Problems in the Application of Rhetorical Structure Theory to Text Generation. Thesis submitted in partial fulfilment of the M. Eng. Sc. (Cognitive Science) Degree. University of Melbourne June 1994 Nick Nicholas Colours of rethoryk been to me queynte; My spirit feeleth noght of swich mateere. — Geoffrey Chaucer, The Franklin’s Tale 726–727. Statement of Authorship. To the best of my knowledge, the work contained within this thesis is entirely my own, except that which is appropriately attributed to other authors. Nor has this work, either in part or in whole, been submitted as assessment to any other degree or diploma. Nick Nicholas June 1994 Acknowledgements. Την ευγνωµοσυνη´ µου και το ϑαυµασµο´ µου πρ´επει να απευϑυνω´ στην επι− σταµεν´ η´ µου, ∆ρα. Λ´εσλι Στ´ερλιγκ, που ε´ιχε την υποµονη´ και τη γνωση´ να µε καϑοδηγησει´ τους περασµ´ενους ´εξι µηνες,´ απο´ τους οπο´ιους τους τ´εσσερεις τους π´ερασα να οδηγω´ σε κυκλους.´ Θα ηϑελα´ επ´ισης να ευχαριστησω´ το ∆ρα. Ροµπερτ´ Ντ´ειλ, που µου προµηϑευσε´ αρκετ´ες χρησιµοτατες´ πηγ´ες την τελευ− τα´ια στιγµη.´ 1 ghotpu’vamvaD tlho’ vIja’ je: vIyo’na watSonvaD, jup QaQ Damo’ ’ej “ghItlhlIj yIruch” reH jIHvaD jatlhmo’; <melbIn> DuSaQ’a’ DIvI’ nawlogh loHvaD, DIvI’ngan QaQ Damo’ ’ej “ghItlhlIj yIruch” jIHvaD lujatlhQo’mo’; moHamaD maHDIrajIvaD, taHmo’ ghaH, yaH wIwavtaHvIS qaStaHvIS DIS bID, ’ej rIQchugh, vaj QIH ru’ neH SIQpu’; jon Hajeq QeDpInvaD, jIHvaD Qu’ nobmo’ ghaH, ’ej pagh lup buD vIghaj ’e’ HeQmoHmo’; tlhIngan Hollo- HvaD, tlhIngan De’wI’ QonoS je jeSwI’ HochvaD, lupmeyvetlh buD’e’ bong vIghajbogh waQmo’ chaH (nem tlhIngan Hol vIpojmeH, chay’ SoQvaD chen mu’tlheghghom pab ’e’ qel- bogh nger (ChSChMTlhGhP’KNg) vIlo’jaj); robet <lojbab> leSevalIy- ervaD, lojban De’wI’ QonoS jeSwI’ HochvaD je, jIHvaD HolQeD lulIHmo’ qaSpa’ latlh Hoch (jonwI’ lI’be’ vImojlaHbej, vaj Do’ vI- mojbe’pu’); ’ej ’Intlha kurtlhImvaD, Qaw’wI’’a’qoqvaD, loHvaD po’meH wIch Dachu’mo’ ’ej Sengmeyvo’ muleHmo’. HochvaD Qapla’ van je!2 1Appreciation and gratitude must go to my supervisor, Dr Lesley Stirling, for having the patience and the know- how to steer me through these past six months, of which I spent four running myself over. I would also like to thank Dr Robert Dale, who provided me with several useful references at the last minute. 2Thanks also to Fiona Watson, for being a good mate and telling me to do my thesis; to the Executive Committee of the Melbourne University Star Trek Club, for being good Trekkies and not telling me to do my thesis; to Mohammad Mahdiraji, for enduring a semester of sharing an office with me while undergoing little if any permanent damage; to Dr John Hajek, for employing me and making sure I should never have an idle moment; to all at the Klingon Language Institute and the Klingon Mailing List, for filling up those idle hours I somehow ended up having anyway (I hope I’ll do a Rhetorical Structure Theory (RST) analysis of Klingon one day!); to Bob ‘Lojbab’ LeChevalier and all those at the Lojban Mailing List, for getting me into linguistics in the first place (I’d have been pathetic as an engineer anyway); and to Indra ‘The Destroyer’ Kurzeme, for being an all-round legend of administrative dexterity and keeping me out of trouble. Success and salutations to you all! Contents. Statement of Authorship. .................................................................................iii Acknowledgements.......................................................................................... iv 1. Introduction..................................................................................................1 1.1. Rhetorical Structure Theory. ............................................................. 2 1.2. Computational Linguistics. .............................................................. 4 1.3. Connectives and Rhetorical Structure. ................................................. 5 1.4. Scott & de Souza’s Hypothesis.......................................................... 7 1.5. The issues considered. ...................................................................... 9 1.5.1. RST Ontology (Chapter 2).................................................9 1.5.2. RST Taxonomy (Chapter 3). .............................................10 1.5.3. The Scott & de Souza programme (Chapter 4). .....................11 2. RST Ontology............................................................................................ 