TEMPORAL RELATIONS: 
REFERENCE OR DISCOURSE COHERENCE? 
Andrew Kehler 
Harvard University 
Aiken Computation Laboratory 
33 Oxford Street 
Cambridge, MA 02138 
kehler@das.harvard.edu 
Abstract 
The temporal relations that hold between events de- 
scribed by successive utterances are often left implicit 
or underspecified. We address the role of two phenom- 
ena with respect to the recovery of these relations: (1) 
the referential properties of tense, and (2) the role of 
temporal constraints imposed by coherence relations. 
We account for several facets of the identification of 
temporal relations through an integration of these. 
Introduction 
Tense interpretation has received much attention in lin- 
guistics (Partee, 1984; Hinrichs, 1986; Nerbonne, 1986, 
inter alia) and natural language processing (Webber, 
1988; Kameyama et al., 1993; Lascarides and Asher, 
1993, inter alia). Several researchers (Partee, 1984; 
Hinrichs, 1986; Nerbonne, 1986; Webber, 1988) have 
sought to explain the temporal relations induced by 
tense by treating it as anaphoric, drawing on Reichen- 
bach's separation between event, speech, and reference 
times (Reichenbach, 1947). Specifically, to account for 
the forward progression of time induced by successive 
simple past tenses in a narrative, they treat the simple 
past as referring to a time evoked by a previous past 
tense. For instance, in Hinrichs's (1986) proposal, ac- 
complishments and achievements x introduce a new ref- 
erence point that is temporally ordered after the time 
of the event itself, "ensuring that two consecutive ac- 
complishments or achievements in a discourse are al- 
ways ordered in a temporal sequence." On the other 
hand, Lascarides and Asher (1993) take the view that 
temporal relations are resolved purely as a by-product 
of reasoning about coherence relations holding between 
utterances, and in doing so, argue that treating sim- 
ple and complex tenses as anaphoric is unnecessary. 
This approach parallels the treatment of pronoun res- 
olution espoused by Hobbs (1979), in which pronouns 
are modeled as free variables that are bound as a by- 
product of coherence resolution. The Temporal Cen- 
tering framework (Kameyama et al., 1993) integrates 
lWe will limit the scope of this paper by restricting the 
discussion to accomplishments and achievements. 
aspects of both approaches, but patterns with the first 
in treating tense as anaphoric. 
We argue that aspects of both analyses are necessary 
to account for the recovery of temporal relations. To 
demonstrate our approach we will address the following 
examples; passages (la-b) are taken from Lascarides 
and Asher (1993): 
(1) a. Max slipped. He spilt a bucket of water. 
b. Max slipped. He had spilt a bucket of water. 
c. Max slipped because he spilt a bucket of water. 
d. Max slipped because he had spilt a bucket of 
water. 
Passage (la) is understood as a narrative, indicating 
that the spilling was subsequent to the slipping. Pas- 
sages (lb-d) are instead understood as the second clause 
explaining the first, indicating that the reverse temporal 
ordering holds. We address two related questions; the 
first arises from treating the simple past as anaphoric. 
Specifically, if a treatment such as Hinrichs's is used 
to explain the forward progression of time in example 
(la), then it must be explained why sentence (lc) is as 
felicitous as sentence (ld). That is, one would predict a 
clash of temporal relations for sentence (lc), since the 
simple pasts induce the forward progression of time but 
the conjunction indicates the reverse temporal ordering. 
The second question arises from assuming that all tem- 
poral relations are recovered solely from reasoning with 
coherence relations. Specifically, because the use of the 
simple past in passage (lc) is as felicitous as the past 
perfect in passage (ld) under the explanation interpre- 
tation (in these cases indicated explicitly by because), 
then it must be explained why passage (la) is not un- 
derstood as an explanation as is passage (lb), where 
in each case the relationship needs to be inferred. We 
present our analysis in the next section, and account 
for these facts in Section 3. 
The Account 
We postulate rules characterizing the referential nature 
of tense and the role of discourse relations in further 
constraining the temporal relations between clauses. 
The rules governing tense are: 
319 
1. Main verb tenses are indefinitely referential, cre- 
ating a new temporal entity under constraints 
imposed by its type (i.e., past, present, or fu- 
ture) in relation to a discourse reference time 2 tR. 
