<?xml version="1.0" standalone="yes"?>
<Paper uid="J88-2005">
  <Title>A COMPUTATIONAL MODEL OF THE SEMANTICS OF TENSE AND ASPECT</Title>
  <Section position="4" start_page="0" end_page="0" type="intro">
    <SectionTitle>
2.1 ACTUAL TEMPORAL REFERENCE
</SectionTitle>
    <Paragraph position="0"> Actual situations are those that are asserted to have already occurred, or to be occurring at the time when a text is produced. This excludes, e.g., situations mentioned in modal, intensional, negated, or frequentative contexts. 8 A predication denotes an actual situation when two criteria are satisfied. First, at least one of the verb's arguments must be interpreted as specific (Dowty 1979, Mourelatos 1981, Vlach 1981). For example, the simple past of fly denotes a specific situation in Sentence 1 but not in (2), because the subject of the verb in (2) is a nonspecific indefinite plural.</Paragraph>
    <Paragraph position="1">  1. John flew TWA to Boston.</Paragraph>
    <Paragraph position="2"> 2. Tourists flew TWA to Boston.</Paragraph>
    <Paragraph position="3">  This paper does not address the interaction of the nature of a verb's arguments with the specificity of references to situations.</Paragraph>
    <Paragraph position="4"> The second criterion is that the situation must be asserted to hold in the real world for some specific time. Predications in modal contexts (including the future; cf. Sentence 3) are excluded because their truth evaluation does not involve specific real-world times, but rather, hypothetical or potential times.</Paragraph>
    <Paragraph position="5"> 3. The oil pressure should/may/will decrease.</Paragraph>
    <Paragraph position="6"> Additionally, frequency adverbials like always may force a temporally nonspecific reading, as in (4). 4. John always flew his own plane to Boston.</Paragraph>
    <Paragraph position="7"> PUNDIT's time component does not currently identify modal contexts, frequency adverbials, or nonspecific verb arguments. However, it does identify predications denoting situation types when the form of the verb itself provides this information.</Paragraph>
    <Paragraph position="8"> In evaluating actual time, PUNDIT distinguishes between examples like (5) and (6) on the basis of the verb and its grammatical categories. An actual use of the sentence in (5), for example, would report that a particular pump participated in a particular event at a specific time.</Paragraph>
    <Paragraph position="9">  Sentences 6 and 7, on the other hand, report on types of recurrent events. In sentence 7, it is the adverb whenever that indicates that the main clause refers to a recurrent type of event rather than to a specific event token situated at a particular time. In (6), it is the lexical aspect of the verb seize in combination with the present tense that provides that information. A further difference between the two examples is that (7) entails that on at least one past occasion the pump actually seized when the engine jacked over, while (6) does not entail that the lube oil pump ever actually seized. We will see in Section 4.1 that (6) would immediately be determined not to evoke actual time on the basis of the lexical aspect of the verb and its inflectional form. Although PUNDIT does not yet handle frequency adverbials, Computational Linguistics, Volume 14, Number 2, June 1988 45 Rebecca J. Passonneau A Computational Model of the Semantics of Tense and Aspect Section 5 illustrates the procedure by which the main clause of (7) would be processed so that its relation to the subordinate clause event could be identified later. Lexical aspect is the inherent semantic content of a lexical item pertaining to the temporal structure of the situation it refers to, and thus plays a major role in computing temporal information. The aspectual categories and their relevance to temporal processing are discussed in Section 2.2.</Paragraph>
    <Paragraph position="10"> It should be noted that other semantic and pragmatic properties also affect temporal analysis. For example, there are conditions under which the present tense of a verb referring to an event, as in (6), is associated with an actual situation. Under the right conditions, first-person performatives (e.g., I warn you not to cross me) accomplish the named event at the moment they are uttered (Austin 1977). Even a sentence like (6) can refer to an actual event if interpreted as a report of a presently unfolding situation, as in a sportscast. Handling tense in these types of discourse would require representing pragmatic features, such as the speaker/addressee relationship, in order to handle the relation of indexicals like tense and person to the speech situation (Jakobson 1957). Section 3 briefly mentions some semantic distinctions pertaining to the verb in addition to lexical aspect, which PUNDIT does handle. Otherwise, however, this paper focuses on temporal analysis of third person descriptions containing verbs whose arguments refer to specific, concrete participants.</Paragraph>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
2.2 TEMPORAL STRUCTURE OF ACTUAL SITUATIONS
</SectionTitle>
      <Paragraph position="0"> Situations are classified on the basis of their temporal structure into three types: states, processes, and transition events. Each situation type has a distinct temporal structure comprised of one or more intervals. Two features are associated with each interval: kinesis and boundedness. Both terms will be defined more fully below, but briefly, kinesis pertains to the internal structure of an interval, or in informal terms, whether something is happening within the interval. Boundedness pertains to the way in which an interval is located in time with respect to other intervals, e.g., whether it is bounded by another interval.</Paragraph>
