addition, a sentence-internal discourse relation holds between the temporal ..... Note that the usual order of events in NARRATION is inverted by the usage of.
Before and after: sentence-internal and -external discourse relations# Frank Schilder and Thora Tenbrink∗ This study presents a corpora investigation (online available texts of written and spoken English) regarding the usage of the discourse connectives before and after in multi-sentence discourse. We focus on sentences that begin with a connective and analyse these along with the preceding text. Our results show that two discourse relations have to be derived for these sentences: a sentence-external and a sentence-internal one. The sentence-external relation holding between the temporal clause and a preceding sentence is responsible for the coherent placement of a before/after-sentence in a wider discourse context. In particular, before and after presuppose one particular discourse relation, viz. ELABORATION. In addition, a sentence-internal discourse relation holds between the temporal clause and the main clause.
Introduction Studies addressing the processing of temporal connectives have largely been confined to the sentence level (cf. Wrobel, 1994). Our investigation of the usage of before and after in natural language corpora, however, points to context effects to be expected at the discourse level. We present evidence that before/after-sentences serve as presupposition triggers that must either be bound by the context or be accommodated. Our main finding is that sentences starting with before or after have to be linked to the previous sentence(s) via the discourse relation ELABORATION. Otherwise a coherent discourse cannot be established. Before and after have traditionally been viewed as the prototypical linguistic expressions denoting temporal order. Consequently, their usage has been studied extensively to infer information on psychological processes and cognitive effects concerning temporal phenomena (see e.g. Coker 1975; Wrobel 1994). In particular, three variables influencing the processing of sentences containing temporal connectives have been focussed on: • Syntax of the complex temporal sentence: Clark & Clark (1968) and others (e.g., Jou & Harris, 1990) argued that the clause structure main clause + subordinate clause is to be regarded as the unmarked or standard case. • Order of mention: The correspondence of the sequence of events in the sentential representation to the intended temporal order of events enhances processing (e.g., Zimmer, 1982). This effect and the previous one may neutralize each other. • Knowledge about the order of events: If the events are described in correspondence to their natural order (e.g., preparing a meal before eating), processing is easier than if the natural order is disturbed. French & Brown (1977), for instance, point out that non-arbitrary (i.e. somehow logically connected) sequences facilitate performance in a task where subjects act out two events joined by either before or after. If there is no natural order, a default assumption is that order of mention corresponds to order of events (Strube, Hemforth & Wrobel, 1988). Moreover, the #
Workshop From Sentence processing to discourse interpretation: Crossing the borders. Utrecht University, Utrecht institute of Linguistics OTS, Utrecht (The Netherlands), 2-3 July 2001 ∗ Department for Informatics, Vogt-Kölln-Str. 30, 22527 Hamburg, Germany.
linguistic context supplies additional knowledge, enabling the listener/reader to develop a mental model of the situation. If the described order of events corresponds to (or fits well into) this mental model, processing is enhanced (Wrobel, 1994). However, the question of which additional non-temporal phenomena before and after are capable of expressing is rarely addressed. The above findings have not led to a detailed investigation of the kinds of relationships and expectations that may be conveyed by before and after in natural discourse. In more theoretically oriented research in the field of formal discourse semantics, it has been observed that before and after trigger presuppositional effects (e.g., Heinämäki, 1974; Lascarides & Oberlander, 1993; Asher & Lascarides, 1998). In (1)
Jane went to England after she won the scholarship.
the fact that Jane won the scholarship is presupposed and still holds even when the sentence is negated. Moreover, Schilder (2001) points out that it is conceivable (but not necessarily true in all meaningful contexts) that Jane went to England because of the scholarship such that a causal relation is implied. More information extracted from the linguistic context of this sentence would decide on the latter point by providing background knowledge. Thus, before and after are not, as previously assumed, purely markers of temporal relationships, but presuppose more complicated relationships between events. On the one hand, this calls for a closer analysis of the non-temporal implications conveyed by before and after regarding the sentence-internal relationship between the temporal and the main clause of a sentence involving one of the two connectives. We deal more closely with this issue in Tenbrink & Schilder (2001). On the other hand, the question arises in which way such sentences are embedded in the wider discourse context. The remainder of this paper will deal with this question by investigating primarily sentences which begin with either before or after in their natural discourse context. Connections between the clauses of a text Research in the fields of discourse analysis and text linguistics pointed to the fact that naturally occurring sentences need to be understood and analysed together with their situational and linguistic context. For example, a sentence like (2)
Peter closed the door.
