TEMPORAL ONTOLOGY IN NATURAL LANGUAGE 
s Marc Moen-~ and Mark Steedman t 
Centre for Cognitive Science *t and Dept. of AT t, Univ. ~ Edinb-rgh, 
and Dept. of Computer and Information Science, Univ. of Pennsylvania t 
ABSTRACT 
A semantics of linguistic categories like tense, aspect, and cer- 
tain temporal adverbials, and a theory of their use in defining 
the temporal relations of events, both require a more complex 
structure on the domain underlying the meaning representa- 
tions than is commonly assumed. The paper proposes an 
ontology based on such notions as causation and consequence, 
rather than on purely temporal primitives. We claim that any 
manageable logic or other formal system for natural language 
temporal descriptions will have to embody such an ontology, 
as will any usable temporal database for knowledge about 
events which is to be interrogated using natural language. 
1. Introduction 
It has usually been assumed that the semantics of temporal 
expressions is directly related to the linear dimensional con- 
caption of time familiar from high-school physics - that is, to 
a model based on the number-line. However, there are good 
reasons for suspecting that such a conception is not the one 
that our linguistic categories are most directly related to. 
When-clauses provide an example of the mismatch between 
linguistic temporal categories and a semantics based on such 
an assumption. Consider the following examples: 
(1) When they built the 39th Street bridge... 
(a) ...a local architect drew up the plans. 
Co) ...they used the best materials. 
(c) ...they solved most of their traffic problems. 
To map the temporal relations expressed in these examples 
onto linear time, and to try to express the semantics of when in 
terms of points or intervals (poss~ly associated with events), 
would appear to imply either that when is multiply ambiguous, 
allowing these points or intervals to be temporally related in at 
least three different ways, or that the relation expressed 
between main and when-clauses is one of "approximate coin- 
cidence". However, neither of these tactics explains the pecu- 
liarity of utterances like the following: 
(2) #When my car broke down, the sun set. 
The oddity of this statement seems to arise because the when- 
clause predicates something more than mere temporal coin- 
cidence, that is, some contingent relation such as a causa/link 
between the two events. Of course, our knowledge of the 
world does not easily support such a link. This aspect of the 
sentence's meaning must stem from the sense-meaning of 
when, because parallel utterances using just after, at approxi. 
mate/y the same t/me as, and the like, which predicate purely 
temporal coincidence, are perfectly felicitous. 
We shall claim that the different temporal relations conveyed 
in examples (1)do not arise from any sense-ambiguity of 
when, or from any "fuzziness" in the relation that it expresses 
between the times refered to in the clauses it conjoins, but from 
the fact that the meaning of when is not primarily temporal at 
an. We shall argue that when has single sense-meaning 
reflecting its role of establishing a temporal focus. The 
apparent diversity of meanings arises from the nature of this 
referent and the organisation of events and states of affairs in 
episodic memory under a relation we shall call "contingency", 
a term related to such notions as causality, rather than temporal 
sequenfiality. This contingent, non-temporal relation also 
determines the ontology of the elementary propositions denot- 
ing events and states of wl~ch episodic memory is composed, 
and it is to these that we turn first. 
2. Temporal and Aspectual Categories 
Utterances of English sentences can, following Vendler, be 
classified into temporal/aspecmal types on the basis of the 
tenses, aspects and adverbials with which they can cooccur (cf. 
Dowry, 1979). This "espocmal type" refers to the relation to 
other happenings in the discourse that a speaker predicates of 
the particular happening that their utterance describes. Thus an 
utterance of Harry reached the top is usually typical of what 
we will call a "culmination" -- informally, an event which the 
speaker views as accompanied by a transition to a new state of 
the world. This new state we will refer to as the "consequent 
state" of the event. Harry hiccupped is not usually viewed by 
speakers as leading to any specific change of state. It typifies 
what we call "point" expressions, that is punctual events whose 
consequences are not at issue. 
Similarly, Harry climbed typifies what we will call for obvious 
reasons a "process": such utterances describe an event as 
extended in time but not characterised by any particular con- 
clusion or culmination. In contrast. Harry climbed to the top 
typically describes a state of affah~ that also extends in time 
but that does have a particular culmination associated with it at 
which a change of state takes place. We classify such an utter- 
ance as a "culminated process". Finally, Harry is at the top 
typically describes a state. 
