Interpreting Tense, Aspect and Time Adverbials: A Compositional

27 Pages·2004·0.34 MB·English

Interpreting Tense(cid:0) Aspect and Time Adverbials(cid:1)
(cid:0)
A Compositional(cid:0) Uni(cid:2)ed Approach
Chung Hee Hwang (cid:0) Lenhart K(cid:1) Schubert
Dept(cid:1) of Computer Science(cid:2) University of Rochester
Rochester(cid:2) New York (cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:2) U(cid:1) S(cid:1) A(cid:1)
Abstract
We extend our theory of English tense(cid:0) aspect and time adverbials (cid:1)Hwang and
Schubert(cid:0) (cid:2)(cid:3)(cid:3)(cid:4)(cid:0) (cid:2)(cid:3)(cid:3)(cid:5)(cid:6) to deal with a wider range of time adverbials(cid:0) including many
adverbials offrequency(cid:0) cardinality(cid:0) duration(cid:0) andtimespan(cid:0) andadverbials oftempo(cid:7)
ral relation involving subordinating conjunctions such as after(cid:0) since(cid:0)and until(cid:8) Our
theory is fully formal in that it derives indexical (cid:9)quasi(cid:7)(cid:10)logical forms from syntactic(cid:7)
semantic rule pairs of a formal grammar(cid:0) and nonindexical logical forms via deindex(cid:7)
ing rules in the form of equivalences and equations(cid:8) The grammar allows for complex
sentences and the semantic rules and deindexing rules are easy to implement compu(cid:7)
tationally(cid:0) producing formulas in Episodic Logic(cid:8)
(cid:0) Introduction(cid:1) A Compositional Alternative
to Reichenbach
Researchers concerned withhigher(cid:0)leveldiscourse structure(cid:1) e(cid:2)g(cid:2)(cid:1)Webber(cid:3)(cid:4)(cid:5)(cid:6)(cid:6)(cid:7)(cid:1)
Passonneau(cid:3)(cid:4)(cid:5)(cid:6)(cid:6)(cid:7) and Song and Cohen(cid:3)(cid:4)(cid:5)(cid:5)(cid:4)(cid:7)(cid:1)have almost invariably relied on
some Reichenbach(cid:3)(cid:4)(cid:5)(cid:8)(cid:9)(cid:7)(cid:0)like conception of tense(cid:2) The syntactic part of this
conception is that there are nine tenses in English(cid:1) namelysimple past(cid:1) present
and future tense(cid:1) past(cid:1) present and future perfect tense(cid:1) and posterior past(cid:1)
present and future tense (cid:10)plus progressive variants(cid:11)(cid:2) The semantic part of the
conception is that each tense speci(cid:12)es temporal relations among exactly three
timesparticulartoatensedclause(cid:1)namelytheeventtime(cid:10)E(cid:11)(cid:1)thereference time
(cid:10)R(cid:11) and the speech time (cid:10)S(cid:11)(cid:2) On this conception(cid:1) informationin discourse is a
matterof(cid:13)extracting(cid:14)oneofthenineReichenbachiantensesfromeachsentence(cid:1)
assertingtheappropriaterelationsamongE(cid:1)RandS(cid:1)andappropriatelyrelating
thesetimestopreviouslyintroducedtimes(cid:1)takingaccountofdiscoursestructure
cues implicit in tense shifts(cid:2) While there is much that is right and insightful
aboutReichenbach(cid:15)sconception(cid:1)thelumpingtogether oftense andaspect isout
ofstep withmodernsyntaxandsemantics(cid:1)providingan unsatisfactorybasis for
a compositionalaccount of intra(cid:0) and intersentential temporal relations(cid:2)
(cid:0)
Portionsof this paper were presentedat theDARPA Workshopon Speechand Natural
Language(cid:0)(cid:1)(cid:2)(cid:2)(cid:3)(cid:0)the(cid:4)(cid:3)thAnnualMeetingoftheACL(cid:0)(cid:1)(cid:2)(cid:2)(cid:5)(cid:0)andtheARPAWorkshoponHu(cid:6)
manLanguageTechnology(cid:0)(cid:1)(cid:2)(cid:2)(cid:4)(cid:7) ThisresearchwassupportedinpartbyNSFResearchGrant
IRI(cid:6)(cid:2)(cid:3)(cid:1)(cid:4)(cid:1)(cid:8)(cid:3)andONR(cid:9)ARPAResearchContractsNo(cid:7)N(cid:3)(cid:3)(cid:3)(cid:1)(cid:10)(cid:6)(cid:11)(cid:5)(cid:6)K(cid:6)(cid:3)(cid:1)(cid:2)(cid:4)andNo(cid:7)N(cid:3)(cid:3)(cid:3)(cid:1)(cid:10)(cid:6)(cid:2)(cid:5)(cid:6)
J(cid:6)(cid:1)(cid:12)(cid:1)(cid:5)(cid:7)
In particular(cid:1) we think that the uniformuse ofE(cid:0)R(cid:0)S triples rests ona very
dubiousbasis(cid:16) (cid:12)rst(cid:1)appealismadetotheintuitionthatintensed perfects(cid:1) there
is an implicitreference timeinvolvedbesides the timeofspeech and the timeof
the described event(cid:2) Then(cid:1) this extra reference time is also imported into the
simple tenses(cid:1) even though for these there is no analogous intuition about the
presence ofsuch a reference time(cid:2) Then somesystematic roleis sought forthese
reference times(cid:1) and di(cid:17)erent researchers (cid:12)nd di(cid:17)erent uses for them(cid:2) Often(cid:1)
the (cid:13)reference time(cid:14)for a simple past sentence is claimed to be the time of the
event introduced by the previous sentence(cid:1) which intuitivelytends to be closely
alignedwith the new event time(cid:2) But this glosses over the fact thatpeople have
quite di(cid:17)erent intuitions about perfect reference times and these past reference
times(cid:2) Moreimportantly(cid:1)itglossesoverthefactthatthesame (cid:13)eventreference(cid:14)
relationsthatarefelttoexistintersententiallyforsimplepasts like(cid:13)Johnpicked
upthephone(cid:2) HecalledMary(cid:14)alsoexist forpastperfects like(cid:13)Johnhadpicked
up the phone(cid:2) He had called Mary(cid:2)(cid:14)In both cases(cid:1) the (cid:13)calling(cid:14)event is felt to