presupposes the existence of a door that has either been mentioned explicitly in the previous sentences or that can be accommodated using background knowledge gained from either the linguistic or the non-linguistic situational context. Moreover, clauses are connected to each other in certain non-arbitrary ways that are labeled discourse or rhetorical relations, and that are considered by various different approaches. We start from Lascarides & Asher's (1993) approach, which focusses specifically on temporal relations between clauses, and show how their account may be enlarged by taking into account syntactic variations. Lascarides and Asher specify the following possible rhetorical relations between sentences: a) EXPLANATION and RESULT: two kinds of causal connections between two events described in a text, with either the first or the second event mentioned in the text causing the other event. b) ELABORATION: the second event mentioned in the text is part of the first. c) NARRATION: The secondly described event is a consequence of (but not strictly speaking caused by) the first one. d) BACKGROUND: The state described in the second clause provides the circumstances under which the event described in the first clause occurred. 2
In Tenbrink & Schilder (2001) we show how the discourse relation between two clauses connected by before and after can be accounted for in Lascarides' and Asher's terms. While both before and after can express the discourse relation NARRATION, only after may alternatively convey an EXPLANATION or RESULT relation. Both ELABORATION and BACKGROUND are blocked because they indicate overlap between the events, while the semantics of before and after contains that two non-overlapping events succeed each other. Here is one example from the CPSA corpus (see below) that illustrates how the discourse relation EXPLANATION (i.e., the event described in the second clause explains why the one described in the first clause happened) is conveyed by the seemingly purely temporal connective after: (3)
[Mandel:] But to understand that this is a next natural progression after many years of work at the local and state level.
In many cases it is not a causal relation but a kind of NARRATION that characterizes the relationship between the clauses. We distinguish three different meaningful kinds: a) Insertion, where the event described by the temporal clause is presupposed, and the inserted one is new to the hearer; b) Regulation, where both events are known from the discourse context, and the utterance's new information concerns the order itself; and c) Dependency, where the later event is based non-causally on the earlier one. This account of how the two events described in clauses connected by before and after are related to each other does not specify the relation of the clause complex1 to the wider discourse context. Schilder (2001) analyses sentences in which order of mention corresponds to temporal order. He shows that sentences beginning with after convey semantic implications with regard to the previous discourse which do not occur in sentences involving before in sentence-mid position. In particular, Schilder observes that sentence-initial after conveys a termination or a sub-part relation with regard to a previously mentioned event. However, sentences beginning with before were not part of the analysis. Thus, the effects of syntactic variation clearly need further investigation. Corpora analysis To address this issue we present a further corpora investigation. Data from various different kinds of online available corpora are analysed. On the one hand, we investigate spoken language by using a sample of the Corpus of Spoken Professional AmericanEnglish (CPSA; http://www.athel.com/corpdes.html), containing press conference transcripts from the White House, and a record of faculty meetings at UNC and Committee Meetings held at various locations around the country to discuss the creation of different kinds of national tests. On the other hand, we consider written texts of both fictional and elaborated scientific content. Two scientific articles were taken from the collection of Publications of the council for research in values and philosophy (CRVP, http://www.crvp.org/). Fiction written by Linda Coleman and Kate Chopin was found at The English Server Fiction Collection (http://eserver.org/). These various kinds of corpora were chosen in order to determine whether the identified concepts are reflected in different kinds of context, and whether they occur independent of speech style. As our research question addresses the occurrence of linguistic phenomena rather than their distribution, quantitative analyses are not carried
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We use 'clause complex' (in preference to 'sentence') to refer to the two clauses connected by before and after, as it may happen that the sentence as a whole contains additional clauses, or that the two clauses occur in two different speaker turns. This kind of syntactic variation does not influence the semantics described.
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out. In the present analysis, we concentrate on sentences beginning with before and after. 1. AFTER We found that in sentences beginning with after, an event previously described by the text is taken up in the after-clause and is elaborated, for instance, by stating the termination of the event: (4)
But that evening Edna finished her dinner alone, with forced deliberation. Her face was flushed and her eyes flamed with some inward fire that lighted them. After finishing her dinner she went to her room.
