails that of States Overlap, 
which entails that of Narration. So there is a 
'Penguin-type' conflict where Law 5 has the most 
specific antecedent. In MASH Law 5's consequent, 
i.e. cause(me(ha), me(hb)), is inferred from these 
premises. The antecedent of Result is thus sat- 
isfied, but the antecedent to Background is not. 
Result does not conflict with Narration, and so 
by Defeasible Modus Ponens, both Result(ha, 5b) 
and Narration(ha, hb) are inferred. 
Note that thanks to the axioms on Background 
and Result and the inconsistency of overlap(el, e~) 
and el -~ e2, these discourse relations are in- 
consistent. This captures the intuition that if 
a causes b, then b could not have been the case 
when a happened. In particular, if Max switching 
off the light caused the darkness, then the room 
could not have been dark when Max switched off 
the light. 
Discourse Popping 
Consider text (9a-e): 
(9) a. Guy experienced a lovely evening last 
night. 
b. He had a fantastic meal. 
9For the sake of simplicity, we ignore the problem of 
inferring that the light is in the room. 
60 
c. He ate salmon. 
d. lie devoured lots of cheese. 
e. He won a dancing competition. 
The discourse structure for (9a-d) involves Cas- 
caded Penguin Principles and Defeasible Modus 
Ponens as before. Use is made of the defeasible 
knowledge that having a meal is normally part of 
experiencing a lovely evening, and eating salmon 
and devouring cheese are normally part of having 
a meal if these events are connected: 
Guy experienced a lovely evening last night 
Elaboration 
He had a fantastic meal 
Elabora~-~f~~boration 
lie ate salmon He devoured 
Narration lots Of cheese 
We study the attachment of (9e) to the preced- 
ing text in detail. Given the concept of openness 
introduced above, the open clauses are (9d), (95) 
and (9a). So by the assumptions on text pro- 
cessing, the reader believes (9d, 9e), (9b, 9e) and 
(9a, 9e). (9d, 9e) verifies the antecedent to Narra- 
tion, but intuitively, (9d) is not related to (9e) at 
all. The reason for this can be explained in words 
as follows: 
• (9d) and (9e) don't form a narrative be- 
cause: 
- Winning a dance competition is nor- 
mally not part Of a meal; 
- So (9e) doesn't normally elaborate (9b); 
- But since (9d) elaborates (95), (9e) can 
normally form a narrative with (9d) 
only if (9e) also elaborates (9b). 
Thcse intuitions can be formalised, where Info(a, fl) 
is a. gloss for "me(a) is having a meal and me(fl) 
is winning a dance competition": 
* Law 9 
(a, ^ I fo( , Z) >  prep(me( ), me(.)) 
• Defeaslbly Necessary Test for 
Elaboration 
(a, ^ > 
-~ Elaboration( a, fl) 
• Constraint on Narration 
Elaboration((~, fll )A-~Eiaboration( a, f12 ) > 
-~ N arration(~t , ~2 ) 
The result is a 'Nixon Polygon'. There is irre- 
solvable conflict between Narration and the Con- 
straint on Narration because their antecedents are 
not logically related: 
Narration(9d, 9e) 
-~Elaboration(9b, 9e) 
Elaboration(9b, 9e) 
l (9d,De) 
(9d, 9e) 
-~prep(me(9b, 9e)) Elaboration(~ 
(9d, 9e) 
Info(9b, 9e) 
Elaboration(9b, 9d) 
The above in MASH yields \]i~Narration(9d, 9e) 
and ~-~Narration(9d, 9e). We assume that be- 
lieving (9d, 9e) and failing to support any dis- 
course relation between (9d) and (9e) is inco- 
herent. So (9d,9e) cannot be believed. Thus 
the Nixon Diamond provides the key to discourse 
'popping', for (9e) must be related to one of the 
remaining open clauses; i.e. (95) or (9a). In fact 
by making use of the knowledge that winning a 
dance competition is normally part of experienc- 
ing a lovely evening if these things are connected, 
Elaboration(9a, 9e) and Narration(9b, 9e) follow 
as before, in agreement with intuitions. 
Conclusion 
We have proposed that distinct natural inter- 
pretations of texts with similar syntax can be ex- 
plained in terms of defeasible rules that represent 
61 
causal laws and Gricean-style pragmatic maxims. 
The distinct discourse relations and event rela- 
tions arose from intuitively compelling patterns 
of defeasible entailment. The Penguin Principle 
captures the intuition that a reader never ignores 
information salient in text that is relevant to cal- 
culating temporal and discourse structure. The 
Nixon Diamond provided the key to 'popping' 
from subordinate discourse structure. 
We have investigated the analysis of texts in- 
volving only the simple past tense, with no other 
temporal markers present. Lascarides & Asher 
(1991) show that the strategy pursued here can 
be applied to the pluperfect as well. Future work 
will involve extending the theory to handle texts 
that feature temporal connectives and adverbials. 

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