be right after the picking up of the phone(cid:2) But if the time of the (cid:13)previously
reported event(cid:14)istobetreated asa(cid:13)reference time(cid:14)insimplepasts(cid:1)itoughtto
be treated as a (cid:13)reference time(cid:14) in past perfects as well(cid:18)in other words(cid:1) past
perfectsshouldhavetwo reference times(cid:10)theperfectreference timeandprevious
event reference time(cid:11)besides the timeofspeech andevent time(cid:19) Sobythe same
reasoning(cid:1) should we then not have two extra reference times for simple tenses(cid:1)
as well(cid:20)
Wethinknot(cid:18)rather(cid:1)wethinkthatthepresence oftheextrareference time
in tensed perfects is due to the presence of the extra perf operator (cid:10)in addition
to the past operator(cid:11)(cid:2) More generally(cid:1) we contend that English past(cid:1) present(cid:1)
future and perfect are separate morphemes making separate contributions to
syntactic structure and meaning(cid:2) (cid:10)Note that perfect have(cid:1) like most verbs(cid:1) can
occur untensed(cid:21) e(cid:2)g(cid:2)(cid:1) (cid:13)She is likely to have left by now(cid:2)(cid:14)(cid:11) The corresponding
operators past(cid:0) pres(cid:0) futr(cid:0) and perf contribute separately and uniformly to the
meaningsof their operands(cid:1) i(cid:2)e(cid:2)(cid:1)formulasat the level of logicalform(cid:2) Thus(cid:1) for
instance(cid:1)thetemporalrelationsimplicitin(cid:13)Johnwillhaveleft(cid:14)areobtainednot
byextracting a (cid:13)future perfect(cid:14) and asserting relations amongE(cid:1) R and S(cid:1) but
ratherbysuccessivelytakingaccountofthemeaningsofthenestedpres(cid:0)futr and
perf operators in the logicalform of the sentence(cid:2) As it happens(cid:1) each of those
operators implicitlyintroduces exactly one episode(cid:1) yielding a Reichenbach(cid:0)like
result in this case(cid:2) (cid:10)But note(cid:16) a simple present sentence like (cid:13)John is tired(cid:14)
would introduce only one episode concurrent with the speech time(cid:1) not two(cid:1) as
inReichenbach(cid:15)sanalysis(cid:2)(cid:11) Evenmoreimportantlyforpresent purposes(cid:1) eachof
(cid:0)
pres(cid:0)past(cid:0) futr andperf istreated uniformlyindeindexingandcontextchange(cid:2)
Equallyimportantly(cid:1)theclausalstructure ofsentences (cid:10)ortheirlogicalforms(cid:11)
is not in general (cid:13)(cid:22)at(cid:1)(cid:14) with a single level of constituents and features(cid:1) but
may contain multiple levels of embedding(cid:2) This substructure can give rise to
arbitrarily complex relations among the contributions made by the parts(cid:1) such
(cid:0)Well(cid:0) almost uniformly(cid:13)we thinkthere are two variantsof perf in English(cid:7) There may
alsobegeneric variantsofpres andpast(cid:7)
astemporalanddiscourse relations amongsubordinate clausal constituents and
events or states of a(cid:17)airs they evoke(cid:2) It is therefore essential that these intra(cid:0)
sententialrelationsbesystematicallybroughttolightandintegratedwithlarger(cid:0)
scale discourse structures(cid:2) Consider(cid:1) for instance(cid:1) the followingpassage(cid:2)
S
(cid:9)(cid:2)(cid:10) John will (cid:11)nd this note when he gets home(cid:8) (cid:0)
(cid:9)(cid:4)(cid:10) He will think(cid:1)a(cid:2) Mary has left(cid:1)b(cid:2)(cid:8) Eb Rb Ra
Ea
Reichenbach(cid:15)s analysis of (cid:10)(cid:23)(cid:11) gives us Eb (cid:1) S(cid:0)Rb (cid:1) Ra(cid:0)Ea(cid:1) where t(cid:0) (cid:1) t(cid:1)
means t(cid:0) is before t(cid:1)(cid:1) as shown above(cid:2) That is(cid:1) John will think that Mary(cid:15)s
leaving took place some time before the speaker uttered sentence (cid:10)(cid:23)(cid:11)(cid:2) This is
incorrect(cid:21) it is not even likely that John would know about the utterance of
(cid:10)(cid:23)(cid:11)(cid:2) Inactuality(cid:1)(cid:10)(cid:23)(cid:11)onlyimpliesthatJohn willthinkMary(cid:15)sleavingtookplace
some time before the time of his thinking(cid:1) i(cid:2)e(cid:2)(cid:1) S (cid:1) Ra(cid:0)Ea and Eb (cid:1) Rb(cid:0)Ra(cid:2)
Reichenbach(cid:15)s system fails to take into account the local context created by
syntactic embedding(cid:2) Attempts have been made to re(cid:12)ne Reichenbach(cid:15)s theory
(cid:10)e(cid:2)g(cid:2)(cid:1)(cid:3)Hornstein(cid:1)(cid:4)(cid:5)(cid:9)(cid:9)(cid:21)Smith(cid:1)(cid:4)(cid:5)(cid:9)(cid:6)(cid:7)(cid:11)(cid:1)butwethinkthesehavegenerallynotgone
far enough in rebuilding the foundations(cid:2)
We have developed a uniform(cid:1) compositional approach to interpretation in
whichaparsetreeleadsdirectly(cid:10)inrule(cid:0)by(cid:0)rulefashion(cid:11)toapreliminary(cid:1)indexi(cid:1)
cal logicalform(cid:10)LF(cid:11)(cid:1)andthisLFisdeindexed byprocessingitinthecurrentcon(cid:1)
text (cid:10)a well(cid:0)de(cid:12)ned structure(cid:11)(cid:2) The relevant context structures are called tense
trees(cid:2) Deindexing simultaneously transforms the LF and the context(cid:16) context(cid:0)
dependent constituents of the LF(cid:1) such as operators past(cid:0) pres and perf and
adverbs liketoday orearlier(cid:1)are replaced byexplicitrelationsamongquanti(cid:12)ed
episodes(cid:21) (cid:10)anaphoraare alsodeindexed(cid:1) but this isnotdiscussed here(cid:11)(cid:21) andnew