12 2.1. What is a rhetorical relation? ............................................................12 2.1.1. The relational criterion. ....................................................14 2.1.2. Testing the relational criterion. ..........................................15 2.1.3. Against Volitionality. ......................................................16 2.1.4. Against Temporals. .........................................................17 2.1.5. For ELABORATION ..........................................................18 2.1.6. Computational implications. .............................................19 2.2. What is rhetorically related? .............................................................20 2.2.1. ........................................................................Clauses. 20 2.2.2. Propositions...................................................................22 2.2.3. Problems with a propositional RST. ...................................24 2.3. ..................................................................................Conclusion. 26 3. RST Taxonomy.......................................................................................... 28 3.1. Longacre’s taxonomy...................................................................... 29 3.2. Halliday & Hasan’s taxonomy. .........................................................31 3.2.1. Knott & Dale’s substitution classes. ...................................34 3.3. Mann & Thompson’s classifications.................................................. 35 3.3.1. The CAUSE Cluster. ........................................................35 3.3.2. The Causation cluster....................................................... 36 3.3.3. Relation pairings............................................................. 38 ENABLEMENT–MOTIVATION.......................................... 39 JUSTIFY–EVIDENCE...................................................... 39 ANTITHESIS–CONCESSION .............................................39 EVALUATION–INTERPRETATION. ....................................40 Other pairings. .............................................................40 3.4. The Tilburg taxonomy. ...................................................................40 3.4.1. The relations the Tilburg taxonomy includes. .......................40 3.4.2. The two CONCESSIONS ....................................................41 3.6. Hobbs’ taxonomy. .........................................................................43 3.7. A synthesis of taxonomies............................................................... 45 3.7.1. The taxonomical parameters. .............................................46 Presentational/Informational............................................ 46 Adversative/Non-Adversative. ..........................................47 Basic/Elaborative. .........................................................48 Deontic/Non-Deontic. ....................................................48 Modal/Non-Modal. ........................................................49 Illocutionary/Epistemic. .................................................49 BACKGROUND. ............................................................50 ANTITHESIS .................................................................51 3.7.2. Explaining connectives using the taxonomy. ........................51 3.8. Computational implications of an RST taxonomy. ..............................54 4. The Scott & de Souza programme.............................................................. 56 4.1. What Scott & de Souza propose: Theoretical heuristics. ........................57 4.1.1. Heuristic 1: ‘Accurate and unambiguous’. ............................57 4.1.2. Heuristic 2: ‘Keep the propositions together’. .......................59 4.1.3. Heuristic 3: ‘Single sentence’. ...........................................60 4.1.4. Heuristic 3: Van Dijk & Kintsch’s model. ...........................61 4.1.5. Heuristic 3: How many propositions per sentence? ................63 4.1.6. Heuristic 3: Clearer text? ..................................................65 4.2. What Scott & de Souza accomplish: Embedding. .................................66 4.2.1. Textual marking for ELABORATIONS.................................. 67 4.2.2. Where should ELABORATIONS be embedded? ........................69 4.2.3. Dangling sentences. .........................................................71 4.2.4. Syntactic complexity of embedding..................................... 72 4.3. What Scott & de Souza accomplish: Parataxis. ....................................73 4.3.1. ....................................................................Evaluation. 75 4.4. Filling in the blanks: Hypotaxis. ......................................................76 4.4.1. Distinguishing adversative relations textually. ......................79 4.5. Blanking out the fillers: Unsignalled rhetorical relations. .......................81 4.5.1. Unsignalled Presentational relations. ...................................81 4.5.2. OLUTIONHOOD. ............................................................S