Overview of the TACITUS Project 
Jerry R. Hobbs 
Artificial Intelligence Center 
SRI International 
1 Aims of the Project 
The specific aim of the TACITUS project is to develop interpretation pro- 
cesses for handling casualty reports (casreps), which are messages in free- 
flowing text about breakdowns of machinery. 1 These interpretation pro- 
cesses will be an essential component, and indeed the principal component, 
of systems for automatic message routing and systems for the automatic 
extraction of information from messages for entry into a data base or an 
expert system. In the latter application, for example, it is desirable to be 
able to recognize conditions in the message that instantiate conditions in 
the antecedents of the expert system's rules, so that the expert system can 
reason on the basis of more up-to-date and more specific information. 
More broadly, our aim is to develop general procedures, together with the 
underlying theory, for using commonsense and technical knowledge in the 
interpretation of written discourse. This effort divides into five subareas: (1) 
syntax and semantic translation; (2) commonsense knowledge; (3) domain 
knowledge; (4) deduction; (5) ~local" pragmatics. Our approach in each of 
these areas is discussed in turn. 
2 Syntax and Semantic Translation 
Syntactic analysis and semantic translation in the TACITUS project are 
being done by the DIALOGIC system. DIALOGIC has perhaps as exten- 
sive a coverage of English syntax as any system in existence, it produces 
IThe TACITUS project is funded by the Defense Advanced Research Projects Agency 
under Office of Naval Research contract N00014-85-C-0013, as part of the Strategic Com- 
puting program. 
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a logical form in first-order predicate calculus, and it was used as the syn- 
tactic component of the TEAM system. The principal addition we have 
made to the system during the TACITUS project has been a menu-based 
component for rapid vocabulary acquisition, that allows us to acquire sev- 
eral hundred lexical items in an afternoon's work. We are now modifying 
DIALOGIC to produce neutral representations instead of multiple readings 
for the most common types of syntactic ambiguities, including prepositional 
phrase attachment ambiguities and very compound noun ambiguities. 
3 Commonsense Knowledge 
Our aim in this phase of the project is to encode large amounts of com- 
monsense knowledge in first-order predicate calculus in a way that can be 
used for knowledge-based processing of natural language discourse. Our ap- 
proach is to define rich core theories of various domains, explicating their 
basic ontologies and structure, and then to define, or at least to charac- 
terize, various English words in terms of predicates provided by these core 
theories. So far, we have alternated between working from the inside out, 
from explications of the core theories to characterizations of the words, and 
from the outside in, from the words to the core theories. Thus, we first pro- 
ceeded from the outside in by examining the concept of ~wear', as in "worn 
bearings", seeking to define ~wear", and then to define the concepts we de- 
fined Uwear" in terms of, pushing the process back to basic concepts in the 
domains of space, materials, and force, among others. We then proceeded 
from the inside out, trying to flesh out the core theories of these domains, 
as well as the domains of scalar notions, time, measure, orientation, shape, 
and functionality. Then to test the adequacy of these theories, we began 
working from the outside in again, spending some time defining, or charac- 
terizing, the words related to these domains that occurred in our target set 
of casreps. We are now working from the inside out again, going over the 
core theories and the definitions with a fine-tooth comb, checking manually 
for consistency and adequacy and proving simple consequences of the ax- 
ioms on the KADS theorem-prover. This work is described in an enclosed 
publication \[1\]. 
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4 Domain Knowledge 
In all of our work we are seeking general solutions that can be used in a wide 
variety of applications. This may seem impossible for domain knowledge. In 
our particular case, we must express facts about the starting air compressor 
of a ship. It would appear difficult to employ this knowledge in any other ap- 
plication. However, our approach makes most of our work even in this area 
relevant to many other domains. We are specifying a number of "abstract 
machines" or "abstract systems", in levels, of which the particular device we 
must model is an instantiation. We define, for example, a "closed producer- 
consumer system". We then define a "closed clean fluid producer-consumer 
system" as a closed producer-consumer system with certain additional prop- 
erties, and at one more level of specificity, we define a "pressurized lube-oil 
system". The specific lube-oil system of the starting air compressor, with 
all its idiosyncratic features, is then an instantiation of the last of these. In 
this way, when we have to model other devices, we can do so by defining 
them to be the most specific applicable abstract machine that has been de- 
fined previously, thereby obviating much of the work of specification. An 
electrical circuit, for example, is also a closed producer-consumer system. 
