  Since Austin introduced the term ``infelicity'', the linguistic literature
has been flooded with its use, but no formal or computational explanation has
been given for it. This thesis provides one for those infelicities that occur
when a pragmatic inference is cancelled.
  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, one 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 infelicitous
utterances that has been implemented in Common Lisp. The algorithm applies
equally to simple and complex utterances and sequences of utterances.
