Review of The Language of Time
|Date: Mon, 05 Dec 2005 02:41:11 +0530
From: Anil Kumar Singh
Subject: The Language of Time
EDITORS: Mani, Inderjeet; Pustejovsky, James; Gaizauskas, Robert
TITLE: The Language of Time
SUBTITLE: A Reader
PUBLISHER: Oxford University Press
Anil Kumar Singh, Language Technology Research Centre (LTRC),
IIIT, Gachibowli, Hyderabad-500019 (A.P.), India.
Natural language (NL) can convey all kinds of information about the
world, abstract or concrete, real or hypothetical. Temporal information
is one of the most important aspects of whatever we want to express
using NL. This book presents a collection of some of the well known
papers on temporal information in NL. The focus seems to be on the
computational point of view. However, papers on linguistic and
philosophical issues have been included to provide enough
background. Since scholars and researchers from diverse fields
(philosophy, linguistics, psychology, natural language processing,
artificial intelligence) have contributed to the study of time in NL, the
editors have made an attempt to represent all of these. The book can
be useful for anyone interested in time and/or language.
The book is divided into four parts and there is a very informative
introduction to each of these parts. These introductions make the
papers accessible to even those without the required background as
they summarize and connect the papers. Some readers who are
looking just for a quick overview of the study of time in NL might want
to read only the introductions.
In all, the book contains 29 important papers. These papers cover
theories as well as computational approaches for studying (or
computing) time in NL. People working in the relevant fields are
already familiar with many of these.
The first part has 8 papers on tense, aspect and event structure. The
second part is about temporal reasoning and it consists of 6 papers.
The third part has 8 papers on temporal structure of discourse. The
fourth part puts together 7 papers on temporal annotation.
The first part starts with Vendler's well known paper ('Verbs and
Times') on verb classification with respect to time (or tense). Vendler
claims (a claim more or less accepted by many authors of the following
papers) that almost all the verbs can be classified into a few classes,
at least in their most dominant sense. The classes that he comes up
with after a very interesting and enjoyable discussion are those of
states, activities (or processes), accomplishments and achievements.
This four way classification is the starting point of a lot of work done
on time in language. Perhaps this is why Vendler's is the first paper in
the book, although it is not the earliest among those included in the
The second paper is on 'the syntax of event structure' by Pustejovsky.
He argues that grammatical phenomena make reference to the
internal structure of events, and that a subeventual analysis for
predicates is able to systematically capture these effects. This paper
represents a major stream in semantics. The purpose is to show that
verbs decompose into distinct event types with internal structure. The
primary components of an event structure are the event type (of the
lexical item), the rules of event decomposition and the rules to map to
lexical structure. Pustejovsky considers three basic event types:
states, processes and transitions. He also relates event structure with
lexical conceptual structure (LCS). Problems like adverbial
modification are discussed in terms of event structure. On the whole it
is a long but very readable paper which makes a convincing case for
looking at verbs as events with an internal structure. As the editors
point out in the introduction, this approach can avoid proliferation of
primitives, which is a major problem with schemes like that of Dowty's
(Dowty, 1979). A limitation is that event structure is not directly related
with temporal structure.
Pustejovsky's paper is followed by Emmon Bach's 'Algebra of Events'.
The starting point for this paper is the close parallels between the
mass count distinction in nominal systems and the aspectual
classification of verbal expressions. Bach expresses one of the
parallels as a proportion: events:processes::things:stuff, drawing on
Link's paper on count-mass-plural domain (Link, 1983). His algebra
deals with the distinctions based mainly on Carlson's classification
(Carlson, 1981), which subdivides achievements in two categories
(happenings and culminations, e.g. recognize and die). It also, in a
way, takes into account the structure of events and processes. Link's
scheme treats sets of individuals ('John and Mary') as super-
individuals formed by a join operation. The super-individual is made of
the stuff that the individuals are made of. Bach's contention is that telic
events are also formed of process 'stuff'.
The fourth paper is Reichenbach's classic work called 'The Tenses of
Verbs' (Reichenbach, 1947). The primary idea is that the times of
events can be located with respect to a deictic centre, which makes
them similar to pronouns (the anaphoric view of tenses). In
Reichenbach's scheme, there can be references to three time points:
the speech time (S), the event time (E) and the reference time (R).
These three time points can be related by 'precedes' or 'simultaneous'
relations. Depending on the relations between S, E and R, we can
define all the possible tenses. For example, E=R
past, whereas S=R=E would be simple present. Only seven out of the
thirteen possible relations are realized in English. This is perhaps the
most influential paper in the book. This approach to analyze tenses
has come to be known as the Reichenbachian approach. Quite apart
from its influence however, the paper is remarkable for the way it is
written. It is not divided into sections, there is no conclusion, nor are
there any references. One wonders how the reviewers would react if a
paper like this was submitted to a natural language processing (NLP)
conference in this age of LaTex stylesheets. Anyway, I found this
paper to be as enjoyable as it is influential.