structural components andepisode tokens (cid:10)and other information(cid:11)are added to
thecontext(cid:2) Thisdualtransformationisaccomplishedbysimplerecursiveequiv(cid:0)
alences andequalities(cid:2) More speci(cid:12)cally(cid:1)they drivethe generationandtraversal
of tense trees in deindexing(cid:2) Our treatment of various kinds of time adverbials
is fully compatibleand integrated with the treatment of tense and aspect(cid:2)
We describe tense trees insection (cid:23) and tense(cid:0)aspect deindexing rules insec(cid:0)
tion (cid:24)(cid:2) We then discuss our compositional approach to the interpretation of
temporal adverbials in section (cid:8)(cid:1) and an extension of our system that accom(cid:0)
modates aspectual class shifts and the interaction between multiple temporal
adverbials in section (cid:25)(cid:2) Concluding remarks are in section (cid:26)(cid:2)
(cid:2) Tense Trees
Tense trees provide that part of a discourse context structure which is needed
to interpret (cid:10)and deindex(cid:11) temporal operators and modi(cid:12)ers within the logical
formofEnglishsentences(cid:2) Theydi(cid:17)erfromsimplelistsofReichenbachianindices
in that they organize episode tokens (cid:10)for described episodes and the utterances
themselves(cid:11)inawaythatechoesthehierarchyoftemporalandmodaloperatorsof
thesentences andclausesfromwhichthetokensarose(cid:2) Tensetrees forsuccessive
sentences are (cid:13)overlaid(cid:14)in such a way that related episode tokens typicallyend
up as adjacent elements of lists at tree nodes(cid:2) For instance(cid:1) tense trees allow
the reference times(cid:27)episodes of the perfect to be automaticallyidenti(cid:12)ed(cid:2) The
traversal of trees and the additionof new tokens is simplyand fullydetermined
by the logicalformsof the sentences being interpreted(cid:2)
Themajoradvantageoftensetreesisthattheyallowsimple(cid:1)systematicinter(cid:0)
pretation(cid:10)bydeindexing(cid:11)oftense(cid:1)aspect(cid:1)andtimeadverbialsintextsconsisting
ofarbitrarilycomplexsentences(cid:1) andinvolvingimplicittemporalreferenceacross
clause and sentence boundaries(cid:2) This includes certain relations implicit in the
ordering of clauses and sentences(cid:2) As has been frequently observed(cid:1) for a se(cid:0)
quence of sentences within the same discourse segment(cid:1) the temporal reference
of a sentence is almostinvariablyconnected to that of the previous sentence in
some fashion(cid:2) Typically(cid:1) the relation is one of temporal precedence or concur(cid:0)
rency(cid:1) depending onthe aspectual class or aktionsart involved(cid:10)cf(cid:2)(cid:1)(cid:13)John closed
his suitcase(cid:21) He walked to the door(cid:14) versus (cid:13)John opened the door(cid:21) Mary was
sleeping(cid:14)(cid:11)(cid:2) However(cid:1) in (cid:13)Mary got in her Ferrari(cid:2) She bought it with her own
money(cid:1)(cid:14)the usualtemporalprecedence is reversed (cid:10)based on worldknowledge(cid:11)(cid:2)
Also(cid:1)other discourse relationscouldbe implied(cid:1)such ascause(cid:1)of(cid:0) explains(cid:0) elab(cid:1)
orates(cid:0) etc(cid:2) Whatever the relation may be(cid:1) (cid:12)nding the right pair of episodes
involved in such relations is of crucial importance for discourse understanding(cid:2)
Echoing Leech(cid:3)(cid:4)(cid:5)(cid:6)(cid:9)(cid:7)(cid:1) we use the predicate constant orients(cid:1) which subsumes
allsuch relations(cid:2) Notethattheorientspredicationscanlaterbe usedtomake
probabilisticordefaultinferencesaboutthetemporalorcausalrelationsbetween
the twoepisodes(cid:1) based ontheir aspectual class and other information(cid:2) We now
describe tense trees more precisely(cid:2)
The form of a tense tree is illustrated in Figure (cid:4)(cid:2) As an aid to intuition(cid:1)
the nodes in Figure (cid:4) are annotated with simple sentences whose indexical LFs
would lead to those nodes in the course of deindexing(cid:2) A tense tree node may
have up to three branches(cid:18)a leftward past branch(cid:1) a downward perfect branch(cid:1)
and a rightward future branch(cid:2) Each node contains a stack(cid:0)like list of recently
introduced episode tokens (cid:10)which we will often refer to simply as episodes(cid:11)(cid:2)
In addition to the three branches(cid:1) the tree may have (cid:10)horizontal(cid:11) embedding
links to the roots of embedded tense trees(cid:2) There are two kinds of these em(cid:0)
bedding links(cid:1) both illustrated in Figure (cid:4)(cid:2) One kind(cid:1) indicated by dashed
lines (cid:10)with the label mod(cid:0)sub(cid:11)(cid:1) is created by subordinating constructions such
as VPs with that(cid:0)complement clauses(cid:2) The other kind(cid:1) indicated by dotted
lines (cid:10)and labelled utt(cid:11)(cid:1) is derived from the surface speech act (cid:10)e(cid:2)g(cid:2)(cid:1) telling(cid:1)
(cid:1)
asking or requesting(cid:11) implicit in the mood of a sentence(cid:2) On our view(cid:1) the
utterances of a speaker (cid:10)or sentences of a text(cid:1) etc(cid:2)(cid:11) are ultimately to be rep(cid:0)
resented in terms of modal predications expressing these surface speech acts(cid:1)