Thus we can interpret Vendler as saying that a part of the 
meaning of any utterance of a sentence is one of a small 
number of temporal/aapectual profiles distinguished on a small 
number of dimensions. They can be summarized as in Figure 
1. 
It is important to be clear that this claim concerns sentences 
used in a conte~, sense-mearLings of sentences or verbs in iso- 
lation are usually compatible with several (or even all possible) 
Vendlerian profiles, as Dowry and Verkuyl have pointed out - 
hence the frequent use of the word "typically" above. The 
details of this taxonomy and the criteria according to which 
utterances can be categorised are therefore less important than 
1 Readen familiar with Vendler's work will realise that we 
have changed his terminology. We have done so both for notational 
convenience and to avoid the considerable confusion that has arisen 
concerning the precise meaning of the old terms. 
+conseq 
-conseq 
EVENTS STATES 
atomic extended 
Harry left early 
At six, John arrived 
Sandra hi.upped 
Paul winked 
Sue built a sandcastle 
The ice melted completely 
Max worked in the garden 
Alice played the piano 
John knows French 
He was in the kitchen 
Hg~el 
the observation that each pri~tive entity of a given type, such 
as the culmination-event of Har~'s reaching the top, carries 
intimations of other associated events and states, such as the 
process by which the culmination was achieved, and the conse- 
quent state that followed. What linguistic devices like tenses, 
aspects, and temporal/aspectual adverbials appear to do is to 
transform entities of one type into these other "contingently" 
related entities, or to turn them into composites with those 
related entities. 
The temporal/aspecmal ontology that underlies these 
phenomena can be defined in terms of the state-transition net- 
work shown in Figure 2. The semantics of tenses, as-pecmal 
auxiliaries and temporal adverbials is defined in terms of func- 
tions which map categories onto other categories, and having 
the important characteristic of "coez~ing" their argument to be 
of the appropriate type. Both the possibilities for coercing an 
input proposition, and the possibilities for the output category. 
are defined by the transition net. In addition, the felicity of a 
particular transition is conditional on support from knowledge 
and context. 
Consider, for example, the combination of a culminated pro- 
cess expression with a for-adverbial, as in 
(3) Sue played the sonata for a few minutes. 
A for.adverbial coerces its input to be of the process variety. 
According to the network in Figure 2, such a transition is feli- 
citoos if the culmination point associated with the event of 
p/ay/ng the sonata is "stripped off'. As a result, there is no 
implication in (3) that Suefln/shed playing the sonata. 
Another routz through the network is possible in order to 
account for examples with for-adverbials: the culminated pro- 
cess, like any other event, can be viewed as an unstructured 
"point". A transition to turn it into a process then results in an 
iteration of occurrences at which Sue plays the sonata. This 
route through the network seems to be ruled out for 
(3) because it finds no support in our knowledge about sonatas 
and about how long they typically last. It does result, however, 
in a likely interpretation for a sentence ~ke 
(4) Sue played the sonata for about eight hours. 
EVENTS J 
I 
atomic \[ extended \[ 
I I 
J +conseq. \[ ~ CULMINATION CULMINATED consequences 
II A ~ / PRoczss. t 1/ /f 
_.o.s.,.-/J J/ t" 
POINT .,~..,.___~....~ PROCESS 
~--eration} ~ (in progress) 
J J 
I " 
Figure 2 
Not all aspecmal/temporal adverbials expressing a time span 
have the same functional type. /n-adverbials, for example, 
coerce their input to be a culminated process expression. This 
means that combination with a culmination expression requires 
the transition to be made to the culminated process node. 
According to the aspe~mal network in Figure 2 this transition 
is a felicitous one ff the context allows a preparatmy process to 
be associated with the culmination, as in (5): 
(5) Laura reached the top in two hours. 
The in-adverbial then defines the length of this preparatory 
period. 
Since the arcs describe what the world has to be like for transi- 
tions m be made felicitously, it is obvious that there are expres- 
sions that will resist certain changes. For example, it will be 
hard to find a context in which an in-adverbial can be com- 
bined with a culmination expression Ytke Harry accideraally 
spilled his coffee, since it is hard to imagine a context in which 
a preparatory process can be associated with an involuntary 
act. 