In this example, the temporal connective after triggers two different discourse relations: on the one hand, the after-clause points back to the previous discourse by elaborating on an event mentioned before. In this case, the elaboration consists of the statement that the event is terminated. On the other hand, the after-clause is connected to the main clause following it via the rhetorical relation NARRATION. The flow of events as characterized in the discourse can be schematized as follows: Edna finished her dinner alone (...) ELABORATION: Termination After finishing her dinner
she went to her room
NARRATION Figure 1: Discourse relations established by sentence-initial after: Termination
A further possibility is that the event taken up and elaborated by the after-clause is not terminated as a whole, but a sub-part of it is terminated, as in: (5)
His unique distinction between common and individual form or essence caused difficulties in theology. (...) After being accused of impairing the unity of God, he retracted his unacceptable theological propositions.
Here, 'being accused of impairing the unity of God' is to be understood as (maybe the temporally latest) part of the 'difficulties in theology' that the person described experiences. Thus, the after-clause is responsible for the relation between the clause itself and the previous discourse, as well as for the relation to the main clause that follows it. The relation to the main clause in this case is a causal one, identified as RESULT in Lascarides and Asher's terms. Figure 2 illustrates these two relationships. These examples illustrate how the after-clause triggers rhetorical relations in two directions, one backward-looking and one forward-looking. With regard to the previous discourse, an ELABORATION relation is established, with either the whole previously mentioned event being terminated by the after-clause, or part of it. With regard to the main clause, after conveys a rhetorical relation (e.g., EXPLANATION or NARRATION) connected to the temporal clause. This effect is illustrated in fig. 3.
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... difficulties in theology ELABORATION: sub-part After being accused ...
he retracted his ... propositions
RESULT Figure 2: Discourse relations established by sentence-initial after: Sub-part relation
[tc=temporal clause; mc=main clause; e=event; R*=restricted rhetorical relation] Figure 3: Discourse trees integrated via the discourse relations established by sentenceinitial after
Fig. 3 indicates how the discourse syntactic tree interacts with the discourse semantic tree. The left tree reflects the syntactic structure of the discourse, as computed via context-free rules (cf. Webber et al. 1999 employ a lexicalised Tree Adjoining Grammar (TAG) for discourse parsing). The right tree represents the discourse tree structure that is derived from the discourse markers. In the discourse tree, the sentence-internal relation R* is restricted to the set of relations discussed earlier. In addition, the sentence-external relation that connects the sentence to the previous discourse has to be ELABORATION which subsumes the two cases of sub-part and termination. In some cases, the after-clause does not explicitly refer back to an event previously mentioned by the given text. Then, the reference of the after-clause needs to be accommodated, i.e. a link must be created between the previous text and the afterclause, using world and background knowledge. In these cases, it is not possible to identify an ELABORATION relation on the surface of the text as the event which is elaborated is not mentioned explicitly. For example, in (6)
Anselm was born of a noble family in 1033, in Aosta, northern Italy. After his education by the Benedictines at Aosta, he entered their order in 1060 at Bec in Normandy.
the after-clause needs to be accommodated, which is done by inferring from the previous sentence that Anselm's life is described as a whole. Note that this sentence could not occur as the beginning of a life description, i.e. without a previous sentence indicating the setting in which the sentence beginning with after is to be accommodated.
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In sentences where after occurs in a later syntactic position, all kinds of rhetorical relations are possible between the clause complex and the previous discourse. There is no need to identify a specific relation of the after-clause to the previous discourse (although there may be one), as the following example illustrates: (7) You know I have a quick temper, but I don't want to quarrel or be rude to a woman, especially my wife; yet I'm driven to it, and feel like ten thousand devils after I've made a fool of myself.
Here, after serves to establish the relationship between the narrator's feelings and the previous event (his having made a fool of himself); however, there is no need to identify this event with anything mentioned previously (see fig. 4). some
relation
to
EXPLANATION
previous discourse
...feel like ten thousand devils
after I've made a fool of myself
Figure 4: Sentence-internal discourse relation established by sentence-mid after
Fig. 5 below illustrates that the connective after in non-initial sentence position only restricts the sentence-internal relation R*. A sentence-external discourse relation connecting the sentence to the previous discourse is unrestricted unless further discourse markers are presented. For the sequence in (7) a CONTRAST relation would be imposed because of the discourse marker yet.
Figure 5: Integrated discourse trees indicating the discourse relation established by sentence-mid after
2. BEFORE In the case of sentence-initial before, the situation mentioned in the main clause temporally precedes the situation referred to by the before-clause. Our corpora investigation reveals that this clause is linked to a previously mentioned situation via ELABORATION, as in: (8)
"I'm taking my clothes," I say slowly, automatically, sadly, and with fear. Before my words are out, it's over. She says something, just a breathless "OK, I'm finished" and she's gone.
