Since Austin introduced the term ``infelicity'', the linguistic literature has been flooded with its use. Today, not only performatives that fail are considered infelicitous but also utterances that are syntactically, semantically, or pragmatically ill-formed. However, no formal or computational explanation has been given for infelicity. This thesis provides one for those infelicities that occur when a pragmatic inference is cancelled.
We exploit a well-known difference between pragmatic and semantic information: since implicatures and presuppositions, i.e., the carriers of pragmatic information, are not specifically uttered, pragmatic inferences are defeasible, while most of semantic inferences are indefeasible. Our contribution assumes the existence of a finer grained taxonomy with respect to pragmatic inferences. It is shown that if one wants to account for the natural language expressiveness, she should distinguish between pragmatic inferences that are felicitous to defeat and pragmatic inferences that are infelicitously defeasible. Thus, it is shown that one should consider at least three types of information: indefeasible, felicitously defeasible, and infelicitously defeasible. The cancellation of the last of these determines the pragmatic infelicities.
A new formalism has been devised to accommodate the three levels of information, called ``stratified logic''. Within it, we are able to express formally notions such as ``utterance U presupposes P'' or ``utterance U is infelicitous''. Special attention is paid to the implications that our work has in solving some well-known existential philosophical puzzles. The formalism yields an algorithm for computing interpretations for utterances, for determining their associated presuppositions, and for signalling infelicities. Its implementation is a Lisp program that takes as input a set of stratified formulas that constitute the necessary semantic and pragmatic knowledge and the logical translation of an utterance or set of utterances and that computes a set of ``optimistic'' interpretations for the given utterances. The program computes for each set of utterances the associated presuppositions and signals when an infelicitous sentence has been uttered.
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