The next paper is 'Tense Logic and the Logic of Earlier and Later'
(Prior, 1968). Prior presents a tense logic which is an extension of E.
J. Lemmon's minimal tense logic Kt. He also relates it to the minimal
calculus of earlier-later relation. He adds two operators G ('is always
going to be') and H ('has always been') to the two operators provided
by Kt: P (past) and F (future). The earlier-later calculus uses
operators T and U such that 'Tap' means 'It is the case at instant a
that p' and 'Uab' means that 'The instant a is earlier than instant b'.
The tense logic is built upon in stages in the paper so as to get four
logical systems of increasing power. This paper is also an important
one, but it is not as easy to read as the some of the other papers in
the book, partly due to the 'somewhat opaque prefix notation'.
The paper by Moens and Steedman counters the idea that linguistic
categories are related to a linear model of time. Their ontology is
based on a mental representation of events structured on more than
purely temporal principles. Their aspectual categories are a modified
form of the Vendlerian classes. States are distinguished from events,
which in turn can be classified in four categories depending on
whether there are consequences and whether the events are atomic
or extended. Aspectual type of a proposition can change due to
modifiers. Such 'aspectual coercion' can be explained in terms of a
transition network, which represents an aspect calculus. Another
central notion in their ontology is that of 'nucleus' ('a structure
comprising a culmination, an associated preparatory process, and a
consequent state'). This nucleus plays a role in many of the
permissible transitions in the network. However, as the editors point
out, the paper doesn't offer rules for compositional semantics.
Dorr and Olsen use LCS representation of Levin's classes. The
aspectual classed are defined in terms of three features (telicity,
dynamicity and durativity). In their system, features can only get
added, not deleted. These aspectual features can be used to help in
machine translation and generation.
The last paper in the first part is by Passonneau, describing the
PUNDIT information extraction system. She focuses only on actual
(realis) events. A three step method is used to find the actual time
associated with an event (if there is any), determine the temporal
structure of the situation, and locating the situation with respect to the
time of text production or to the times of other situations. The features
used for events are situation type (state, process, or transition event),
kinesis (active or stative) and boundedness (bounded, unbounded, or
The second part begins with a long but enjoyable paper (except
perhaps the example about a Republican President) by McDermott.
He presents a 'temporal logic for processes and plans'. One important
idea in this logic is that of temporal chronicles ('a complete possible
history of the universe'). The chronicles are arranged in the form of a
tree in which branching occurs only towards the future. This is meant
to model future possibility. McDermott defines events as sets of
intervals over which a proposition is minimally (at least once) true.
'A logic based calculus of events' is presented by Kowalsky and
Sergot. They concentrate on applying event calculus for database
updates as well as simple narratives. Instead of rejecting an update
conflicting with the existing information, the update is accepted and
the conflicting information is withdrawn. Some predicates introduced
are 'Holds', 'HoldsAt', 'initiates', 'terminates' and 'happens'.
Indeterminant temporal anchoring and granularity is taken into
account by Chittaro and Combi in framework called 'Temporal
Granularity and Indeterminacy Event Calculus (TGIC)'. Event calculus
determines the maximal validity intervals (MVIs) over which
properties hold. Their definition of MVI accommodates a more general
concept of event.
Here is another very influential paper, this time by James F. Allen. It
presents a 'general theory of action and time'. This theory seeks to
take care of actions that involve non-activity, actions not
decomposable into subactions and actions which occur simultaneously
and interact with others. Allen uses thirteen basic interval relations
like 'before', 'after', 'meet', 'during', etc. There are three classes
(property, event and process) and one metalanguage predicate
(HOLDS, OCCUR, OCCURRING) associated with each of these
Galton's paper is a critical revision of Allen's work. He 'reinstates'
instants to give a combined instant-interval scheme in which instants
either fall within or limit intervals. One of the reasons for this revision
of Allen's theory is to accommodate continuous change. The HOLD
and OCCURS predicated are each split into three (HOLDS-ON,
The paper by Hobbs and Pustejovsky on TimeML (originally DAML-
Time), a markup language for annotating temporal information in a
discourse. They also relate it to OWL-Time ontology. It takes into
account the formal theories of time that have been suggested during
the last many decades. This paper can be a good introduction to
TimeML, which might become a standard.