such as (cid:3)Speakertell Hearer (cid:10)That (cid:28)(cid:11)(cid:7) or (cid:3)Speakerask Hearer(cid:10)Whether (cid:28)(cid:11)(cid:7)(cid:2)
Speaker and Hearer are indexical constants to be replaced by the speaker(cid:10)s(cid:11)
and the hearer(cid:10)s(cid:11) of the utterance context(cid:2) The two kinds of embedding links
(cid:3)
Thereisalsoathirdkindoflink(cid:14)labelledsub(cid:15)(cid:0)aswillbeshowninsection(cid:10)(cid:7)
utterance pres
node s(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)sHe is home
utt (cid:1)(cid:2)
(cid:1) (cid:2)
pas(cid:1)t perf (cid:2)futr
(cid:1) (cid:2)
mod(cid:7)sub
Heleft s(cid:1) s (cid:2)s(cid:0) (cid:0) (cid:0) (cid:0) (cid:0) (cid:0)s pres
(cid:2) He has left He will leave(cid:2)
(cid:2) (cid:2)
(cid:2) (cid:2)
perf futr perf futr
(cid:2) (cid:2)
s (cid:2)s s (cid:2)s
He had left He would He will She will think
leave have left that he will leave
perf
sg
He would have left
Figure (cid:4)(cid:2) A Tense Tree
require slightly di(cid:17)erent tree traversal techniques as willbe seen later(cid:2)
A set of trees connected by embedding links is called a tense tree structure
(cid:10)though we often refer loosely to tense tree structures as tense trees(cid:11)(cid:2) At any
time(cid:1) exactly one node of the tense tree structure for a discourse is in focus(cid:1)
and the focal node is indicated by sg(cid:2) Note that the (cid:13)tense tree(cid:14) in Figure (cid:4)
is in fact a tense tree structure(cid:1) with the lowest node in focus(cid:2) By default(cid:1) an
episode added to the right end of a list at a node is (cid:13)oriented(cid:14) by the episode
which was previously rightmost(cid:2) For episodes stored at di(cid:17)erent nodes(cid:1) we can
read o(cid:17) their temporal relations fromthe tree roughly as follows(cid:2) At any given
(cid:0) (cid:0)
moment(cid:1) for a pair of episodes e and e that are rightmost at nodes n and n(cid:1)
(cid:0)
respectively(cid:1) where n is adaughter ofn(cid:1)ifthe branch connecting the twonodes
(cid:0) (cid:2) (cid:0)
isapast branch(cid:1) (cid:3)e before e(cid:7) (cid:21) ifitis aperfect branch(cid:1) (cid:3)e impinges(cid:0)one(cid:7)(cid:10)as we
(cid:0) (cid:0)
explain later in sections (cid:24)(cid:1) this yields entailments (cid:3)e before e(cid:7) if e is bounded
(cid:0) (cid:0) (cid:3)
and (cid:3)e extends(cid:0)to e(cid:7) if e is unbounded(cid:1) respectively illustrated by (cid:13)John has
(cid:0)
left(cid:14) and (cid:13)John has been busy(cid:14)(cid:11)(cid:21) if it is a future branch(cid:1) (cid:3)e after e(cid:7)(cid:21) and if it
(cid:0)
is an embedding link(cid:1) (cid:3)e at(cid:0)about e(cid:7)(cid:2) These orienting relations and temporal
relationsarenotextracted post hoc(cid:1) butratherareautomaticallyasserted inthe
course of deindexing using the rules shown later(cid:2)
As a preliminary example(cid:1) consider the following passage and a tense tree
annotated with episodes derived fromit by our deindexing rules(cid:16)
(cid:4)Or(cid:0)sometimes(cid:0)same(cid:0)time (cid:14)cf(cid:7)(cid:0)(cid:16)JohnnoticedthatMarylooked pale(cid:17)vs(cid:1) (cid:16)Maryrealized
thatsomeonebroke her vase(cid:17)(cid:15)(cid:7) This is notdecidedin an ad hoc manner(cid:0)but as a resultof
systematicallyinterpretingthecontext(cid:6)chargedrelationbefT(cid:7) Moreonthislater(cid:7)
(cid:5)Technically(cid:0)boundedness is de(cid:18)nedforformulas(cid:0)ratherthanepisodes(cid:7) However(cid:0) we can
alsospeakofbounded(cid:14)orunbounded(cid:15)episodes(cid:0)namelythosewhosecharacterizingformulas
arebounded(cid:14)orunbounded(cid:15)(cid:7)
u(cid:0)(cid:0)u(cid:3) sg(cid:2)(cid:2)(cid:2)(cid:2)u(cid:2)t(cid:2)(cid:2)t(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s
(cid:1)
past
(cid:10)(cid:24)(cid:11) John picked up the phone(cid:2) epick(cid:0)ehads(cid:1)(cid:1)
(cid:10)(cid:8)(cid:11) He had told Mary that he would call her(cid:2)
perf
mod(cid:0)sub
etell s(cid:0) (cid:0) (cid:0) (cid:0)s
(cid:1)
past
ewd s(cid:1)
(cid:2)
futr
(cid:2)(cid:2)
s
ecall
u(cid:0) and u(cid:1) at the root node are utterance episodes for sentences (cid:10)(cid:24)(cid:11) and (cid:10)(cid:8)(cid:11)
respectively(cid:2) Intuitively(cid:1) the temporal content of sentence (cid:10)(cid:8)(cid:11) is that the event
of John(cid:15)s telling(cid:1) etell(cid:1) took place before some time ehad(cid:1) which is at the same
timeas the event of John(cid:15)s picking up the phone(cid:1) epick(cid:21) and the event ofJohn(cid:15)s
calling(cid:1)ecall(cid:1)islocatedafter sometimeewd(cid:1)whichistheatthesametimeasthe
(cid:10)past perfect(cid:11) event of John(cid:15)s telling(cid:1) etell(cid:2) For the most part(cid:1) this information
can be read o(cid:17) directly fromthe tree(cid:16) (cid:3)epick orients ehad(cid:7)(cid:1)(cid:3)etell before ehad(cid:7) and
(cid:3)ecall after ewd(cid:7)(cid:2) In addition(cid:1) the deindexing rules yield (cid:3)ewd same(cid:0)time etell(cid:7)(cid:2)
Fromthis(cid:1) one mayinfer (cid:3)etell before epick(cid:7) and (cid:3)ecall after etell(cid:7)(cid:1) assumingthat
the orientsrelation defaults to same(cid:1)time here(cid:2)