A similar problem arises in connection with the following 
example: 
(6) John ran in a few minutes 
The process expression John ran has to be changed into a cul- 
minated process expression before combination with the in- 
adverbial is possible. One way in which the network in Figure 
2 will permit the change from a process to a culminated pro- 
cess is ff the context allows a culmination point m be associ- 
ated with the process itself. General world knowledge makes 
this rather hard for a sentence like John ran, except in the case 
where John habitually runs a particular distance, such as a 
measured mile. If the in-adverbial had conveyed a specific 
duration, such as in four minutes, then the analysis would make 
sense, as Dowty has poimed out. However, the unspecific in a 
few minutes continues to resist this interpretation. 
However, another route is also possible for (6): the process of 
John running can be made into an atomic point' and thence 
into a culmination in its own right. This culmination can then 
acquire a preparatory process of its own -- which we can think 
of as preparing to run -. to become the culminated process 
which the adverbial requires. This time, there is no conflict 
with the content of the adverbial, so this reading is the most 
accessible of the two. 
Progressive auxiliaries coerce their input to be a process 
expression. The result of the application is a progressive state, 
which describes the process as being in progress. This means 
that, when a culmination expression like reach the top is used 
with a progressive, a transition path has to be found from the 
culmination node to the process node. According to the transi- 
tion network, this involves first adding a preparatory process to 
the culmination, and then stripping off the culmination point. 
As a result, the progressive sentence only describes the 
preparation as ongoing and no longer asserts that the original 
culmination even occurred. There would be no contradiction 
in continuing 
(7) Harry was reaching the top 
asin 
(8) Harry was roaching the top but he slipped 
and fell before he got there. 
• As Moens & Steedman (1986) point out, the fact that accord- 
ing to the present theory, progressives coerce their input to be a 
process, so that any associated culmination is su'ipped away 
and no longer conuibutes to truth conditions, provides a reso- 
lution of the "imperfective paradox" (Dowry 1979), without 
appealing to theory-external consmaets like "inertia worlds". 
A porfect, as in 
(9) Harry has reached the top 
refers to the consequent state of the culminetion. It requires its 
input category to be either a culmination or a culminated pro- 
cess, and maps this expression into its consequent state. Infor- 
mal evidence that it does so can be obtained by noticing that 
perf~ts are infelicitous if the salient consequences are not in 
force. The most obvious of these consequences for (9) is that 
Hahn still be at the top, although as usual there are other possi- 
bilities. 
Since the transition network includes loops, it will allow us to 
define indefinitely complex temporal/aspectoal categories, like 
ti~ one evoked by the following sentence: 
(10) It took me two days to play the "Minute Waltz" 
in less than sixty seconds for more than an hour. 
The culminated process expression play the Minute Waltz can 
combine sn'aightforwardly with the in-adverbial, indicating 
how long it takes to reach the culmination point of finishing 
playing the Mimae Waltz. Combination with the for.adverbial 
requires this expression to be mined into a process - the most 
obvious route through the network being that through the point 
node. The resulting culminated process expression describes 
the iterated process of playing the Minute Waltz in less than 
$ix~ seconds as lasting for more than an hour. The expression 
it took me..., finally, is like an in-adverbial in that it is looking 
for a culminated process expression to combine with. It finds 
one in the expression to play the Minute Waltz in less than 
sixty seconds for more than an hour but combination is ham- 
pered by the fact that there is a conflict in the length of time the 
adverbials describe. In the case of (10), the whole culminated 
process is instead viewed as a culmination in its own right (via 
the path through the point node). Knowledge concerning such 
musical feats then supplies an appropriate preparatory process 
which we can think of as practicising. The adverbial/t took 
me two days then defines the temporal extent of this prepara- 
tory process needed to reach the point at which repeatedly 
playing that piece of music so fast for such a considerable 
length of time became a newly acquired skilL 
This basic framework thus allows for a unified semantics of a 
wide variety of aspectual adverbials, the progressive, the per- 
fect, and iterative expressions in English. It is also used to 
explain the effect of bare plurals and certain varieties of nega- 
tion on the overall temporal structure of discourse (Moens 
forthcoming). 
