FOREWORD 
Bonnie Lynn Webber 
Department of Computer and Information Science 
University of Pennsylvania, Philadelphia, PA 19104-6389 
1 INTRODUCTION 
The phenomena of tense and aspect have long been of 
interest to linguists and philosophers. Linguists have 
tried to describe their interesting morphological, syn- 
tactic, and semantic properties in the various languages 
of the world, while philosophers have tried to charac- 
terize formally their truth conditions. (For some recent 
collections of papers, the reader is referred to Tedeschi 
and Zaenen 1981; Hopper 1982; Dahl 1985; and LoCas- 
io and Vet 1985.) Recently, computational linguists 
have joined in the act, their interest being sparked by a 
desire to characterize--at the level of processing--how 
we understand and describe complex events in a chang- 
ing world. Here, two kinds of questions converge--one 
concerning the problem of encoding event descriptions, 
the other to do with manipulating references to events. 
In approaching the first question, researchers of all 
linguistic stripes (computational linguists, philosophers 
of language, psycholinguists, and linguists of the "un- 
marked case") have begun to turn their attention from 
how languages convey information about individuals (or 
sets of individuals) and their properties to how they 
convey information about events and situations chang- 
ing over time. In approaching the second question, 
computational linguists have become interested in de- 
veloping systems that can converse with users about 
events and situations (e.g., for planning) or can process 
accounts of events and situations (e.g., for summarizing 
and/or integrating messages). Last year, following the 
appearance of a number of papers on this topic at the 
1987 Conference of the Association for Computational 
Linguistics at Stanford, it was suggested that a special 
issue of Computational Linguistics should be devoted 
to the topic of tense and aspect, in order to examine 
what appeared to be an emerging consensus on these 
questions within the computational-linguistics commu- 
nity. This issue is the result of that suggestion, and 
many of the papers collected below constitute exten- 
sions of the papers presented at the Stanford meeting. 
The papers demonstrate both practical and theoreti- 
cal advances in our understanding of tense and aspect. 
With respect to those approaches that have been imple- 
mented (cf. Hinrichs, Passonneau), the papers demon- 
strate how far we have come in developing general 
methods for extracting and representing event-related 
information from text and embedding those methods in 
question-answering and text-processing systems. With 
respect to theoretical issues involved in how we under- 
stand and describe events in a changing world (a subject 
of all the papers included here), the papers demonstrate 
the significance of ideas of processing, knowledge rep- 
resentation, and common-sense reasoning drawn from 
artificial intelligence and computational linguistics. It is 
these ideas that are computation's unique contribution 
to our understanding of tense and aspect, augmenting 
existing contributions from linguistics and philosophy. 
2 REICHENBACH'S TYPOLOGY OF TENSE 
Many of the papers collected here build upon Hans 
Reichenbach's 1947 account of the underlying structure 
of the English tense system. Reichenbach started from 
the observation that the collection of English tenses 1 
could not be explained either (a) simply in terms of the 
absolute time of an event or (b) in terms of the relative 
time of that event to the (purported) time of utterance of 
the tensed clause. Rather, he proposed that the inter- 
pretation of tense requires three separate sense- 
semantic entities: point of speech (S), point of the event 
(E), and point of reference (R). 2 The interpretations of S 
and E are self-evident. R is the time "talked about" or 
"focused upon" (or, equivalently, the "temporal per- 
spective" from which the described event is viewed). It 
may be the same as S, as in present perfect (Example 
la) or simple present tense (Example lb). 
1. a. John has climbed Aconcagua and Mt. McKin- 
ley.(E<R= S) 
b. John is in the lounge. (E = R = S) 
(Here, "<" indicates "temporally prior to".) It may be 
the same as E, as in the simple past tense (Example 2a) 
or simple future tense (Example 2b). 
2. a. John climbed Aconcagua. (E = R < S) 
b. John will climb Aconcagua. (S < E = R) 
It may be in between E and S, as in the past perfect: 
3. John had climbed Aconcagua. (E < R < S) 
or following both E and S (looking back to them), as in 
the future perfect: 
4. John will have climbed Mt. McKinley. (S < E < R) 
Reichenbach's account is imprecise in some cases, 
incomplete in some, and only seductively suggestive in 
others. While all the authors in this volume have taken. 
Computational Linguistics, Volume 14, Number 2, June 1988 1 
Bonnie Webber Foreword 
Reichenbach's account as their starting point, they 
differ in what they attempt to flesh out, pin down, or 
correct, and in the details of how they do so. Neverthe- 
less, there is a remarkable degree of agreement across 
these accounts. If together they do not yet provide a 
complete solution to the problem of how we understand 
and describe events, they may still comprise a new 
stage from which further progress may be made. 
