Anaphoric Binding: an integrated overview Ant´onioBranco University of Lisbon NLX|Natural Language and Speech Group, Department of Informatics Faculdade de Ci^encias,Campo Grande, 1749-016 Lisboa, Portugal [email protected] The interpretation of anaphors depends on their antecedents as the semantic value that an anaphor eventually conveys is co-specified by the value of its an- tecedent. Interestingly, when occurring in a given syntactic position, different anaphors may have different sets of admissible antecedents. Such differences are the basis for the categorization of anaphoric expressions according to their anaphoric capacity, being important to determine what are the sets of admissible antecedents and how to represent and process this anaphoric capacity for each type of anaphor. From an empirical perspective, these constraints stem from what appears as quite cogent generalisations and exhibit a universal character, given their cross linguistic validity. From a conceptual point of view, in turn, the relations among binding constraints involve non-trivial cross symmetry, which lends them a modular nature and provides further strength to the plausibility of arXiv:2103.06924v1 [cs.CL] 11 Mar 2021 their universal character. This kind of anaphoric binding constraints appears thus as a most significant subset of natural language knowledge, usually referred to as binding theory. This paper provides an integrated overview of these constraints holding on the pairing of nominal anaphors with their admissible antecedents that are based on grammatical relations and structure. Along with the increasing interest on neuro-symbolic approaches to natural language, this paper seeks to contribute to revive the interest on this most intriguing research topic. Ant´onioBranco 1 Introduction It is an inherent feature of natural languages that their expressions have the potential to convey a range of semantic values which their usage in context will constrain in such a way that some value is eventually circumscribed and expressed. This is exemplified in the minimal contrast below, where out of its possible semantic values, the expression flying planes express one of such values in each context. (1) a. Flying planes are complex machines. b. Flying planes is a difficult task. Like other natural language expressions, anaphors are also semantically polyvalent. They form however a class of expressions whose context sensi- tiveness is rather peculiar in as much as for them to eventually express a semantic value, more than being circumscribed from an intrinsic repertoire of potential values, that value is co-specified by the semantic value of other expressions, which are for this reason termed as their antecedents. This is exemplified in the contrasts below with the anaphor it, that could be continued at will. This anaphor inherently contributes the information that its denotation is singular and non human. Yet in each different context, it eventually conveys a different semantic value as a result of that value being co-specified by the semantic value of a different antecedent (in italics), (2) a. John pulled off the wheel. It was heavy. b. Paul bought a computer. It has a touch screen. c. Peter got a ticket that Paul wanted to buy. It is for Saturday night. d. ... Adding to this semantic peculiarity, the dependency of anaphors with respect to their antecedents may exhibit the syntactic peculiarity of being a long-distance relation. This is illustrated with the set of contrasts below, that could be continued at will, where the anaphor her and its antecedent anaphor Mary can be separated by a string of words of arbitrary length. 2 Anaphoric Binding: an integrated overview (3) a. Mary thinks that Peter saw her. b. Mary thinks that John knows that Peter saw her. c. Mary thinks that Paul believes that John thinks that Peter saw her. d. ... In large enough contexts, an anaphoric expression may have more admis- sible antecedents than the antecedent that happen to eventually co-specify its interpretation. This receives a minimal example in the sentence below, where the anaphor herself has two admissible antecedents, out of which one will eventually end up being the selected antecedent given the respective utterance context (not represented below). (4) Claire described Joan to herself. Interestingly, when occurring in a given syntactic position, different anaphoric expressions may have different sets of admissible antecedents. This is illustrated in the emblematic examples below, with two anaphoric expressions from English | herself and her | occurring in the same position, each with different sets of admissible antecedents. (5) Mary's friend knows that Paula's sister described Joan to herself / her. For the expression herself, either Joan or Paula's sister is an admissible antecedent. For her, its set of admissible antecedents includes instead Paula, Mary's friend and Mary. Further examples with two anaphoric expressions occurring in the same position and each with different sets of admissible antecedents are the following: (6) Mary's friend knows that Paula's sister saw her / the little girl. For the expression the little girl, either Paula or Mary is an admissible antecedent. For her, its set of admissible antecedents includes additionally Mary's friend. Such differences in terms of sets of admissible antecedents are the basis for the partition of nominal anaphoric expressions into different groups according to their anaphoric capacity. It has been an important topic of research to determine how many such groups or types of anaphoric expressions there are, what are the sets of admissible antecedents for each 3 Ant´onioBranco type, what expressions belong to which type in each language, and how to represent and process this anaphoric capacity. The regularities emerging with this inquiry have been condensed in a handful of anaphoric binding constraints, or principles, which seek to cap- ture the relative positioning of anaphors and their admissible antecedents in grammatical representations. From an empirical perspective, these con- straints stem from what appears as quite cogent generalisations and exhibit a universal character, given their cross linguistic validity.1 From a concep- tual point of view, in turn, the relations among binding constraints involve non-trivial cross symmetry, which lends them a modular nature and provides further strength to the plausibility of their universal character.2 Accordingly, anaphoric binding principles appear as one of the most significant subsets of grammatical knowledge, usually termed as Binding Theory. ? This paper provides a condensed yet systematic and integrated overview of these grammatical constraints on anaphoric binding, that is of the gram- matical constraints holding on the pairing of nominal anaphors with their admissible antecedents. The integration into grammar of these anaphoric binding constraints in a formally sound and computationally tractable way, as well as of the appropriate semantic representation of anaphors are also covered in the present summary. On the basis of such overview, the ultimate goal of this paper is to provide an outlook into promising avenues for future research on anaphoric binding and its modelling | both from symbolic and neural perspectives. ? In the next section, Section 2, the empirical generalisations captured in the binding constraints are introduced, together with the relevant auxiliary notions and parameterisation options.3 The key ingredients for the integration of binding constraints into grammar are discussed in Section 3, and a detailed account of this integration is provided in the following Section 4 | which is further illustrated with the support of the working example in the Appendix. 1 (Branco 2000c), i.a. 2 (Branco 2005a). 3 To support this presentation, the frameworks adopted are Head-Driven Phrase Structure Grammar (Pollard & Sag 1994), for syntax, and Minimal Recursion Semantics (Copestake et al. 2005) and Underspecified Discourse Representation Theory (Frank & Reyle 1995), for semantics. The adoption of or transposition to other sufficiently expressive and well defined frameworks will be quite straightforward. 4 Anaphoric Binding: an integrated overview Section 5 is devoted to discuss how the account of anaphoric binding presented in the previous sections ensures a neat interface of grammar with reference processing systems, and thus supports a seamlessly articulation of binding constraints with anaphora resolution. In the penultimate section, Section 6, additional binding constraints are introduced, that hold from the perspective of the antecedents, rather from the perspective of the anaphors, together with the respective supporting empirical evidence. The final Section 7 is devoted to provide an outlook into promising avenues for future research that may further enhance our understanding of and our coping with anaphoric binding and its modelling | both symbolic and neural. 2 Empirical generalisations Since the so called integrative approach to anaphora resolution was set up,4 it is common wisdom that factors determining the antecedents of anaphors divide into filters, or hard constraints, and preferences, or soft constraints. The former exclude impossible antecedents and help to circumscribe the set of admissible antecedents; the latter interact to converge on the eventual antecedent among the admissible antecedents. So-called binding principles are a notorious subset of hard constraints on anaphora resolution: they capture generalisations concerning the constraints