All of the permissible transitions between aspectual categories 
illustrated in Figure 2 appear to be related to a single elemen- 
tsry contingency-based event structure which we call a 
"nucleus". A nucleus is defined as a structure comprising a 
culmination, an associated preparatory process, and a conse- 
quentstate, hcanberepresented~.orially as in~gure3. 2 
preparatory process consequent state 
IIIIII/111111111111111111111111111111111111 
I 
culmination 
Figure 3 
Any or all of these elements may be compound: the prepara- 
tion may consist of a number of discrete steps, for example the 
stages of climbing, having lunch or whatever, leading to the 
culmination of reaching the top. The consequent state may 
also be compound. Most importantly, we shall see that it 
includes the further events, if any, that are in the same 
sequence of contingently related events as the culmination. 
Similarly, the culmination may itself be a complex event - 
such as the entire culminated process of climbing a mountain. 
(In this case, the associated preparatory process and conse- 
quant state will be quite different ones to those internal to the 
culminated process itself.) The device is intended to embody 
the proposal that when we hear about an event like climbing a 
moun~a/n in conjunction with some coercive aspectuzl 
category which forces it to undergo a transition, then the alter- 
natives that are available are: 
a) to decompose the core event into a nucleus and to make 
a transition to one of the componants, such as the 
prepuratory activity of climbing or to the consequent 
state of having climbed the mountain; or 
b) to treat the entire event as a culmination, to c(m~oose it 
into a nucleus with whatever preparation and conse- 
quences the context provides for the activity of climbing 
a mountain, and to make the transition to either one of 
those. 
We further claim that those are the on/y alternatives. 
The concept of a nucleus not only explains the Iransitions of 
Figure 2, but also provides an answer to the question raised in 
the introduction concerning the apparent vagaries in the mean- 
ing of when-clauses. 
3. When-clauses 
The aspects and temporal/aspecmal adverbials considered 
above all act to modify or change the aspecmal class of the 
core proposition, subject to the limits imposed by the network 
in Figure 2, which we claim is in turn determined by the organ- 
isation of episodic memory. However. tenses and certain 
other varieties of adverbial adjuncts have a rather different 
character. Tense is widely regarded as an anaphoric category. 
requiring a previously established temporal referent. The 
referent for a present tense is usually the time of speec.~ but 
the referent for a past tense must be explicitly established. 
Such a referent is usually established using a second type of 
"temporal" adverbial, such as once upon a time. attire o'clock 
last Saturday, while I was cleaning my teeth, or when I woke 
up this morning. 
Most accounts of the anaphoric nature of tense have invoked 
Reichenbach's (1947) trinity of underlying times, and his con- 
cept of the "positional" use of the reference time which he 
2 A similar ~ent structure is proposed by Passonneau (1987). 
called "R". Under these accounts (reviewed in Steedman, 
1982), the adjuncts establish a reference time to which the 
reference lime of a main clause and subsequent same-tensed 
clauses may attach or refer, in much the same way that various 
species of full noun phrases establish referants for pronouns. 
However, in one respect, the past tense does not behave like a 
pronotm. Use of a pronoun such as "she" does not change the 
referent to which a subsequent use of the same pronoun may 
refer, whereas using a past tense may. In the following exam- 
ple. the teml~al reference point for sentence (b) seems to 
have moved on from the time established by the adjunct in (a): 
(II) a. At exactly five o'clock, Harry walked in. 
b. He sat down. 
This fact has caused theorists such as Dowry (1986), Hinrichs 
(1984) and Partee (1984) to stipulate that the ref~,~,ce time 
autonomously advances during a narrative. However. such a 
stipulation (besides creating problems for the theory vis-~i-v/s 
those narratives where reference time seems not to advance) 
seems to be unnecessmT, since the amount by which it 
advances still has to be determined by context. The concept of 
a nucleus that was invoked above to explain the varieties of 
aspecmal categories offers us exactly what we need to explain 
both the fact of the advance and its extent. We simply need to 
assume that a main clause event such as Harry walked in is 
interpreted as an entire nucleus, complete with consequent 
state, for by definition the consequent state includes whatever 
other events were contingent upon Harry walking in, including 
whatever he did next. Provided that the context (or the 
heerer's assumptions about the world) suppolls the idea that a 
subsequent main clause identifies this next contingent event, 
then it will provide the temporal referent for that main clause. 
