Building Representations of Common Ground with Intelligent Agents: Papers from the 2011 AAAI Fall Symposium (FS-11-02) Modal Verbs in the Common Ground: Discriminating among “Actual” and “Non-Actual” Uses of Could and Would for Improved Text Interpretation Lori Moon ([email protected]) University of Illinois Urbana-Champaign Urbana, IL 61801 USA Abstract properties. Rather, sentence (3) is counterfactual and pre- supposes the negation of the antecedent proposition, which, Modal verbs occur in contexts which convey information about non-actual states of affairs as well as in contexts which if the presupposition is retained, requires the consequent convey information about the actual world of the discourse. clause to be evaluated in a world other than the actual world Modeling the semantic interpretation of non-actual states of the discourse (Lewis 1973). of affairs is notoriously complicated, sometimes requiring In sentence (4), the italicized clause must be interpreted modal logic, belief revision, non-monotonic reasoning, and relative to the speaker’s beliefs about another agent’s inten- multi-agent autoepistemic models. This work presents lin- tion rather than an actual event (Verstraete 2008). Sentence guistic features which disambiguate those instances of the (4) can be followed with the clause, ‘but hei doesn’t’[walk past tense modal verbs could and would which occur in con- to school], without a resulting contradiction, as can sentence texts where the proposition in the scope of the modal is not (3). However, if the same clause is appended to the end of true in the actual world of the discourse from those instances which presuppose or entail that an event in their scope oc- sentence (1) or sentence (2), a contradiction results. Nega- curred in the actual world of the discourse. It also illustrates tion of the proposition that Jeron walks to school is not pos- the complexity of the role of modal verbs in semantic inter- sible in sentences (1) and (2) because the use of the clause pretation and, consequently, the limitations of state of the art containing the modal constitutes an assertion that the propo- inference systems with respect to modal verbs. The features sition in the scope of the modal actually occurred. In sen- suggested for improving modal verb interpretation are based tences (3) and (4), the speaker does not make an assertion on the analysis of corpus data and insights from the linguistic that a ‘Jeron-walking-to-school’ event ever occurred, and the literature. corresponding proposition, therefore, is not entailed. Modal verbs pose special problems for implementation Introduction tasks which are concerned with meaning representation. Modal verbs such as could and would allow various inter- Consider an agent which encounters one of the past tense pretations depending on the surrounding linguistic context. modal verbs above while processing a text. In the first two The clause, ‘he would walk to school’ appears in the four uses, the agent needs to incorporate into the common ground sentences below with four different meanings: the fact that the event in the scope of the modal verb occurred repeatedly during a time span which precedes the speech 1. When Jeroni was five, hei would walk to school (every time (for sentence (1)) and at a time which is future rela- day). tive to a past reference point, but past relative to the speech 2. Jeroni stood by the door waiting for hours, later hei time for sentence (2). would walk to school. The interpretation of the non-actual uses is more com- 3. If Jeroni were you, hei would walk to school. plicated to model in the common ground. Lewis (1973:9- 13) demonstrated that counterfactual conditionals are vari- 4. Jeroni bought a house close to campus so that hei would ably strict conditionals and require interpretation in a non- walk to school. monotonic logic. Purpose uses of past tense modal verbs, In sentences (1) and (2), the modal verb occurs in a con- such as that in sentence (4), describe the speaker’s belief text which entails that the proposition in the scope of the about the another agent’s intentions (Verstraete 2008) and modal (corresponding to the constituents underlined in (1) would require interpretation in a multi-agent expansion of an above) refers to an event which actually occurred: the sen- auto-epistemic logic with beliefs and intentions (e.g., (Rao tences entail that there was a past event of Jeron walking to and Georgeff 1991)). school in the actual world of the discourse. As part of a larger project determining which interpreta- In contrast, the modal verbs in sentences (3) and (4) are tion models are required for modal verb interpretation and in non-actual contexts and do not have the same entailment how to link them to natural language expressions, this study Copyright c 2011, Association for the Advancement of Artificial describes the linguistic features distinguishing actual and Intelligence (www.aaai.org). All rights reserved. non-actual uses of could and would. This distinction is of 30 special importance for systems which