      <Paragraph position="1"> This approach to the compositional semantics of temporal reference is similar in spirit to interval semantics in the attempt to account for the semantic effects of aspectual class (Dowty 1986, Dowty 1982, Dowty 1979, Taylor 1977). However, interval semantics captures the distinct temporal properties of situations by specifying a truth-conditional relation between a full sentence and a unique interval. The goal of PUNDIT's temporal analysis is not simply to sort references to situations into states, processes, and events, but more specifically to represent the differences between the three situation types by considering in detail the characteristics of the set of temporal intervals that they hold or occur over (Allen 1984:132). Thus, instead of specifying a single set of entailments for each of the three situation types, the temporal semantics outlined here specifies what prop-erty of an interval is entailed by what portion of the input sentence, and then compositionally constructs a detailed representation of a state, ' process, or event from the intervals and their associated features. The critical difference from interval semantics is that while intervals are the fundamental unit from which situation representations are constructed, it is proposed here that intervals have properties that differentiate them from one another.</Paragraph>
      <Paragraph position="2">  The three situation types--states, processes, and transition events--are distinguished from one another entirely on the basis of the grammatically encoded means provided by the language for talking about how and when they occur. Peopl e certainly can and do conceptualize finer differences among real-world situations and can even describe these differences, given sufficient time or space. But certain gross distinctions are unavoidably made whenever people mention things happening in the world. Here and in the next section we will examine the temporal distinctions encoded in the form of the verb, often referred to as aspect, which are here referred to as temporal structure. Part of the temporal structure, that which Talmy (1985) described as the pattern of distribution of action through time, is represented in the time arguments for the three situation types. Another part of the temporal structure, its event time, is the component of temporal structure that gets located in time by tense and the perfect. All the relevant distinctions of temporal structure are represented in terms of intervals and moments of time.</Paragraph>
      <Paragraph position="3"> States. Very briefly, a state is a situation that holds over some interval of time, which is both stative and unbounded. A stative interval is one in which, with respect to the relevant predication, there is no change across the interval for which the situation holds. Thus stative intervals are defined here much as stative predications are defined in interval semantics: An interval I over which some predication q/ holds is stative iff it follows from the truth of 4, over I that q/ is true at all subintervals of I (Dowty 1986:42).</Paragraph>
      <Paragraph position="4"> Sentence 8 is an example of a typical stative predication whose verb phrase is headed by an adjective. During the interval for which the predicate low holds over the entity pressure, each subinterval is equivalent to any other subinterval with respect to the asserted situation; thus its kinesis is stative.</Paragraph>
      <Paragraph position="5"> 8. The pressure is low.</Paragraph>
      <Paragraph position="6"> Some of the diagnostic tests for stative predications are that they cannot be modified by rate adverbials (*The pressure was quickly low), nor referenced with do it anaphora (The pressure was very low. *The temperature also did it~that.) While inability to occur with the progressive suffix has often been cited as another diagnostic, it is a less reliable one. Dowty 1979 identifies a class of locative stative predications that occur in the 46 Computational Linguistics, Volume 14, Number 2, June 1988 Rebecca J. Passonneau A Computational Model of the Semantics of Tense and Aspect progressive (e.g., The socks are lying under the bed.) Predicates denoting cognition or behavior have often been classified as statives but may occur in the progressive with reference to a cognitive or behavioral process. 9 Although such verbs do not appear in the current domain, they would be treated differently from pure stative verbs.</Paragraph>
      <Paragraph position="7"> The intervals associated with states are also inherently unbounded, although a temporal bound could be provided by an appropriate temporal adverbial (e.g., The pressure was normal until the pump seized). ~deg When an unbounded interval is located with respect to another point in time, it is assumed to extend indefinitely in both directions around that time, as with the punctual adverbial in (9). The moment within the interval that is explicitly located by tense and the punctual adverbial is the situation's event time, depicted as a circle in the middle of the interval, with arrows representing that the interval extends indefinitely into the past and toward the present.</Paragraph>
      <Paragraph position="8"> 9. The pressure was low at 0800.</Paragraph>
      <Paragraph position="9"> Situation type: state Kinesis: stative Boundedness: unbounded This sentence would be true if the pressure were low for only an instant coincident with 0800, but it is not asserted to hold only for that instant; one thus assumes that it was low not only at the named time, but also prior and subsequent to it. In this sense, the interval is unbounded, as represented graphically above.</Paragraph>
      <Paragraph position="10"> Processes. A process is a situation which holds over an active interval of time. Active intervals contrast with stative intervals in that there is change within the interval, a useful distinction for interpreting manner adverbials indicating rate of change, e.g., slowly and rapidly. Since states denote the absence of change over time, they cannot be modified by rate adverbials; processes can be.</Paragraph>
      <Paragraph position="11"> The definition of active intervals is also adapted from the characterization of process predications in interval semantics: An interval I over which some predication qJ holds is active iff it follows from the truth of ~b at I that ~b is true over all subintervals of I down to a certain limit in size (Dowty 1986:42).</Paragraph>
      <Paragraph position="12"> Active intervals can be unbounded or unspecified for boundedness, depending on whether the verb is progressive. In (10), the active interval associated with the alarm sounding is unbounded and bears the same relationship to the named clock time as does the stative interval in (9) above.</Paragraph>
      <Paragraph position="13"> 10. The alarm was sounding at 0800.</Paragraph>
    </Section>
  </Section>
class="xml-element"></Paper>