For instance, a main verb past tense introduces a 
new temporal entity t under the constraint prior- 
to(t, tR). For simple tenses tR is the speech time, 
and therefore simple tenses are not anaphoric. 
2. Tensed auxiliaries in complex tenses are anaphor- 
ic, identifying tR as a previously existing tempo- 
ral entity. The indefinite main verb tense is then 
ordered with respect to this tR. 
The tenses used may not completely specify the implicit 
temporal relations between the described events. We 
claim that these relations may be further refined by 
constraints imposed by the coherence relation operative 
between clauses. We describe three coherence relations 
relevant to the examples in this paper and give temporal 
constraints for them. 3 
Narration: The Narration relation is characterized by 
a series of events displaying forward movement of 
time, such as in passage (la). As did Lascarides 
and Asher (1993), we capture this ordering as a 
constraint imposed by the Narration coherence re- 
lation itself.- 4 
(2) If Narration(A, B) then ta < tB 
Parallel: The Parallel relation relates utterances that 
share a common topic. This relation does not 
impose constraints on the temporal relations be- 
tween the events beyond those provided by the 
tenses themselves. For instance, if passage (la) 
was uttered in response to the question What bad 
things happened to Maz today? (inducing a Paral- 
lel relation instead of Narration), a temporal or- 
dering among the sentences is no longer implied. 
Explanation: The Explanation relation denotes a 
cause-effect relationship with reversed clause or- 
dering, as in sentences (lb-d). Therefore, the sec- 
ond event is constrained to preceding the first: 
(3) If Ezplanation(A,B) then tB < tA 
To summarize the analysis, we claim that tense oper- 
ates as indefinite reference with respect to a possibly 
anaphorically-resolved discourse reference time. The 
temporal relations specified may be further refined as 
2This term is borrowed from Kameyama et al. (1993). 
3We assume here that the two clauses in question are 
related directly by a coherence relation. This may not be 
the case; for instance the use of a past perfect may signal 
the start of an embedded discourse segment, as in Web- 
ber's flower shop example (Webber, 1988; Kameyama et al., 
1993). How this account is to be extended to address coher- 
ence at the discourse segment level is the subject of future 
work. 
4The Cause-Effect relation also has this ordering 
constraint. 
a by-product of establishing the coherence relationship 
extant between clauses, Narration being but one such 
relation. 
We now 
repeated 
(4) a. 
b. 
c. 
d. 
Examples 
analyze the examples presented in Section 1, 
below, using this approach: 
Max slipped. He spilt a bucket of water. 
Max slipped. He had spilt a bucket of water. 
Max slipped because he spilt a bucket of water. 
Max slipped because he had spilt a bucket of 
water. 
The implicit ordering on the times indefinitely evoked 
by the simple pasts in passage (4a) results solely from 
understanding it as a Narration. In passage (4b), the 
auxiliary had refers to the event time of the slipping, 
and thus the past tense on spill creates a temporal en- 
tity constrained to precede that time. This necessitates 
a coherence relation that is consistent with this tem- 
poral order, in this case, Explanation. In passage (4c), 
the times evoked by the simple pasts are further or- 
dered by the Explanation relation indicated by because, 
resulting in the backward progression of time. In pas- 
sage (4d), both the tense and the coherence relation 
order the times in backward progression. 
Restating the first problem noted in Section 1, if 
treating the simple past as anaphoric is used to account 
for the forward progression of time in passage (4a), then 
one would expect the existence of the Explanation re- 
lation in passage (4c) to cause a temporal clash, where 
in fact passage (4c) is perfectly felicitous. No clash of 
temporal relations is predicted by our account, because 
the use of the simple pasts do not in themselves imply 
a specific ordering between them. The Narration rela- 
tion orders the times in forward progression in passage 
(4a) and the Explanation relation orders them in back- 
ward progression in passage (4c). The Parallel relation 
would specify no ordering (see the potential context for 
passage (4a) given in Section 2). 