3 OVERVIEW 
There are five papers included in this issue: by Erhard 
Hinrichs (now at the University of Illinois), by Marc 
Moens and Mark Steedman (University of Edinburgh), 
by Sasha Nakhimovsky (Colgate University), by Becky 
Passoneau (UNISYS) and by myself (University of 
Pennsylvania). All the papers have gone through exten- 
sive revision in response to internal exchange among the 
authors, in addition to the usual process of external review. 
Hinrichs describes a meaning-representation lan- 
guage (MRL) for temporal expressions based on higher- 
order Intensional Logic, that is being used in the 
JANUS system, a natural-language understanding and 
generation system under joint development at BBN 
Laboratories and ISI. He shows how this MRL, which 
employs temporal indices based on Reichenbach's 
points of speech, event, and reference, avoids problems 
that classical tense logics have with even basic temporal 
adverbials like "yesterday", with negation, with quan- 
tification, and with multiple-clause sentences describing 
multiple events. He also shows how this MRL permits 
accurate description of discourse entities evoked by 
tensed clauses, for use in resolving anaphoric noun 
phrases (NPs) and other context-sensitive expressions. 
Moens and Steedman propose and argue for a tripar- 
tite ontology of events based more on notions of causa- 
tion and consequence than on purely tempor~d primi- 
tives. Their proposal allows a simple solution to what 
has been called "the imperfective paradox", to the 
semantics of "when" clauses, and to the changing 
sense of an event description as various temporal ad- 
verbials are added to it. They also propose Reichen- 
bachian-style analyses for the various "future tenses" 
in English those constructions used to describe events 
that are expected, possible, or intended subsequent to 
speech time. In that, they fill in a gap in Reichenbach's 
own analysis, which considers only the simple future 
tense and is equivocal even on that. 
Nakhimovsky describes how our common-sense 
knowledge of events manifests itself in language and 
how this knowledge is used in understanding narratives. 
Nakhimovsky discusses three types of common-sense 
knowledge involved in how we describe events and 
how, in turn, we understand event descriptions: knowl- 
edge of the internal structuring of events, knowledge of 
their duration and the durations of their various sub- 
parts, and knowledge of how events can be viewed. 
(The first and third types of knowledge link with discus- 
sions in Moens and Steedman and in Passonneau.) At a 
practical level, Nakhimovsky shows how such knowl- 
edge could be stored in the lexicon. 
Pa,;sonneau describes temporal processing in PUN- 
DIT, a natural-language text ur~derstanding system de- 
veloped by researchers at UNISYS. Its purview is 
clauses that describe actual events such as are fre- 
quently found in status reports. In particular, it ad- 
dresses the problem of processing the distinctive con- 
tribufions of those linguistic elements used in describing 
events of different types. For each clause describing an 
actual event, Passonneau characterizes the temporal 
structure of that event and its temporal location vis-h- 
vis speech time or, when relevant, the event described 
in an adjoining temporal adverbial. Like Moens and 
Steedman, Passonneau employs a tripartite structure 
for events in characterizing temporal structure. This 
paper makes beautifully clear many of the factors 
involved in correctly characterizing the internal tempo- 
ral structure of events. 
Finally, I focus on the discourse-related properties of 
tensed clauses and show that they are very similar to 
those of anaphoric definite NPs. Specifically, they 
depend on our knowledge of plausible relations between 
events (as anaphoric definite NPs depend on our knowl- 
edge of plausible relations between entities), and they 
respond in somewhat similar fashion to a listener's 
perceptions of discourse structure. In demonstrating the 
former dependency, I employ a tripartite event struc- 
ture drawn from the work of Moens and Steedman and 
from Passonneau. This paper contributes to a long- 
standing discussion in linguistics as to whether tense 
should be classified as an anaphor, as well as identifying 
additional knowledge and mechanisms needed for proc- 
essing reports and other narrative texts. 
NOTES 
1. geichenbach regarded simple past, present, and future, and past, 
present, and future perfect as tenses. While others have considered 
the perfect and the future as belonging to the systems of aspect and 
modality, rather than tense, Reichenbach's usage remains a con- 
venient shorthand. 
2. Reichenbach points out in a footnote that Jespersen, more than 20 
years before, proposed a related three-point semantics for past 
perfect and future perfect but did not extend a similar analysis to 
the other English tenses. 

REFERENCES 
Dahl, O. 1985 Tense and Aspect Systems. Basil Blackwell, Oxford, 
England. 
Hopper, P. 1982 Tense-aspect: between semantics and pragmatics. 
John Benjamins, Amsterdam. 
Dowty, D., ed. 1986 Linguistics and Philosophy 9(1). Special Issue on 
Tense and Aspect in Discourse. 
LoCasio, V. and Vet, C. 1985 Temporal Structure in Sentence and 
Discourse. Foris Publications, Dordrecht. 
Reichenbach, H. 1947, reprint 1966 The Elements of Symbolic Logic. 
The Free Press, New York, NY. 
Tedeschi, P. and Zaenen, A. 1981 Syntax and Semantics. Volume 14: 
Tense and Aspect. Academic Press, NewYork, NY. 