How does (cid:3)epick orients ehad(cid:7) default to (cid:3)epick same(cid:0)time ehad(cid:7)(cid:20) One of the
most important features of our account is that the tendency of past perfect
(cid:13)reference time(cid:14)toalignitselfwithapreviouslyintroduced pastevent isjust an
instance of a general tendency of atelic episodes to align themselves with their
orientingepisode(cid:2) Thisis the sametendency noted previously for(cid:13)Johnopened
the door(cid:2) Mary was sleeping(cid:2)(cid:14) In the present tense tree(cid:1) ehad is an episode
evoked bythe past tense operator whichis part of the meaningofhad in (cid:10)(cid:8)(cid:11)(cid:2) It
is an atelic episode(cid:1) since this past operator logically operates on a sentence of
form(cid:10)perf(cid:28)(cid:11)(cid:1)andsuchasentence describes astate inwhich(cid:28)hasoccurred(cid:18)in
this instance(cid:1) a state in which John has told Mary that he will call her(cid:2) It is
this atelicity of ehad which (cid:10)by default(cid:11) leads to a same(cid:1)time interpretation of
orients(cid:2)
We remarkedthat the relation(cid:3)ewd same(cid:0)timeetell(cid:7) isobtained directly from
the deindexing rules(cid:2) We leave it to the reader to verify this in detail (cid:10)see Past
and Futr rules stated in section (cid:24)(cid:11)(cid:2) We note only that ewd is evoked by the
past tense component of would in (cid:10)(cid:8)(cid:11)(cid:1) and denotes a (cid:10)possible(cid:11) state in which
Johnwill callMary(cid:2) Its atelicity(cid:1)and the factthat the subordinate clause in(cid:10)(cid:8)(cid:11)
(cid:4)
is (cid:13)past(cid:0)dominated(cid:1)(cid:14) causes (cid:3)ewd befT etell(cid:7) to be deindexed to (cid:3)ewd same(cid:0)time
etell(cid:7)(cid:2)
(cid:6)Anodeispast(cid:0)dominated ifthereisapast branchinitsancestry(cid:14)whereembeddinglinks
alsocountasancestrylinks(cid:15)(cid:7)
(cid:3) Deindexing with Tense Trees
We now discuss show how tense trees are modi(cid:12)ed as discourse is processed(cid:1) in
particular(cid:1)howepisode tokens are stored at appropriatenodes ofthe tense tree(cid:1)
and how context(cid:0)independent(cid:1) (cid:13)deindexed(cid:14) episodic logical forms (cid:10)ELFs(cid:11)(cid:1) with
orientsand temporal ordering relations incorporated into them(cid:1) are obtained(cid:2)
The processing of the (cid:10)indexical(cid:11) LF of a new utterance always begins with the
root node of the current tense tree (cid:10)structure(cid:11) in focus(cid:2) The processing of the
top(cid:0)level operator immediately pushes a token for the surface speech act onto
the episode list of the root node(cid:2) A typical indexical LF (cid:10)derivation of indexical
LFs is discussed in section (cid:8)(cid:11) looks like(cid:16)
(cid:10)decl (cid:10)past (cid:10)(cid:0) (cid:3)Mary answer(cid:7)(cid:11)(cid:11)(cid:11) (cid:13)Mary did not answer(cid:2)(cid:14)
(cid:10)decl standsfordeclarative(cid:21)itsdeindexingruleintroduces the surface speech act
of type (cid:13)tell(cid:14)(cid:11)(cid:2) As mentioned earlier(cid:1) our deindexing mechanism is a compo(cid:0)
sitional one in which operators past(cid:0) futr(cid:0) perf(cid:0) (cid:0)(cid:0) That(cid:0) decl(cid:0) etc(cid:2)(cid:1) contribute
separately to the meaning of their operands(cid:2) As the LF is recursively trans(cid:0)
formed(cid:1) the tense and aspect and modal operators encountered(cid:1) past(cid:0) perf and
futr(cid:1) in particular(cid:1) cause the focus to shift (cid:13)downward(cid:14) along existing branches
(cid:10)or new ones if necessary(cid:11)(cid:2) That is(cid:1) processing a past operator shifts the cur(cid:0)
rent focus down to the left(cid:1) creating a new branch if necessary(cid:2) The resulting
tense tree is symbolized as (cid:0)T(cid:2) Similarly perf shifts straight down(cid:1) and futr
shifts down to the right(cid:1) with respective results (cid:1)T and (cid:2)T(cid:2) pres maintainsthe
current focus(cid:2) Certain operators embed new trees at the current node(cid:1) written
(cid:1)(cid:2)T (cid:10)e(cid:2)g(cid:2)(cid:1) That(cid:11)(cid:1) or shift focus to an existing embedded tree(cid:1) written (cid:2)(cid:2)T (cid:10)e(cid:2)g(cid:2)(cid:1)
decl(cid:11)(cid:2) Focus shifts to a parent or embedding node are symbolized as (cid:3)T and
(cid:4)T respectively(cid:2) As a (cid:12)nal tree operation(cid:1) dT denotes storage of episode token
eT (cid:10)a new episode symbol not yet used in T(cid:11) at the current focus(cid:1) as rightmost
elementofitsepisodelist(cid:2) Aseachnodecomesintofocus(cid:1)itsepisodelistandthe
lists at certain nodes on the same tree path provide explicit reference episodes
intermsofwhichpast(cid:0) pres(cid:0) futr(cid:0) perf(cid:0)timeadverbials(cid:1)and implicit(cid:13)orienting(cid:14)
relations are rewritten nonindexically(cid:2) Eventually the focus returns to the root(cid:1)
and at this point(cid:1) we have a deindexed ELF(cid:1) as well as a modi(cid:12)edtense tree(cid:2)
Before we proceed with deindexing rules(cid:1) we need to mention some basic
features of EL(cid:1) our semantic representation(cid:2) In EL we take it that utterances
characterize situations or episodes(cid:1) and central to EL are the two episodic op(cid:0)