In its ability to refer to entities that have not been explicitly 
mentioned, but whose existence has merely been implied by 
the presence of an entity that has been mentioned, tense 
appears more like a definite NP like the mus/c in the following 
example than like a p~o,~oun, as Webber (1987) points out. 
(12) I went to a party last night. The music was wonderful. 
A similar move is all that is required to explain the puzzle con- 
cerning the apparent ambiguity of when-clauses with which the 
paper began. A when-clause behaves rather like one of those 
phrases that are used to explicitly change topic, like and your 
father in the following example from Isard, (1975): 
(13) And your father, how is he? 
A when-clause introduces a novel temporal referent into focus 
whose unique identifiability in the bearer's memory is simi- 
larly presupposed. However, again the focussed temporal 
referent is an entire nucleus, and again an event main clause 
can attach itself anywhere within this structure that world 
knowledge will allow. For example, consider the example (1) 
with which we began (repeated here): 
(14) When they built the 39th Street bridge... 
(a) ...a local architect drew up the plans. 
Co) ...they used the best materials. 
(c) ...they solved most of their traffic problems. 
Once the core event of the when-clause has been identified in 
memory, the hearer has the same two alternatives described 
before: either it is decomposed into a preparatory process, a 
culmination and a consequent state, or the entire event is 
ueated as itself the culmination of another nucleus. Either 
4 
way, once the nucleus is established, the reference time of the 
main clause has to be situated somewhere within it - the exact 
location being determined by knowledge of the entities 
involved and the episode in question. So in example (a) the 
entire culminated process of building the bridge becomes a 
culmination (via a path in Figure 2 which passes through the 
"point" node) which is associated in a nucleus with prepara- 
tions for, and consequences of, the entire business, as in Figure 
4: 
they prepare they have built 
to build the bridge 
IIIIIIIII/1111111 1 IIII///I//////////I 
I 
they build 
the bridge 
Figure 4 
The drawing up of the plans is then, for reasons to do with 
knowledge of the world, situated in the preparatory phase. 
In example (b), in contrast, the building of the bridge is 
decomposed into a quite different preparatory process of build- 
ing, a quite different culmination of completing the bridge end 
some consequences which we take to be also subtly distinct 
f~rom those in the previous case, as in Figure 5. The use of the 
best materials is then, as in (a), situated in the preparatory pro- 
cess - but it is a different one this time. 
they build they have completed 
the bridge 
I/I///I///I//I/////////I/////////// 
I 
they complete the 
bridge 
Figure S 
Example (c) is like (a) in giving rise to the nucleus in Figure 4, 
but pragmatics demands that the main clause be situated some- 
where in the consequent state of building the bridge. 
Thus, a main clause event can potentially be situated anywhere 
along this nucleus, subject to support f3"om knowledge about 
the precise events involved, But example (2) is still strange, 
because it is so hard to think of any relation that is supported in 
this way: 
(15) #When my car broke down, the sun set 
The when-clause defines a nucleus, consisting of whatever pro- 
cess we can think of as leading up to the car's break-down, the 
break-down itself and its possible or actual consequences. It is 
not clear where along this nucleus the culmination of the sun 
set could be situated: it is not easy to imagine that it is a func- 
tional part of the preparatory process typically associated with 
a break-down, and it is similarly hard to imagine that it can be 
a part of the consequent state, so under most imaginable cir- 
cumstances, the utterance remains bizarre. 
The constraints when places on possible inteqa~etations of the 
relation between subordinate and main clause are therefore 
quite strong. First, general and specific knowledge about the 
event described in the when-clause has to support the associa- 
tion of a complete nucleus with it. Secondly. world knowledge 
also has to support the contingency relation between the events 
in subordinate end main clause. As a result, many constructed 
examples sound strange or are considered to be infelicitous, 
because too much context has to be imported to make sense of 
them. 