5 Deduction 
The deduction component of the TACITUS system is the KLAUS Auto- 
mated Deduction System (KADS), developed as part of the KLAUS project 
for research on the interactive acquisition and use of knowledge through 
natural language. Its principal inference operation is nonclausal resolu- 
tion, with possible resolution operations encoded in a connection graph. 
The nonclausal representation eliminates redundancy introduced by trans- 
lating formulas to clause form, and improves readability as well. Special 
control connectives can be used to restrict use of the formulas to either for- 
ward chaining or backward chaining. Evaluation functions determine the 
sequence of inference operations in KADS. At each step, KADS resolves on 
the highest-rated link. The resolvent is then evaluated for retention and 
links to the new formula are evaluated for retention and priority. KADS 
supports the incorporation of theories for more efficient deduction, includ- 
ing deduction by demodulation, associative and commutative unification, 
many-sorted unification, and theory resolution. The last of these has been 
used for efficient deduction using a sort hierarchy. Its efficient methods for 
performing some reasoning about sorts and equality and the facility for or- 
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dering searches by means of an evaluation function make it particularly well 
suited for the kinds of deductive processing required in a knowledge-based 
natural language system. 
6 Local Pragmatics 
We have begun to formulate a general approach to several problems that 
lie at the boundary between semantics and pragmatics. These are problems 
that arise in single sentences, even though one may have to look beyond the 
single sentence to solve them. The problems are metonymy, reference, the 
interpretation of compound nominals, and lexical and syntactic ambiguity. 
All of these may be called problems in "local pragmatics". Solving them con- 
stitutes at least part of what the interpretation of a text is. We take it that 
interpretation is a matter of reasoning about what is possible, and there- 
fore rests fundamentally on deductive operations. We have formulated very 
abstract characterizations of the solutions to the local pragmatics problems 
in terms of what can be deduced from a knowledge base of commonsense 
and domain knowledge. In particular, we have devised a general algorithm 
for building an expression from the logical form of a sentence, such that a 
constructive proof of the expression from the knowledge base will constitute 
an interpretation of the sentence. This can be illustrated with the sentence 
from the casreps 
Disengaged compressor after lube oil alarm. 
To resolve the reference of "alarm" one must prove constructively the ex- 
pression 
(3 x)alarm(x) 
To resolve the implicit relation between the two nouns in the compound 
nominal "lube oil alarm" (where "lube oil" is taken as a multiword), one 
must prove constructively from the knowledge base the existence of some 
possible relation, which we may call nn, between the entities referred to by 
the nouns: 
C3 ^ .,be-oaCv) ^ 
A metonymy occurs in the sentence in that "after" requires its object to be 
an event, whereas the explicit object is a device. To resolve a metonymy 
that occurs when a predicate is applied to an explicit argument that fails to 
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satisfy the constraints imposed by the predicate on its argument, one must 
prove constructively the possible existence of an entity that is related to the 
explicit argument and satisfies the constraints imposed by the predicate. 
Thus, the logical form of the sentence is modified to 
... A after(d,e) A q(e,x) A alarm(x) A ... 
and the expression to be proved constructively is 
(3e)event(e) A q(e,x) A alarmCx) A ... 
In the most general approach, nn and q are predicate variables. In less 
ambitious approaches, they can be predicate constants, as illustrated below. 
These are very abstract and insufficiently constrained formulations of 
solutions to the local pragmatics problems. Our further research in this 
area has probed in four directions. 
(1) We have been examining various previous approaches to these prob- 
lems in linguistics and computational linguistics, in order to reinterpret them 
into our framework. For example, an approach that says the implicit rela- 
tion in a compound nominal must be one of a specified set of relations, such 
as "part-of', can be captured by treating "nn" as a predicate constant and 
by including in the knowledge base axioms like 
(v x, y)part-of(y, x) ..(x, y) 
In this fashion, we have been able to characterize succinctly the most com- 
mon methods used for solving these problems in previous natural language 
systems, such as the methods used in the TEAM system. 