Restating the second problem noted in Section 1, if 
temporal relations can be recovered solely from reason- 
ing with coherence relations, and the use of the simple 
past in passage (4c) is as felicitous as the past perfect 
in passage (4d) under the Explanation interpretation, 
then one asks why passage (4a) is not understood as an 
Explanation as is passage (4b), where in each case the 
relationship needs to be inferred. We hypothesize that 
hearers assume that speakers are engaging in Narration 
in absence of a specific cue to the contrary. The use 
of the past perfect (as in passage (4b)) is one such cue 
since it implies reversed temporal ordering; the use of 
an explicit conjunction indicating a coherence relation 
other than Narration (as in passages (4c-d)) is another 
such cue. While passage (4a) could be understood as an 
Explanation on semantic grounds, the hearer assumes 
Narration since no other relation is cued. 
320 
We see several advantages of this approach over that 
of Lascarides and Asher (1993, henceforth L&A). First, 
L&A note the incoherence of example (5) 
(5) ? Max poured a cup of coffee. He had entered the 
room. 
in arguing that the past perfect should not be treated 
as anaphoric: 
(6) Theories that analyse the distinction between the 
simple past and pluperfect purely in terms of dif- 
ferent relations between reference times and event 
times, rather than in terms of event-connections, 
fail to explain why \[(4b)\] is acceptable but \[(5)\] is 
awkward. (Lascarides and Asher, 1993, pg. 470) 
Example (5) indeed shows that coherence relations need 
to be utilized to account for temporal relations, but it 
does not bear on the issue of whether the past per- 
fect is anaphoric. The incoherence of example (5) is 
predicted by both their and our accounts by virtue of 
the fact that there is no coherence relation that corre- 
sponds to Narration with reverse temporal ordering. ~ 
In addressing this example, L&A specify a special rule 
(the Connections When Changing Tense (CCT) Law) 
that stipulates that a sentence containing the simple 
past followed by a sentence containing the past perfect 
can be related only by a subset of the otherwise possi- 
ble coherence relations. However, this subset contains 
just those relations that are predicted to be possible by 
accounts treating the past perfect as anaphoric; they 
are the ones that do not constrain the temporal order 
of the events against displaying backward progression 
of time. Therefore, we see no advantages to adopting 
their rule; furthermore, they do not comment on what 
other laws have to be stipulated to account for the facts 
concerning other possible tense combinations. 
Second, to explain why the Explanation relation can 
be inferred for passage (4b) but not for passage (4a), 
L&A stipulate that their causal Slipping Law (stating 
that spilling can cause slipping) requires that the CCT 
Law be satisfied. This constraint is imposed only to 
require that the second clause contain the past per- 
fect instead of the simple past. However, this does not 
explain why the use of the simple past is perfectly co- 
herent when the Explanation relationship is indicated 
overtly as it is in sentence (4c), nor do they adequately 
explain why CCT must be satisfied for this causal law 
and not for those supporting similar examples for which 
they successfully infer an unsignaled Explanation rela- 
tion (see discussion of example (2), pg. 463). 
Third, the L&A account does not explain why the 
past perfect cannot stand alone nor discourses gener- 
ally be opened with it; consider stating sentence (7) in 
isolation: 
(7) Max had spilt a bucket of water. 
5For instance, in the same way that Explanation corre- 
sponds to Cause-Effect with reverse temporal ordering. 
Intuitively, such usage is infelicitous because of a depen- 
dency on a contextually salient time which has not been 
previously introduced. This is not captured by the L&A 
account because sentences containing the past perfect 
are treated as sententially equivalent to those contain- 
ing the simple past. On the other hand, sentences in the 
simple past are perfectly felicitous in standing alone or 
opening a discourse, introducing an asymmetry in ac- 
counts treating the simple past as anaphoric to a pre- 
viously evoked time. All Of these facts are explained by 
the account given here. 
Conclusion 
We have given an account of temporal relations whereby 
(1) tense is resolved indefinitely with respect to a possi- 
bly anaphorieally-resolved discourse reference time, and 
(2) the resultant temporal relations may be further re- 
fined by constraints that coherence relations impose. 
This work is being expanded to address issues pertain- 
ing to discourse structure and inter-segment coherence. 
Acknowledgments 
This work was supported in part by National Science 
Foundation Grant IRI-9009018, National Science Foun- 
dation Grant IRI-9350192, and a grant from the Xerox 
Corporation. I would like to thank Stuart Shieber and 
Barbara Grosz for valuable discussions and comments 
on earlier drafts. 

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