Semantic Ambiguity and Perceived Ambiguity

I explore some of the issues that arise when trying to establish a connection between the underspecification hypothesis pursued in the NLP literature and work on ambiguity in semantics and in the psychological literature. A theory of underspecification is developed ‘from the first principles’, i.e., starting from a definition of what it means for a sentence to be semantically ambiguous and from what we know about the way humans deal with ambiguity. An underspecified language is specified as the translation language of a grammar covering sentences that display three classes of semantic ambiguity: lexical ambiguity, scopal ambiguity, and referential ambiguity. The expressions of this language denote sets of senses. A formalization of defeasible reasoning with underspecified representations is presented, based on Default Logic. Some issues to be confronted by such a formalization are discussed. 1 The Combinatorial Explosion Puzzle The alternative syntactic readings of a sentence such as (1) probably number in the hundreds, whereas sentences such as (2) would have hundreds of thousands scopally distinct readings if all permutations of scope-taking sentence constituents were considered admissible readings. Yet, human beings appear able to deal with these sentences effortlessly. (1) We should move the engine at Avon, engine E1, to Dansville to pick up the boxcar there, then move it from Dansville to Corning, load some oranges, and then move it on to Bath. (2) A politician can fool most voters on most issues most of the time, but no politician can fool all voters on every single issue all of the time. This paper will appear in K. van Deemter and S. Peters (eds), Semantic Ambiguity and Underspecification, CSLI.