(2) We have been investigating constraints on the most general formula- 
tions of the problems. There are general constraints, such as the Minimality 
Principle, which states that one should favor the minimal solution in the 
sense that the fewest new entities and relations must be hypothesized. For 
example, the argument-relation pattern in compound nominals, as in "lube 
oil pressure", can be seen as satisfying the Minimality Principle, since the 
implicit relation is simply the one already given by the head noun. In addi- 
tion, we are looking for constraints that are specific to given problems. For 
example, whereas whole-part compound nominals, like "regulator valve", 
are quite common, part-whole compound nominals seem to be quite rare. 
This is probably because of a principle that says that noun modifiers should 
further restrict the possible reference of the noun phrase, and parts are 
common to too many wholes to perform that function. 
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(3) A knowledge base contains two kinds of knowledge, "type" knowledge 
about what kinds of situations are possible, and "token" knowledge about 
what the actual situation is. We are trying to determine which of these 
kinds of knowledge are required for each of the pragmatics problems. For 
example, reference requires both type and token knowledge, whereas most 
if not all instances of metonymy seem to require only type knowledge. 
(4) At the most abstract level, interpretation requires the constructive 
proof of a single logical expression consisting of many conjuncts. The deduc- 
tion component can attempt to prove these conjuncts in a variety of orders. 
We have been investigating some of these possible orders. For example, one 
plausible candidate is that one should work from the inside out, trying first 
to solve the reference problems of arguments of predications before attempt- 
ing to solve the compound nominal and metonymy problems presented by 
those predications. In our framework, this is an issue of where subgoals for 
the deduction component should be placed on an agenda. 
7 Implementation 
In our implementation of the TACITUS system, we are beginning with the 
minimal approach and building up slowly. As we implement the local prag- 
matics operations, we are using a knowledge base containing only the axioms 
that are needed for the test examples. Thus, it grows slowly as we try out 
more and more texts. As we gain greater confidence in the pragmatics op- 
erations, we will move more and more of the axioms from our commonsense 
and domain knowledge bases into the system's knowledge base. Our initial 
versions of the pragmatics operations are, for the most part, fairly standard 
techniques recast into our abstract framework. When the knowledge base 
has reached a significant size, we will begin experimenting with more general 
solutions and with various constraints on those general solutions. 
8 Future Plans 
In addition to pursuing our research in each of the areas described above, 
we will institute two new efforts next year. First of all, we will begin to 
extend our work in pragmatics to the recognition of discourse structure. 
This problem is illustrated by the following text: 
Air regulating valve failed. 
Gas turbine engine wouldn't turn over. 
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Valve parts corroded. 
The temporal structure of this text is 3-1-2; first the valve parts corroded, 
and this caused the valve to fail, which caused the engine to not turn over. 
To recognize this structure, one must reason about causal relationships in 
the model of the device, and in addition one must recognize patterns of 
explanation and consequence in the text. 
The second new effort will be to build tools for domain knowledge ac- 
quisition. These will be based on the abstract machines in terms of which 
we are presently encoding our domain knowledge. Thus, the system should 
be able to allow the user to choose one of a set of abstract machines and 
then to augment it with various parts, properties and relations. 
Researchers 
The following researchers are participating in the TACITUS project: John 
Bear, William Croft, Todd Davies, Douglas Edwards, Jerry Hobbs, Kenneth 
Laws, Paul Martin, Fernando Pereira, Raymond Perrault, Stuart Shieber, 
Mark Stickel, and Mabry Tyson. 
Publication 
1. Hobbs, Jerry R., William Croft, Todd Davies, Douglas Edwards, and 
Kenneth Laws, "Commonsense Metaphysics and Lexical Semantics", Pro- 
ceedings, 2~th Annual Meeting of the Association for Computational Lin- 
guistics, New York, June 1986., pp. 231-240. 
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