erators (cid:13)(cid:29)(cid:14) and (cid:13)(cid:29)(cid:29)(cid:14)(cid:2) Roughly(cid:1) (cid:3)(cid:28) (cid:29) (cid:3)(cid:7) means that (cid:28) is true in episode (cid:3) (cid:10)or(cid:1)
(cid:28) describes (cid:3)(cid:11)(cid:1) and (cid:3)(cid:28) (cid:29)(cid:29) (cid:3)(cid:7) means that (cid:28)(cid:1) and only (cid:28)(cid:1) is true in episode (cid:3)
(cid:5)
(cid:10)or(cid:1) (cid:28) characterizes (cid:3)(cid:11)(cid:2) As mentioned(cid:1) each of the deindexing rules for tense(cid:0)
aspect operators introduces an episode intothe logicalformandpredicates that
the episode is characterized (cid:10)(cid:13)(cid:29)(cid:29)(cid:14)(cid:11) by the operand (cid:10)after recursive deindexing(cid:11)(cid:2)
(cid:10)Use of (cid:13)(cid:29)(cid:14) will be seen later when deindexing of adverbials is discussed(cid:2)(cid:11) The
(cid:7)
Like(cid:16)situations(cid:17)(cid:0)(cid:16)episodes(cid:17)isagenerictermthatmaystandforevents(cid:0)states(cid:0)processes(cid:0)
eventualities(cid:0)etc(cid:7) Operatorssimilartoourepisodiconesarethe(cid:16)support(cid:17)relation(cid:14)(cid:0)j(cid:19)(cid:20)(cid:15)
insituationsemantics(cid:21)Barwise(cid:0)(cid:1)(cid:2)(cid:11)(cid:2)(cid:22)andtheeventuality(cid:16)type(cid:17)condition(cid:14)(cid:0)(cid:23) (cid:20) (cid:15) inDRT
(cid:21)KampandReyle(cid:0)(cid:1)(cid:2)(cid:2)(cid:4)(cid:22)(cid:7)
square(cid:0)bracketed(cid:1) in(cid:12)xedformisthe preferred sentence syntaxinEL(cid:2)Ingeneral(cid:1)
(cid:3)(cid:4)n (cid:5) (cid:4)(cid:0)(cid:6)(cid:6)(cid:6)(cid:4)n(cid:1)(cid:0)(cid:7) is an equivalent way of writing (cid:10)(cid:5) (cid:4)(cid:0)(cid:6)(cid:6)(cid:6)(cid:4)n(cid:11)(cid:1) which is in turn
equivalent to (cid:10)(cid:6)(cid:6)(cid:6)(cid:10)(cid:10)(cid:5) (cid:4)(cid:0)(cid:11)(cid:4)(cid:1)(cid:11)(cid:6)(cid:6)(cid:6)(cid:4)n(cid:11)(cid:2) Also used in EL are restricted quanti(cid:12)ers of
form(cid:10)Q(cid:1)(cid:16)(cid:28)(cid:30)(cid:11)(cid:1)whereQisaquanti(cid:12)er(cid:1)(cid:1)isavariable(cid:1)andrestriction(cid:28)andma(cid:0)
trix(cid:30)areformulas(cid:2) FordetailsofsyntaxandsemanticsofEL(cid:1)see (cid:3)Hwang(cid:1)(cid:4)(cid:5)(cid:5)(cid:23)(cid:21)
(cid:6)
Hwang and Schubert(cid:1) (cid:4)(cid:5)(cid:5)(cid:24)(cid:7)(cid:2) We now show some of the basic deindexing rules(cid:2)
Decl(cid:1) (cid:9)decl (cid:12)(cid:10)T (cid:0) (cid:9)(cid:1)eT(cid:13)(cid:1)(cid:1)eT same(cid:7)time NowT(cid:6) (cid:2)(cid:1)LastT immediately(cid:7)precedes eT(cid:6)(cid:6)
(cid:1)(cid:1)Speakertell Hearer(cid:9)That (cid:12)(cid:0)(cid:1) dT(cid:10)(cid:6)(cid:14)(cid:14) eT(cid:6)(cid:10)
(cid:0) (cid:0)
Tree transformation(cid:13) (cid:9)decl (cid:12)(cid:10) T (cid:15) (cid:4) (cid:9)(cid:12) (cid:9)(cid:2)(cid:3) dT(cid:10)(cid:10)
Pres(cid:1) (cid:9)pres (cid:12)(cid:10)T (cid:0) (cid:9)(cid:1)eT(cid:13)(cid:1)(cid:1)eT at(cid:7)about EmbT(cid:6)(cid:2) (cid:1)LastTorients eT(cid:6)(cid:6) (cid:1)(cid:12)dT (cid:14)(cid:14) eT(cid:6)(cid:10)
(cid:0) (cid:0)
Tree transformation(cid:13) (cid:9)pres (cid:12)(cid:10) T (cid:15)(cid:9)(cid:12) (cid:9)dT(cid:10)(cid:10)
Past(cid:1) (cid:9)past (cid:12)(cid:10)T (cid:0) (cid:1)eT(cid:13)(cid:1)(cid:1)eT befT EmbT(cid:6)(cid:2) (cid:1)Last(cid:2)T orients eT(cid:6)(cid:6) (cid:1)(cid:12)c(cid:0)T(cid:14)(cid:14) eT(cid:6)(cid:10)
(cid:0) (cid:0)
Tree transformation(cid:13) (cid:9)past (cid:12)(cid:10) T (cid:15)(cid:2)(cid:9)(cid:12) (cid:9)d(cid:0)T(cid:10)(cid:10)
Futr(cid:1) (cid:9)futr(cid:12)(cid:10)T (cid:0) (cid:9)(cid:1)eT(cid:13)(cid:1)(cid:1)eT after EmbT(cid:6) (cid:2)(cid:1)Last(cid:3)T orients eT(cid:6)(cid:6) (cid:1)(cid:12)c(cid:1)T (cid:14)(cid:14) eT(cid:6)(cid:10)
(cid:0) (cid:0)
Tree transformation(cid:13) (cid:9)futr (cid:12)(cid:10) T (cid:15) (cid:2)(cid:9)(cid:12) (cid:9)d(cid:2)T(cid:10)(cid:10)
Perf(cid:1) (cid:9)perf (cid:12)(cid:10)T (cid:0) (cid:9)(cid:1)eT(cid:13)(cid:1)(cid:1)eT impinges(cid:7)on LastT(cid:6)(cid:2) (cid:1)Last(cid:4)Torients eT(cid:6)(cid:6) (cid:1)(cid:12)c(cid:2)T(cid:14)(cid:14) eT(cid:6)(cid:10)
(cid:0) (cid:0)
Tree transformation(cid:13) (cid:9)perf (cid:12)(cid:10) T (cid:15) (cid:2)(cid:9)(cid:12) (cid:9)d(cid:1)T(cid:10)(cid:10)
That(cid:1) (cid:9)That (cid:12)(cid:10)T (cid:0) (cid:9)That (cid:12)(cid:5)(cid:1)T(cid:10)
(cid:0) (cid:0)
Tree transformation(cid:13) (cid:9)That (cid:12)(cid:10) T (cid:15) (cid:4) (cid:9)(cid:12) (cid:9)(cid:4)(cid:3)T(cid:10)(cid:10)
Asmentionedearlier(cid:1)SpeakerandHearerintheDecl(cid:0)rulearetobereplaced
bythe speaker(cid:10)s(cid:11) andthe hearer(cid:10)s(cid:11) oftheutterance(cid:2) Note thateachequivalence
pushes the dependence on context one level deeper into the LF(cid:1) thus deindexing
the top(cid:0)level operator(cid:2) The symbols NowT(cid:1) LastT and EmbT refer respectively
to the speech time for the most recent utterance in T(cid:1) the last(cid:0)stored episode
at the current focal node(cid:1) and the last(cid:0)stored episode at the current embedding
node(cid:2) When nosuch stored episodes existforLastT(cid:1)certainother episodes may
be substituted for LastT(cid:21) and within certain subtrees(cid:1) EmbT is interpreted as
