Interfaces and the grammar

Among a number of common-language terms that are used by theoretical linguists in ways that are sometimes puzzling, the terms “interface” and “computation” have recently become salient. The linguistic mapping between a signal, such as speech, and meaning is itself sometimes referred to as an “interface”. However, most people think that both the speech wave and meaning are translated into discrete symbolic representations, such as strings of lexical items on the one hand, and either logical forms or model theoretic-representations on the other. Thus it is natural to think of these levels of representation as interfaces of the linguistic system as a whole to other extra-linguistic systems such as articulation and perception on the one hand, and the process of reasoning about the world on the other. We then need a different word to refer to the mapping between the two interface levels. It is natural to think of this as a “computation. ” One of the principal contributions of the generative linguistic approach to the problem of characterizing the computation in this sense has been the identification of (competence) grammar as a component. There are obvious evolutionary and developmental reasons to expect this competence grammar-based computation to be rather direct, and in particular to eschew any further intermediate “hidden” interface levels of representation, particularly those of any kind not monotonically related to the two primary interface levels. The theories presented in Chomsky 1957 and 1965 can be viewed as adhering to this parsimonious view of the grammar in spirit, once it is realized that most transformations are “structure-preserving” (Emonds 1976), and can therefore be “base-generated” (Bresnan 1978; Brame 1978), hence eliminating the need for intermediate levels of representation such as “Deep Structure”. (The Augmented Transition Network grammar (ATN) of Woods 1973 was a clever contemporary realization of this idea in computational terms. ) A great deal of subsequent work by linguists and non-linguists has clung to a view of grammar as a unitary monotonic syntax-semantics interface, including Gazdar’s (1981) pioneering demonstration that the vast majority of syntactic phenomena could be captured with nothing more powerful than Context Free Grammar (CFG), and Montague’s (1974) demonstration that meaning representation could be perspicuously captured in terms of intensional logic, using the λ-calculus as a “glue language” to build representations in lock-step with syntactic derivation. Montague famously observed that in principle the level of logical form could be dispensed with in favor of direct computation via surface derivation on the model theory (although this only seems to work if the model theory itself is regarded as an interface level, distinct from the physical world). He urged that, even if logical form was in practice needed by mortals to keep track of the semantics, it should always be transparent to model theory—no interpretation without denotation. However, in term of both linguistic explanation, psychological plausibility (particularly in terms of the manifest possibility of evolution and acquisition of language), and practical computation, the use of logical forms appears to be essential. In that case, their eliminability in principle in favor of direct representation of the model theory does not seem a particularly helpful observation. In fact, we shall see that it is traditional surface syntactic structure, rather than logical form, that should be eliminated as a representational level. In psychological and computational terms, the objection to eliminating logical form is that, while unsupervised probabilistic induction of grammars from unanalyzed text to any desired level of confidence is in principle possible, for grammars of the size and ambiguity characteristic of human language, such “unsupervised” grammar induction is of a computational complexity that makes it quite impracticable. The only successful wide coverage parsers, such as those of Collins (1997) and Charniak (2000),