In all of the cases discussed so far, the main clause has been an 
event of some variety. When the main clause is stative, as in 
the following examples, the effect is much the same. That is to 
say, the when-clause establishes a nucleus, end the stative is 
asserted or ,-,ehed wherever world knowledge permits within 
the nucleus. The only difference is that statives are by 
definition unbounded with respect to the reference time that 
they are predicated of, end outlast it. It follows that they can 
usually fit in almost anywhere, end therefore tend not to coerce 
the when-clause, or to induce the causal/contingent interpreta- 
lions that we claim characteriso the corresponding sentences 
with events as main clauses: 
(16) When they built that bridge 
..l was still a young lad. 
...my grandfather had been dead for several years. 
...my aunt was having an affair with the milkman. 
...my father used to play squash. 
However, world knowledge may on occasion constrain the 
relation of a stative main clause, and force it to attach to or 
describe a situation holding over either the preparatory process 
or the consequent state of the subordinate clause, as in the fol- 
lowing examples (cf. Smith 1983): 
(17) When Harry broke Sue's vase, 
...she was in a good mood. 
...she was in a bad mood. 
4. Towards a Formal Representation 
We have argued in this paper that a principled end unified 
semantics of natural language categories like tense, aspect and 
aspectual/temporal adverbials requires an ontology based on 
contingency rather than temporality. The notion of "nucleus" 
plays a crucial role in this ontology. The process of temporal 
reference involves reference to the appropriate part of a 
nucleus, where appropriateness is a function of the inherent 
meaning of the core expression, of the coercive nature of co- 
occurring linguistic expressions, end of particular end general 
knowledge about the area of discourse. 
The identification of the correct ontology is also a vital prelim- 
inary to the construction and management of temporal data- 
bases. Effective exchange of information between people and 
machines is easier if the data structures that are used to orgen- 
ise the information in the machine correspond in a natural way 
to the conceptual structures people use to organize the same 
information. In fact, the penalties for a bad fit between data- 
structures and human concepts are usually crippling for any 
attempt to provide natural language interfaces for data base 
systems. Information extracted from natural language text can 
only be stored to the extent that it fits the preconceived for- 
mats, usually resulting in loss of information. Conversely. 
such data structures cannot easily be queried using natural 
language if there is a bad fit between the conceptual structure 
implicit in the query and the conceptual structure of the data- 
base. 
The "contingency-based" ontology that we are advocating here 
has a number of implications for the construction and manage- 
ment of such temporal databases. Rather than a homogeneous 
5 
database of dated points or intervals, we should partition it into 
distinct sequences of causally or otlun~vise contingently related 
sequences of events which we will call "episodes", each lead- 
ing to the satisfaction of a particular goal or intention. This 
partition will quite incidentally define a partial temporal order- 
ing on the events, but the primary purpose of such sequences is 
more related to the notion of a plan of action or an explanation 
of an event's occurrence than to anything to do with time itself. 
It follows that only events that are continganfly related neces- 
sarily have well defined temporal relations in memory. 
A first atxempt to investigate this kind of system was reported 
by Steedman (1982), using a program that verified queries 
against a database structured according to some of the princi- 
ples outlined above. These principles can be described using 
Kowalski's event-calculus (Kowalski & Sergot 1986). In this 
f~amework, there are primitives called events, the occurrence 
of which usually implies the existence of periods of time over 
which states hold. In the terms of the present paper, these 
"events" are either "points" or "culminations" (depending on 
whether they are in fact associated with consequent states - 
see section 2). For example, in the world of academic promo- 
lions which Kowalski and Sea'got take as an example, an evant 
description like John was promoted from the rank of lecturer 
to the rank of professor is a culmination which implies that 
there was a period of time, ended by this event, during which 
John had the rank of lecturer, and there is a period of time, 
started by that same event, during which John had the rank of 
professor. 
The events in the event calculus are given unique identifiers, 
but are not necessarily associated with absolute time. More- 
over, they can be partially ordered with respect to each other, 
or occur simultaneously. Events themselves may also be 
described only partially;, later information can be added when 
it becomes available. These features, which they share with 
the corresponding primitives in a number of other formalisms, 
such as those of McDermott (1982), Allen (1984) and Lansky 
(1986), an constitute an advance over temporal representation 
formalisms based on the situation calculus (McCarthy & Hayes 
1969)o 
Although Kowalski's events are undecomposable points or 
culminations, they can be used to describe extended events 
such as our processes, in terms of a pair identifying their start- 
ing point and to the point at which they stop (in the case of 
processes) or their culmination (in the case of culminated 
processes). This means that a process expression like John ran 
will introduce two events, one indicating the start of the pro- 
cess and one indicating the endpoint. Just like the point events 
considered by Kowalski and Sergot, these events have certain 
properties or states associated with them. The starting-point of 
the process referred to by uttering John ran marks the begin- 
ning of a progressive state that we refer to when we use a pro- 
gressive like John is running, a state which is terminated by 
the corresponding endpoint event. 