Here, the event of saying something is taken up by the before-clause, and the main clause describes what happens before this event is fulfilled. Thus, the before-clause gets a non-veridical flavour: it is rendered irrelevant whether the narrator really finishes her sentence. Far more important for the flow of events is that something happens before that (see fig. 6). 6
Figure 6: Discourse relations established by sentence-initial before
ELABORATION ... I say slowly, automatically, sadly, and with fear.
something
happens
before the event is
Before my words are out it's over. She says ....
fulfilled
'Inverted' NARRATION: Insertion
Note that the usual order of events in NARRATION is inverted by the usage of before. This observation can be captured by a precise definition of the discourse syntax/semantic interface, as illustrated by the schema in figure 7. The rhetorical relation R* is true for R*(e2,e1), but not for R*(e1,e2) as for the tree invoked by the connective after in fig. 3 and 5.
Figure 7: Integrated discourse trees indicating the discourse relations established by sentence-initial before
Sometimes the whole clause complex serves to elaborate on the previous text: (9)
Aquinas and Scotus spoke as though the distinction of ideas in God were prior to the production of creatures. Before creation, the ideas have no positive reality but are simply 'nonbeings' in the divine mind.
... as though the distinction of ideas in God were prior to the production of creatures. ELABORATION Before creation, the ideas have no positive reality... Figure 8: Clause complex beginning with before elaborating on the previous text
In both these examples, the existence of before at the beginning of the sentence presupposes an ELABORATION relation with regard to the previous discourse, as illustrated by figure 7. In sentence-mid position, however, no specific relation to the previous sentences is required, as in: (10) Scotus refined Thomistic ideas in a crucible of critique before adopting them.
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Here, before establishes a DEPENDENCY relation with regard to the main clause, but it does not point back to the previous discourse (see fig. 9).
Figure 9: Integrated discourse trees indicating the discourse relation established by sentence-mid before
Discussion Our corpora investigation reveals an effect of the position of the temporal clause with regard to the relationship to the previous discourse. The occurrence of before and after at the beginning of a clause complex establishes an ELABORATION relation regarding the previous context. Thus, it is presupposed that the event mentioned by the temporal clause must have been mentioned before, otherwise it could not be elaborated on. In addition, a rhetorical relation (NARRATION or a causal relationship) between the temporal and the main clause can be identified. In sentences where the main clause precedes the temporal clause, no ELABORATION relation with regard to the previous discourse is required. Thus, in those cases there is no need to identify a possible referent to which the temporal clause points back. Some explanation of this may be given by what Halliday (1994) calls the thematic structure of a text. Producers of text start their utterance from a specific point of view, which can be regarded as the theme of the clause or clause complex. In the unmarked case, this corresponds to what is already known to the hearer (in other words, GIVEN). Therefore, if the event mentioned in the temporal clause has already occurred in the previous discourse, it is a good candidate to start a sentence with. Halliday states that, in spoken discourse, it is possible to change the GIVEN/NEW structure by highlighting the NEW element(s) of a sentence prosodically. In written discourse, however, specific linguistic devices are required to mark the beginning of a sentence as NEW. In the absence of such devices, the theme (i.e., the beginning) of a sentence is usually the GIVEN part. This explains why the following re-phrased text sounds slightly odd, compared with the original in example (4) above: (11) But that evening Edna finished her dinner alone, with forced deliberation. Her face was flushed and her eyes flamed with some inward fire that lighted them. She went to her room after finishing her dinner.
The last sentence seems to suggest that she finished her dinner once again. This effect can be circumvented by emphasizing She went to her room prosodically. Likewise, the clauses of example (10) cannot be exchanged without implying that the adoption of the Thomistic ideas must have been mentioned previously or at least known by the reader beforehand: (12) Before adopting Thomistic ideas, Scotus refined them in a crucible of critique.