Though Dowty's 1979 paper is not included, there is one by him
on 'the effect of aspectual class on the temporal structure of
discourse'. His analysis is based on a compositional theory of aspect,
i.e., aspectual classes of lexical items combine to give the aspectual
classes of sentences. He also uses a pragmatic principle called
Temporal Discourse Interpretation Principle (TDIP) which basically
says that the reference time advances forward with each utterance,
unless there is a temporal adverbial. He discusses some other
pragmatic principles like the perspective of the narrator, expectations
about discourse conventions (Grice's maxims) and background
Lascarides and Asher take further the work on including pragmatics in
discourse (with respect to temporal relations). There work is more
formal, based on the notion of 'defeasible reasoning' (based on
default knowledge that can be overridden).
In this chapter, Bell extends the Labovian analysis of temporal
discourse structure. They focus on news stories and claim that the
narrative in these stories can be segmented into 'abstract', 'attribution'
and 'story', each of which can have further segmentation. The stories
also contain many 'episodes', which in turn contain events.
Here Webber extends the Reichenbachian notion of tense as
discourse anaphor. She also related this to the work on discourse by
Grosz and Sidner. The main elements of her approach are temporal
focus, a tripartite ontology of events (preparation, culmination and
consequence), 'specification' of entities by anaphors, and a 'focus
stack' mechanism for resumption of dialog segments.
Song and Cohen present an algorithm for using tense interpretation in
analyzing simple narratives. They also use a modified Reichenbachian
scheme for representing tenses which is more precise and
unambiguous (uses one SRE triple instead of three structures for
future perfect). Their algorithm is based on temporal focus, a tense
hierarchy (for English) and constraints on coherent tense sequences.
The automatic temporal reference resolution system described by
Wiebe et al. works on the ambiguous output of a semantic parser. The
domain they have considered is scheduling dialogs (''How about
two?'', ''Twelve to two.''). In their model, a 'temporal unit' is associated
with the current and the preceding utterance. A temporal unit has
fields which are partially ordered with respect to specificity. Some
resolution rules are also used.
Hwang and Schubert use a novel 'fine structure' of discourse,
namely 'tense trees'. Tense trees differ from simple
Reichenbachian 'lists'. Given that the sentences are not 'flat' (as
seems to have assumed by some, as far as studies of temporal
structure in discourse are concerned), tense trees can be used to
more effectively (compositionally) analyze tense and aspect.
Hitzeman et al. use an HPSG implementation of discourse grammar
for determining the temporal structure of the discourse. They try to
take into account the mutually constraining effect of tense, aspect,
temporal adverbials and rhetorical relations to reduce ambiguity.
Another suggestion is to use underspecified representation of
temporal or rhetorical structure, again for reducing ambiguity.
Note: The page numbers in the references given on the first page of
the book for chapters 23-26 (from the ACL 2001 workshop) seem to
be wrong. See the references below for the correct information.
Wilson et al. present an annotation framework for automatically
marking up temporal information. The annotation scheme is TIMEX2,
which has been incorporated into TimeML. Their system tries to
achieve cross-lingual reusability. Some updates since the publication
of the paper are also mentioned.
Another technique for temporal annotation is described by Katz and
Arosio. In their scheme ('a radically simplified semantic formalism'),
each verb is associated with a temporal interval and there are
relations among these intervals. The relations are encoded with
directed 'secondary edges' representing 'precedence' and 'inclusion'
relations and their duals. They also discuss the relations between
annotations, i.e., whether they are equivalent, consistent, or
inconsistent, or whether one subsumes the other.
Another temporal extraction system has been described by Filatova
and Hovy. They break up news stories into clauses representing
events and then assign time stamps to these events. Tense
information in the clauses is used to help in timestamping.
This paper (Schilder and Habel) describes a semantic tagging system
for extracting temporal information from news messages. The tagger
looks for dates, prepositional phrases and situational verbs. It marks
chunks of texts containing temporal information and also tries to
extract this information. Temporal relations (based on Allen's theory)
are identified partly by prepositions.
This chapter presents in detail the specification language TimeML.
While the other paper (Chapter 14) is more theory oriented, this one is
more about the definition of TimeML and guidelines for annotation
using this language. It is compulsory reading for anyone involved with
actual annotation of temporal information, or even with annotation in
Li et al. present a framework for mapping linguistic patterns to
temporal relations. It uses 'temporal concept frames' (activity related
or time related), temporal relations (relative or absolute), temporal
indicators (time related words), rules for temporal references and
rules for rules for resolving conflicts. They have tried their system for
Chinese with good results.
The last paper (Setzer et al.) is about comparing and evaluating
temporal annotations using the idea of 'temporal closure', which can
help in deciding whether two annotations are equivalent or not. The
paper also suggests ways to make manual annotation easier by
computing closure of an annotation. A precise way of evaluating
annotations is presented.
This book brings together a variety of approaches, theoretical as well
practical, for dealing with time in NL. The papers are among the most
relevant. They have been arranged in an order which makes sense.