This duality between events and states (which was also 
exploited in Steedman, 1982), is very useful for representing 
the kind of ontology that we have argued natural language 
categories reflect. But one shortcoming of Kowalski's event 
calculus is the absence of other than temporal relations 
between the events. The best worked out event-based model 
that takes into account causal as well as temporal relations is 
Lansky's (1986). The representation she presents is based on 
GEM (Lamsky & Owieki 1983), a tool for the specification and 
verification of concurrent programs. GEM retries events and 
explicitly represents both their causal and temporal relations. 
It also provides mechanisms for structuring events into so- 
called "locations of activity", the boundaries on these locations 
being boundaries of causal access - as in our episodes. In Lan- 
sky (1986), the GEM tool is used to build event-based 
knowledge representations for use in planners. She suggests 
the use of three accessibility relations: temporal precedence 
(<), causality or contingency (@). and simultaneity ($). 
These relations have the following properties: 
< : irreflexive, antisymmetric, transitive 
@ : irreflexive, antisymmetric, inlransitive 
$ : reflexive, symmetric, transitive 
Because we follow Lansky in making the 
causality/contingency relation @ intransitive, we avoid certain 
notorious problems in the treatment of when-clauses and per- 
fects, which arise because the search for possible consequences 
of an event has to be restricted to theftrs~ event on the chain of 
contingencies. Thus, when (18a) and (b) are asserted, it would 
be wrong to infer (c): 
(18) (a) When John left. Sue cried 
(b) When Sue cried, her mother got upset 
(c) When John left, Sue's mother got upset 
The reason is exactly the same as the reason that it would be 
wrong to infer that Sue's mother got upset because John left, 
and has nothing to do with the purely temporal relations of 
these events. It should also be noted that the notion of causal- 
ity or contingency used here (in line with Lansky's proposals) 
is weaker than that used in other representation schemes (for 
example that of McDermott 1982) in that causality is here 
decoupled from eventuality: if an event A stands in a causal 
relation to event B, then an occurrence of A will not automati- 
cally lead to an occurrence of B: John laying the foundations 
of the house is a prereclulSlto for or enables him to build the 
walls and roof but does not "cause" it in the more Iraditional 
sense of the word and doe~ not automatically or inevitably lead 
to him building the walls. 
5. Conclusion 
Many of the apparent anomalies and ambiguities that plague 
current semantic accotmts of temporal expressions in natural 
language stem from the assumption that a linear model of time 
is the one that our linguistic categories are most directly related 
to. A more principled semantics is possible on the assumption 
that the temporal categories of tense, aspect, aspecmal adverbi- 
als and of propositions themselves refer to a mental representa- 
tion of events that is smJcmred on other than purely temporal 
principles, and to which the notion of a nucleus or contingently 
related sequence of preparatory process, goal event and conse- 
quent state is central. 
We see this claim as a logical preliminary to the choice of any 
particular formal representation. However, certain properties 
of the event-based calculi of Kowalski and Sergot. and of Lan- 
sky. seem to offer an appropriate representation for a 
3 A Prolog program incorporating the above exmnsion to the 
event calculus is under construction and will be presented in Moens 
(forthcoming). 
semantics of this kind. 
ACKNOWLEDGEMENTS 
We thank Ethel Schuster and Bonnie Lynn Webber for reading 
and commenting upon drafts. Parts of the research were sup- 
ported by: an Edinburgh University Graduate Studentship; an 
ESPRIT grant (project 393) to CCS, Univ. Edinburgh; a Sloan 
Foundation grant to the Cognitive Science Program, Univ. 
Pennsylvania; and NSF grant IRI-10413 A02, ARC) grant 
DAA6-29- 84K-0061 and DARPA grant N0014-85-K0018 to 
CIS, Univ. Pennsylvania. 
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