want to determine Terms and Approach which actions some agent actually performed or avoid de- Some non-actual uses of modal verbs have been associ- riving real world entailments from non-actual contexts (e.g., ated with linguistic expressions such as clauses with irrealis the PASCAL RTE Challenge), for Natural Language Infer- mood or non-veridical operators (Giannakidou 2009) (for a ence systems (see Schubert et. al 2010 for the presentation concise summary of the complexity of these mappings, see of such a system and for a recent concise summary of pre- Marques (2009:182-190)). The non-actual contexts here re- vious work), as well as for tasks extracting facts from re- fer more broadly to contexts in which the proposition in the port documents or encyclopedic sources in order to build a scope of the modal has some morphological marking in- general-purpose knowledge base. dicating that it describes non-actual information (e.g., the presence of tense marking indicating the subjunctive mood Project Design and Goals in English) or which, due to some clausal or extra-clausal information, describes a non-actual state of affairs. The primary goal of this work is to determine what linguis- The notion of actual refers to uses of sentences corre- tic expressions co-occur with the past tense modal verbs sponding to propositions which are either presupposed or in actual versus non-actual contexts. Generally, theoreti- entailed to be true in the actual world of the discourse. cal linguistic studies work with sets of linguistic expres- These propositions are assumed to enter the agent’s common sions or morphological markings in order to determine what ground as facts about the world of the discourse. Non-actual meanings they systematically express. The approach in this uses are a much broader class and include sentences corre- project differs slightly, in that it first classifies naturally oc- sponding to propositions which are only true relative to a curring data with respect to its entailment properties, and participant’s belief model, for example, or only true relative second looks for evidence of the expected linguistic patterns to a non-actual possible world. These propositions do not as well as phenomena which have previously gone unno- enter the common ground as facts about the actual world of ticed. the discourse, but there are fairly reliable presuppositions or Linguistic descriptions of the entailment patterns of inferences associated with them which do provide informa- modal verbs (see (Portner 2009) for references) and their tion about the actual world. association with cross-linguistic patterns has been an active The actual/non-actual distinction differs from area of investigation in the linguistics literature (e.g., (Iatri- (non)veridicality as follows: An operator (e.g., a modal dou 2000), (Giannakidou 2009), (von Stechow and Grønn verb, a propositional attitude verb) is considered to be 2008), and others). There has not, however, been a link veridical if the proposition in its scope is presupposed made between this theoretical research and the practical task or entailed to be true in at least one belief world of the of mapping the generalizations to features which can be speaker’s (or subject’s) epistemic model (Giannakidou straightforwardly encoded in broad-coverage tasks. The gap (2009:1889)). Therefore (non)veridicality is a cross-cutting is largely due to the complexity and variety of interpreta- notion with respect to (non)actuality in that it includes tions modal verbs require, and the high-level nature of the propositional attitude verbs which enter the shared common relevant linguistic features. ground as beliefs about another’s beliefs or, more generally, Previous implementations which attempted to character- beliefs about a proposition under the category ‘veridical’, ize whether modal verbs were (non)actual overgeneralized making it not equivalent to our notion of ‘actual’. by associating lexical modal verbs uniformly with an en- Modal verbs have traditionally been claimed to have a sin- tailment pattern (e.g., MacCartney et. al 2006, MacCart- gle meaning which takes two arguments: a proposition and ney(2009:57)). The importance of expressions which signal a context (Kratzer 1977). The proposition corresponds to non-actual contexts continues to be recognized in the textual the sentence in which the modal occurs (underlined in (5) entailment community, but the work has focused more on below), and the context refers to accessible worlds and an categories which can be associated with non-functional lex- ordering relation on the accessible worlds (Kratzer 1981). ical expressions. For example, recent work has been done on Negative Polarity Items and downward entailing expres- 5. He could do more to help. sions (Danescu-Niculescu-Mizil and Lee 2010), and on em- The seeming ambiguities, such as that between epistemic bedding verbs (non-factive, implicative, etc.) and their rela- and deontic readings, are said to arise from a difference in tions to presuppositions (e.g., (Clausen and Manning 2009)).