the embedding node of the (cid:13)superordinate tree(cid:14) (cid:10)see section (cid:8)(cid:2)(cid:24)(cid:11)(cid:2) As already
mentioned(cid:1)befT inthe Past(cid:0)rulewillbereplaced byeither before orsame(cid:1)time(cid:1)
depending on the aspectual class of its (cid:12)rst argument and on whether the focal
node of T is past(cid:0)dominated(cid:2) In the Perf(cid:0)rule(cid:1) LastT becomes the analogue
of the Reichenbachian reference time for the perfect(cid:2) The impinges(cid:1)on relation
con(cid:12)nes its (cid:12)rst argumenteT (cid:10)the situationor event described by the sentential
operand of perf(cid:11) to the temporal region preceding the second argument(cid:2) As in
the case of orients(cid:1) its more speci(cid:12)c import depends on the aspectual types
of its arguments(cid:2) If eT is a state or process(cid:1) impinges(cid:1)on implicates that it
persists to the reference time(cid:27)episode(cid:1) i(cid:2)e(cid:2)(cid:1) (cid:3)eT extends(cid:0)to LastT(cid:7)(cid:2) If eT is an
event (cid:10)e(cid:2)g(cid:2)(cid:1) an accomplishment(cid:11)(cid:1)impinges(cid:1)on entails that it occurred sometime
(cid:8)See (cid:21)Hwang(cid:0) (cid:1)(cid:2)(cid:2)(cid:5)(cid:22) for the rest of our deindexingrules(cid:7) Some of the omittedones are(cid:23)
Fpres(cid:14)(cid:16)futuralpresent(cid:0)(cid:17)as in (cid:16)Johnhas a meetingtomorrow(cid:17)(cid:15)(cid:0)Prog (cid:14)progressiveaspect(cid:15)(cid:0)
Pred(cid:14)predication(cid:15)(cid:0)K(cid:0)K(cid:0)(cid:0)KaandKe(cid:14)(cid:16)kinds(cid:17)(cid:15)(cid:0)thosefordeindexingvariousoperatorssuchas
negation(cid:0)etc(cid:7) Deindexingrulesforadverbialsareinsection(cid:10)(cid:7)
beforethe reference time(cid:27)episode(cid:1)i(cid:2)e(cid:2)(cid:1)(cid:3)eT before LastT(cid:7)(cid:1)andimplicatesthatits
maine(cid:17)ects persist to the reference time(cid:2)
Toseethedeindexingmechanismatwork(cid:1)letusconsidersentences(cid:10)(cid:24)(cid:11)and(cid:10)(cid:8)(cid:11)
again(cid:2) TheLFsbeforedeindexingareshownin(cid:10)(cid:24)a(cid:1)(cid:8)a(cid:11)below(cid:10)where thelabelled
arrows mark points we will refer to(cid:11)(cid:21) the (cid:12)nal(cid:1) context(cid:0)independent ELFs are in
(cid:10)(cid:24)b(cid:1)(cid:8)b(cid:11)(cid:2) Thetransformationfrom(cid:10)a(cid:11)(cid:15)sto(cid:10)b(cid:11)(cid:15)sandthecorrespondingtense tree
transformations are done with the deindexing rules shown earlier(cid:2) Anaphoric
processingispresupposed here(cid:2) The snapshotsofthe tense tree whileprocessing
(cid:10)(cid:24)a(cid:11) and (cid:10)(cid:8)a(cid:11) with a null initial context(cid:1) at points (cid:3)a(cid:31)(cid:3)g(cid:1) are shown below the
formulas(cid:2)
(cid:9)(cid:5)(cid:10)John picked up the phone(cid:8)
a(cid:8) (cid:9)decl (cid:9)past (cid:1)John pick(cid:7)up Phone(cid:6)(cid:10)(cid:10)
(cid:6)a (cid:6)b (cid:6)c
b(cid:8) (cid:9)(cid:1) u(cid:2)(cid:13)(cid:1)u(cid:2) same(cid:7)time Now(cid:1)(cid:6)
(cid:1)(cid:1)Speaker tell Hearer (cid:9)That
(cid:9)(cid:1) e(cid:2)(cid:13)(cid:1)e(cid:2) before u(cid:2)(cid:6) (cid:1)(cid:1)John pick(cid:7)up Phone(cid:6) (cid:14)(cid:14) e(cid:2)(cid:6)(cid:10)(cid:10)(cid:6)
(cid:14)(cid:14) u(cid:2)(cid:6)(cid:10)
(cid:9)(cid:16)(cid:10)He had told Mary that he would call her(cid:8)
a(cid:8) (cid:9)decl (cid:9)past (cid:9)perf (cid:1)John tell Mary (cid:9)That (cid:9)past (cid:9)futr (cid:1)John call Mary(cid:6)(cid:10)(cid:10)(cid:10)(cid:6)(cid:10)(cid:10)(cid:10)
(cid:6)d (cid:6)e (cid:6)f (cid:6)g
b(cid:8) (cid:9)(cid:1) u(cid:4)(cid:13)(cid:1)(cid:1)u(cid:4)same(cid:7)time Now(cid:2)(cid:6)(cid:2)(cid:1)u(cid:2) immediately(cid:7)precedes u(cid:4)(cid:6)(cid:6)
(cid:1)(cid:1)Speaker tell Hearer (cid:9)That
(cid:9)(cid:1) e(cid:4)(cid:13)(cid:1)(cid:1)e(cid:4)before u(cid:4)(cid:6) (cid:2) (cid:1)e(cid:2)orients e(cid:4)(cid:6)(cid:6)
(cid:1)(cid:9)(cid:1)e(cid:5)(cid:13)(cid:1)e(cid:5) impinges(cid:7)on e(cid:4)(cid:6)
(cid:1)(cid:1)John tell Mary (cid:9)That
(cid:9)(cid:1) e(cid:16)(cid:13)(cid:1)e(cid:16) same(cid:7)time e(cid:5)(cid:6)
(cid:1)(cid:9)(cid:1) e(cid:17)(cid:13)(cid:1)e(cid:17) aftere(cid:16)(cid:6) (cid:1)(cid:1)John call Mary(cid:6) (cid:14)(cid:14) e(cid:17)(cid:6)(cid:10)
(cid:14)(cid:14) e(cid:16)(cid:6)(cid:10)(cid:10)(cid:6)
(cid:14)(cid:14) e(cid:5)(cid:6)(cid:10)
(cid:14)(cid:14) e(cid:4)(cid:6)(cid:10)(cid:10)(cid:6)
(cid:14)(cid:14) u(cid:4)(cid:6)(cid:10)
at a at b at c at d at e at f at g
u(cid:0) u(cid:0) u(cid:0) u(cid:0)(cid:0)u(cid:3) u(cid:0)(cid:0)u(cid:3) u(cid:0)(cid:0)u(cid:3) u(cid:0)(cid:0)u(cid:3)
s(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)sg s(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s sg(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s s(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s s(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s s(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s sg(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)(cid:2)s
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
sg(cid:1)e(cid:0) s(cid:1)e(cid:0) s(cid:1)e(cid:0)(cid:0)e(cid:3) s(cid:1)e(cid:0)(cid:0)e(cid:3) (cid:1)se(cid:0)(cid:0)e(cid:3) (cid:1)se(cid:0)(cid:0)e(cid:3)