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Here, a natural interpretation would be that, although the reader probably knows that Scotus adopted Thomistic ideas, (s)he might need to be informed about the fact that he refined them before that happened. Conclusion In naturally occurring discourse, the first part of a sentence or other clause complex is the one which, in the absence of specific linguistic or prosodic markers, establishes the coherence of the text with regard to the previous discourse. If a temporal clause is in the so-called 'thematic' position, the rhetorical relation which is established concerning the previous discourse is preferably ELABORATION. In clauses beginning with after, a previously mentioned event is taken up and elaborated by stating the termination of the whole or part of the event. In clauses beginning with before, a previously mentioned event is taken up, and the following main clause serves to inform about what happens before this event is finished. These phenomena were found in all different kinds of corpora (i.e., written and spoken discourse involving differing kinds of speech style) which we investigated. Two conclusions may be drawn from these findings: Firstly, in order to capture the impact of syntactic variation on discourse coherence, an approach to rhetorical relations like that of Lascarides and Asher (1993) may be specified further by combining it with a formal tool that is capable of taking into account syntactic information, such as discourse trees involving main and subordinated clauses. Secondly, as regards sentence processing, hypotheses can be spelled out with regard to the position of before and after in discourse. Where before and after occur in the beginning of a sentence, an ELABORATION relation is to be expected, presupposing that the event mentioned in the temporal clause has been mentioned before. Thus, a search for this event in memory is to be expected, which may take time. If no such event is found in the linguistic context, it needs to be accommodated. Where before and after occur in sentence-mid position, such a search is not necessary, although it may be triggered by other phenomena occuring in the clause, such as anaphora. However, sentence-mid before and after may well serve to trigger other kinds of discourse phenomena which still need to be investigated. With regard to the latter point, we have not carried out psycholinguistic experiments to test our hypothesis. For the moment, we simply point out that, to our knowledge, such experiments have so far neglected to take into account the kinds of rhetorical relations – in both directions, sentence-internal and -external – that are suggested by the usage of before and after in natural discourse, apart from purely temporal information. References Asher, N. & A. Lascarides. 1998. The Semantics and Pragmatics of Presupposition, Journal of Semantics, 15, pp 239-299, Oxford University Press. Clark, H.H. & Clark, E.V. 1968. Semantic distinctions and memory for complex sentences. Quarterly Journal of Experimental Psychology, 20, 129-138. Coker, P. 1975. On the acquisition of temporal terms: before and after. Papers and Reports on Child Language Development 10, 166-177. Committee on Linguistics, Stanford University. French, L. & Brown, A.L. 1977. Comprehension of 'before' and 'after' in logical and arbitrary sequences. Journal of Child Language 4, 247-256.
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Halliday, Michael A.K. 1994. An Introduction to Functional Grammar. London: Edward Arnold. Heinämäki, O. 1974. Semantics of English Temporal Connectives. Dissertation, Dept. of Linguistics, Univ. of Texas, Austin. Jou, J. & Harris, R.J. 1990. Event order versus syntactic structure in recall of adverbial complex sentences. Journal of Psycholinguistic Research, 19, 21-42. Lascarides, A. & N. Asher. 1993. Temporal Interpretation, Discourse Relations and Common Sense Entailment. Linguistics and Philosophy 16, 437-493. Lascarides, A. & J. Oberlander. 1993. Temporal Connectives in a Discourse Context. In Proceedings of the European Chapter of the Association for Computational Linguistics (EACL93), pp 260--268, Utrecht, The Netherlands. van der Sandt, R.A. 1992. Presupposition Projection as Anaphora Resolution. Journal of Semantics, 9, 333-377. Schilder, F. 2001. Presupposition Triggered by Temporal Connectives. In: M. Bras-Grivart and L. Vieu (eds), Semantic and Pragmatic Issues in Discourse and Dialogue: Experimenting with Current Theories. Elsevier (forthc.). Strube, G., Hemforth, B. & Wrobel, H. 1988. Kognitive Modellierung und empirische Analyse von Prozessen der Satzverarbeitung (I) Bochum: DFG-Projekt P-PARS-PSY, RuhrUniversität Bochum, Psychologie der Informationsverarbeitung. Tenbrink, T. & F. Schilder. 2001. (Non-)temporal concepts conveyed by before, after, and then in dialogue. In: Proceedings of BIDIALOG 2001, 14-16 June 2001, Bielefeld, pp 228-238. Webber, B., A. Knott, M. Stone & A. Joshi. Discourse Relations: A Structural and Presuppositional Account using Lexicalised TAG. 1999 Meeting of the Association for Computational Linguistics, College Park MD, June 1999. Wrobel, H. 1994. Sprachverstehen als kognitiver Prozess: Zur Rezeption komplexer Temporalsätze. Opladen: Westdeutscher Verlag. Zimmer, H.D. 1982. Rezeption und Produktion komplexer Temporalsätze. Sprache und Kognition, 1, 90-103.
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