The introductions are excellent too. Perhaps the division into four
parts and an introduction for each of them is best way to bring
together a lot of diversity. However, one wonders whether a small
introduction to each paper (instead of a long one to each part),
together with a chapter in the beginning looking at the big picture
wouldn't have been better. As it is, the readers might miss the
connections between the successive chapters in some cases. And
one can also complain about some missing papers (like Dowty, 1979),
but that is unavoidable in any collection of papers. On the whole, it
was rewarding to read this book. It might become a compulsory
reading for people working in the relevant disciplines, and there are
quite a few of them (people as well disciplines).
Dowty D. R. (1979). Word Meaning and Montague Grammar: the
Semantics of Verbs and Times in Generative Semantics and
Montague's PTQ. Reidel.
REFERENCES TO THE ORIGINAL PUBLICATION OF THE
CONTENTS OF THIS BOOK
1. Vendler Z. (1967). Verbs and Times. Ch. 4 of 'Linguistics in
Philosophy', 97-121. Cornell University Press.
2. Pustejovsky James (1991). The Syntax of Event Structure.
Cognition, 41. Elsevier.
3. Bach Emmon (1986). The Algebra of Events. 'Linguistics and
Philosophy', 9, 5-16. Swets and Zeitlinger Publishers.
4. Reichenbach Hans (1947). The Tenses of Verbs. Section 51
of 'Elements of Symbolic Logic', 287-298. The Macmillan Company,
5. Prior A. N. (1968). Tense Logic and the Logic of Earlier and Later.
Chapter 11 of 'Papers on Time and Tense', 116-134. Oxford
University Press, Oxford.
6. Moens Marc and Steedman Mark (1988). Temporal Ontology and
Temporal Reference. Computational Linguistics 14(2), 15-28.
Association for Computational Linguistics.
7. Dorr Bonnie J. and Olsen Mari Broman (1997). Deriving Verbal and
Compositional Lexical Aspect for NLP Applications. Proceedings of the
35th Annual Meeting of the Association for Computational Linguistics,
151-158. Association for Computational Linguistics.
8. Passonneau Rebecca J. (1988). A Computational Model of the
Semantics of Tense and Aspect. Computational Linguistics, 14(2), 15-
28. Association for Computational Linguistics.
9. McDermott Drew (1982). A Temporal Logic for Reasoning About
Processes and Plans. Cognitive Science 6, 101-155. Cognitive
10. Kowalski Robert and Sergot Marek (1986). A Logic-Based
Calculus of Events. New Generation Computing, 4, 67-94. Ohmsha
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and Montanari A. (eds.), 'Spatial and Temporal Granularity: Papers
from the AAAI Workshop'. Technical Report WS-00-08. The AAAI
12. Allen James F. (1984). Towards a General Theory of Action and
Time. Artificial Intelligence, 23, 123-154. Elsevier.
13. Galton Antony (1990). A Critical Examination of Allen's Theory of
Action and Time. Artificial Intelligence, 42, 159-188. Elsevier.
14. Hobbs Jerry R. and Pustejovsky James (2003). Annotating and
Reasoning About Time and Events. Proceedings of the AAAI Spring
Symposium on Logical Formalization of Commonsense Reasoning.
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15. Dowty David R. (1986). The Effects of Aspectual Class on the
Temporal Structure of Discourse: Semantics or Pragmatics?
Linguistics and Philosophy, 9, 37-61. Swets and Zeitlinger.
16. Lascarides Alex and Asher Nicholas, (1993). Temporal Relations,
Discourse Structure, and Commonsense Entailment. Linguistics and
Philosophy, 16, 437-493. Kluwer Academic Publishers.
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23. Wilson George, Mani Inderjeet, Sundheim Beth and Ferro Lisa
(2001). A Multilingual Approach to Annotating and Extracting
Temporal Information. Proceedings of the ACL 2001 Workshop
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24. Katz Graham and Arosio Fabrizio (2001). The Annotation of
Temporal Information in Natural Language Sentences. Proceedings of
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Processing, 104-111. Association for Computational Linguistics.
25. Filatove Elena and Hovy Eduard, (2001). Assigning Time-Stamps
to Event-Clauses. Proceedings of the ACL 2001 Workshop Temporal
and Spatial Information Processing, 88-95. Association for
26. Schilder Frank and Habel Christopher (2001). From Temporal
Expressions to Temporal Information: Semantic Tagging of News
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27. Pustejovsky James, Ingria Robert, Sauri Roser, Castano Jose,
Littman Jessica, Gaizauskas Rob, Setzer Andrea, Katz Graham, Mani
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| ABOUT THE REVIEWER:
ABOUT THE REVIEWER
Anil Kumar Singh is working towards PhD in Computational Linguistics
at the Language Technology Research Centre (LTRC), IIIT,
Hyderabad, India. His research interests are in dealing with temporal
information in NL, statistical NLP, corpus linguistics and, last but not
the least, NL engineering. But there is much more to life than research