A Theory of Parallelism and the Case of VP Ellipsis

We provide a general account of parallelism in discourse, and apply it to the special case of resolving possible readings for instances of VP ellipsis. We show how seyeral problematic examples are accounted for in a natural and straightforward fashion. The generality of the approach makes it directly applicable to a variety of other types of ellipsis and reference. 1 T h e P r o b l e m o f V P E l l i p s i s VP ellipsis has received a great deal of attention in theoretical and computational linguistics (Asher, 1993; Crouch, 1995; Dalrymple, Shieber, and Pereira, 1991; Fiengo and May, 1994; Gawron and Peters, 1990; Hardt, 1992; Kehler, 1993; Lappin and McCord, 1990; Priist, 1992; Sag, 1976; Webbet, 1978; Williams, 1977, inter alia). The area is a tangled thicket of examples in which readings are mysteriously missing and small changes reverse judgments. It is a prime example of a phenomenon at the boundary between syntax and pragmatics. VP ellipsis is exemplified in sentence (1). (1) John revised his paper before the teacher did. This sentence has two readings, one in which the teacher revised John's paper (the strict reading), and one in which the teacher revised his own paper (the sloppy reading). Obtaining an adequate account of strict/sloppy ambiguities has been a major focus of VP ellipsis research. This is challenging because not all examples are as simple as sentence (1). In fact, sentence (1) is the first main clause of one of the more problematic cases in the literature: (2) John revised his paper before the teacher did, and Bill did too. Whereas one might expect there to be as many as six readings for this sentence, Dalrymple et ai. (1991, henceforth DSP) note that it has only five readings; the reading is absent in which (3) John revised John's paper before the teacher revised John's paper, and Bill revised John's paper before the teacher revised Bill's paper. Previous analyses have either generated too few or too many readings, or have required an appeal to additional processes or constraints external to the actual resolution process itself. Examples like (2) test the adequacy of an analysis at a fine-grained level of detail. Other examples test the generality of an analysis, in terms of its ability to account for phenomena similar to VP ellipsis and to interact with other interpretation processes that may come into play. For instance, strict/sloppy ambiguities are not restricted to VP ellipsis, but are common to a wide range of constructions that rely on parallelism between two eventualities, some of which are listed in Table 1. Given the ubiquity of strict/sloppy ambiguities, one would expect these to be a by-product of general discourse resolution mechanisms and not mechanisms specific to VP ellipsis. Any account applying only to the latter would miss an important generalization. In this paper, we give an account of resolution rooted in a general computational theory of parallelism. We demonstrate the depth of our approach by showing that unlike previous approaches, the algorithm generates the correct five readings for example (2) without appeal to additional mechanisms or constraints. We also discuss how other 'missing readings' cases are accounted for. We show the generality of the approach by demonstrating its handling of several other examples that prove problematic for past approaches, including a source-ofellipsis paradox, so-called extended parallelism cases, and sloppy readings with events cases. Of the phe-