e(cid:4) sg e(cid:4) s(cid:0) (cid:0) (cid:0) (cid:0)sg e(cid:4) s(cid:0) (cid:0) (cid:0) s(cid:0) e(cid:4) s(cid:0) (cid:0) (cid:0) s(cid:0)
(cid:1) (cid:1)
s(cid:1)e(cid:5) s(cid:1)e(cid:5)
(cid:2) (cid:2)
(cid:2)sge(cid:6) (cid:2)se(cid:6)
What is important here is(cid:1) (cid:12)rst(cid:1) that Reichenbach(cid:0)like relations are introduced
compositionally(cid:2) In addition(cid:1)the recursive rules take correct account of embed(cid:0)
ding(cid:2) For instance(cid:1) the embedded (cid:13)past(cid:0)future(cid:14) in (cid:10)(cid:8)(cid:11) is correctly interpreted
as future relativized to John(cid:15)s (cid:10)past(cid:11) telling time(cid:2) But beyond that(cid:1) episodes
evoked by successive sentences(cid:1) or by embedded clauses within the same sen(cid:0)
tence(cid:1) are correctly connected to each other(cid:2) In particular(cid:1) note that the orient(cid:0)
ingrelation between John(cid:15)s picking up the phone(cid:1) e(cid:4)(cid:1) and the reference timee(cid:23)
for the telling event is automatically incorporated into the deindexed formula
(cid:10)(cid:8)b(cid:11)(cid:2) Thus we have established inter(cid:0)clausal connections automatically(cid:1) which
in other approaches require heuristic discourse processing(cid:2) This was a primary
motivationfor tense trees(cid:2)
The orients relation is essentially an indicator that there could be a more
speci(cid:12)c discourse relationbetween the argumentepisodes(cid:2) As mentioned(cid:1)itcan
usually be particularized to one or more temporal(cid:1) causal(cid:1) or other (cid:13)standard(cid:14)
discourse relations(cid:2) Existingproposalsforgetting these discourse relationsright
appear to be of two kinds(cid:2) The (cid:12)rst uses the aspectual classes of the predicates
involvedto decide on discourse relations(cid:1)especially temporalones(cid:1) e(cid:2)g(cid:2)(cid:1)(cid:3)Partee(cid:1)
(cid:4)(cid:5)(cid:6)(cid:8)(cid:21) Dowty(cid:1) (cid:4)(cid:5)(cid:6)(cid:26)(cid:21) Hinrichs(cid:1) (cid:4)(cid:5)(cid:6)(cid:26)(cid:7)(cid:2) The second approach emphasizes inference
based on world knowledge(cid:1) e(cid:2)g(cid:2)(cid:1) (cid:3)Lascarides and Asher(cid:1) (cid:4)(cid:5)(cid:5)(cid:24)(cid:7)(cid:2) Our approach
fullycombines the use of aspectual class informationand world knowledge(cid:2) For
example(cid:1)in (cid:13)Mary got in her Ferrari(cid:2) She bought it with her own money(cid:1)(cid:14)the
successivelyreported(cid:13)achievements(cid:14)arebydefaultinchronologicalorder(cid:2) Here(cid:1)
however(cid:1) this default interpretation of orientsis reversed by world knowledge(cid:16)
one owns things after buying them(cid:1) rather than before(cid:2) But sometimes world
knowledge is mute on the connection(cid:2) For instance(cid:1) in (cid:13)John raised his arm(cid:2)
A great gust of wind shook the trees(cid:1)(cid:14) there seems to be no world knowledge
supporting temporal adjacency or a causal connection(cid:2) Yet we tend to infer
both(cid:1)perhaps attributingmagicalpowers to John (cid:10)precisely because ofthe lack
of support for a causal connection by world knowledge(cid:11)(cid:2) So in this case default
conclusionsbasedonorientsseemdecisive(cid:2) Inparticular(cid:1)wewouldassumethat
(cid:0)
ifeande areachievementsoraccomplishments(cid:1)where eistheperformanceofa
(cid:0) (cid:0) (cid:0)
volitionalaction and e is not(cid:1) then (cid:3)e orients e(cid:7) suggests (cid:3)e right(cid:0)before e(cid:7) and
(cid:0) (cid:7)
(cid:10)less (cid:12)rmly(cid:11) (cid:3)e cause(cid:0)of e(cid:7)(cid:2)
Thetense treemechanism(cid:1)andparticularlythewayinwhichitautomatically
supplies orienting relations(cid:1) is well suited for longer narratives(cid:1) including ones
with tense shifts(cid:2) For example(cid:1) in (cid:10)(cid:25)(cid:11) below (cid:10)from (cid:3)Allen(cid:1) (cid:4)(cid:5)(cid:6)(cid:9)(cid:7) with slight
simpli(cid:12)cation(cid:11)(cid:1)even though fb(cid:31)dg would normallybe considered a subsegment
of the main discourse fa(cid:1)eg(cid:1) both the temporal relations within each segment
and the relations between segments (cid:10)i(cid:2)e(cid:2)(cid:1) that the substory temporallyprecedes
themainone(cid:11)are automaticallycaptured byourrules(cid:2) Forinstance(cid:1) e(cid:0)(cid:0)e(cid:0)(cid:8) and
e(cid:0)(cid:0)arerecognizedassuccessive episodes(cid:1)bothpreceded atsometimeinthepast
by e(cid:2)(cid:0)e(cid:4)(cid:0)e(cid:6)(cid:1) and e(cid:9)(cid:1) in that order(cid:2)
(cid:9)(cid:17)(cid:10) a(cid:8) Jack and Sue wentfe(cid:0)g to a hardware store
b(cid:8) as someone hadfe(cid:1)g stolenfe(cid:2)g their lawnmower(cid:8)
c(cid:8) Sue hadfe(cid:3)g seenfe(cid:4)g a man take it and hadfe(cid:5)g chasedfe(cid:6)g him down the street(cid:0)
d(cid:8) but he hadfe(cid:7)g drivenfe(cid:8)g away in a truck(cid:8)
e(cid:8) Afterlookingfe(cid:0)(cid:9)g in the store(cid:0)they realizedfe(cid:0)(cid:0)g that they couldn(cid:18)t a(cid:19)ord
a new one(cid:8)
(cid:9)
Our approach to plausible inferencein EL in general(cid:0) and to such default inferences in
particular(cid:0)isprobabilistic(cid:7)