TACITUS: A Message Understanding System 
Jerry ~. Hobbs, Douglas Appelt, John Bear, 
Mark Stickel, and Mabry Tyson 
SRI International 
Menlo Park, California 
(415) 859-2220 
TACITUS is a general and domain-independent natural language processing system, 
used so far primarily for message processing. It performs a syntactic analysis of the 
sentences in the text, producing a logical form. Next, inferential pragmatics processing is 
applied to the logical form to solve problems of schema recognition, reference resolution, 
metonymy resolution, and the interpretation of vague predicates. An analysis component 
then produces the desired output for the application. TACITUS has been applied to 
several quite different domains, including naval equipment failure reports, naval operations 
reports, and terrorist reports. 
The syntactic component is the DIALOGIC system, developed originally for the TEAM 
transportable, natural language interface to databases. The parser is bottom-up and 
produces all the parses at once, together with their logical forms. Its grammar is among the 
largest computer grammars of English in existence, giving nearly complete coverage of such 
phenomena as sentential complements, relative clauses, adverbials, sentence fragments, 
and the most common varieties of conjunction. Selectional constraints are applied, and 
there are a large number of heuristics for selecting the preferred parses of ambiguous 
sentences. The logical form produced is an "ontologically promiscuous" version of first- 
order predicate calculus, in which relations of grammatical subordination are represented. 
Optionally and where possible, the logical forms for different parses are merged into a 
neutral representation. 
Pragmatics processing is based on abductive inference, implemented in the Prolog 
Technology Theorem Prover (PTTP), using a knowledge base encoding commonsense and 
domain-specific knowledge in the form of predicate-calculus axioms. The fundamental idea 
is that the interpretation of a sentence is the minimM proof from the knowledge base of 
the logical form of the sentence together with the constraints predicates impose on their 
arguments, allowing for coercions, where one merges redundancies where possible and 
makes assumptions where necessary. This formulation leads to an elegant, unified solution 
to the problems of schema recognition, reference resolution, metonymy resolution, and the 
interpretation of vague predicates. The output of this component is an elaborated logical 
form with the relevant inferences drawn and the identities of entities explicitly encoded. 
Finally, an analysis component takes the interpretation produced by the pragmatics 
component and generates the required output. For the equipment failure reports, this is 
a diagnosis of the problem described. For the naval operations reports and the terrorist 
reports, this is entries for a database. With very little effort, analysis components could 
be constructed for a number of other applications, such as message routing and message 
prioritizing. 
A number of convenient knowledge-acquisition facilities have been implemented for 
TACITUS. These include a menu-based lexical acquisition component, a sort hierarchy 
editor, and a component allowing entry of axioms in a subset of English. 
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