To support the sharing and reuse of formally represented knowledge among AI systems, it is useful to define the common vocabulary in which shared knowledge is represented. A specification of a representational vocabulary for a shared domain of discourse — definitions of classes, relations, functions, and other objects — is called an ontology. This paper describes a mechanism for defining ontologies that are portable over representation systems. Definitions written in a standard format for predicate calculus are translated by a system called Ontolingua into specialized representations, including frame-based systems as well as relational languages. This allows researchers to share and reuse ontologies, while retaining the computational benefits of specialized implementations. We discuss how the translation approach to portability addresses several technical problems. One problem is how to accommodate the stylistic and organizational differences among representations while preserving declarative content. Another is how to translate from a very expressive language into restricted languages, remaining system-independent while preserving the computational efficiency of implemented systems. We describe how these problems are addressed by basing Ontolingua itself on an ontology of domain-independent, representational idioms.

An ontology is a set of definitions of content-specific knowledge representation primitives: classes, relations, functions, and object constants. Ontolingua is mechanism for writing ontologies in a canonical format, such that they can be easily translated into a variety of representation and reasoning systems. This allows one to maintain the ontology in a single, machine-readable form while using it in systems with different syntax and reasoning capabilities. The syntax and semantics are based on the KIF knowledge interchange format [11]. Ontolingua extends KIF with standard primitives for defining classes and relations, and organizing knowledge in object-centered hierarchies with inheritance. The Ontolingua software provides an architecture for translating from KIF-level sentences into forms that can be efficiently stored and reasoned about by target representation systems. Currently, there are translators into LOOM, Epikit, and Algernon, as well as a canonical form of KIF. This paper describes the asic approach of Ontologia to the ontology sharing problem, introduces the syntax, and describes the semantics of a few ontological commitments made in the software. Those commitments, that are reflected in the ontological syntax and the primitive vocabulary of the frame ontology, include: a distinction between definitional and nondefinitional assertions; the organization of knowledge with classes, instances, sets, and second-order relations; and assertions whose meaning depends on the contents of the knowledge base. Limitations of Ontologia's "conservative" approach to sharing ontologies and alternative approaches to the problem are discussed.

Qualitative reasoning can, and should, be decomposed into a model-building task, which creates a qualitative differential equation (QDE) as a model of a physical situation, and a qualitative simulation task, which starts with a QDE, and predicts the possible behaviors following from the model. In support of this claim, we present QPC, a model builder that takes the general approach of Qualitative Process Theory [ Forbus, 1984 ] , describing a scenario in terms of views, processes, and influences. However, QPC builds QDEs for simulation by QSIM, which gives it access to a variety of mathematical advances in qualitative simulation incorporated in QSIM. We present QPC and its approach to Qualitative Process Theory, provide an example of building and simulating a model of a non-trivial mechanism, and compare the representation and implementation decisions underlying QPC with those of QPE [ Falkenhainer and Forbus, 1988; Forbus, 1990 ] . Introduction There have been a variety of producti...

Access-Limited Logic (ALL) is a theory of knowledge representation which formalizes the access limitations inherent in a network structured knowledge-base. Where a deductive method such as resolution would retrieve all assertions that satisfy a given pattern, an access-limited logic retrieves only those assertions reachable by following an available access path. The time complexity of inference in ALL is a polynomial function of the size of the accessible portion of the knowledge-base, rather than an exponential function of the size of the entire knowledge-base (as in much past work). Access-Limited Logic, though incomplete, still has a well defined semantics and a weakened form of completeness, Socratic Completeness, which guarantees that for any fact which is a logical consequence of the knowledge-base, there is a series of preliminary queries and assumptions after which a query of the fact will succeed. Algernon implements Access-Limited Logic. Algernon is impo...

We have previously argued that the syntactic structure of natural language can be exploited to construct powerful polynomial time inference procedures. This paper supports the earlier arguments by demonstrating that a natural language based polynomial time procedure can solve Schubert's steamroller in a single step. This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the work described in this paper was provided in part by Misubishi Electric Research Laboratories, Inc. Support for the laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-85-K-0124. This paper appeared in AAAI-91. A postscript electronic source for this paper can be found in ftp.ai.mit.edu:/pub/dam/aaaib.ps. A bibtex reference can be found in ftp.ai.mit.edu:/pub/dam/dam.bib. 1 Introduction Schubert's steamro...

Access-Limited Logic (ALL) is a language for knowledge representation which formalizes the access limitations inherent in a network structured knowledge-base. Where a deductive method such as resolution would retrieve all assertions that satisfy a given pattern, an access-limited logic retrieves all assertions reachable by following an available access path. In this paper, we extend previous work to include negation, disjunction, and the ability to make assumptions and reason by contradiction. We show that the extended ALL neg remains Socratically Complete (thus guaranteeing that for any fact which is a logical consequence of the knowledgebase, there exists a series of preliminary queries and assumptions after which a query of the fact will succeed) and computationally tractable. We show further that the key factor determining the computational difficulty of finding such a series of preliminary queries and assumptions is the depth of assumption nesting. We thus demonstrate the existenc...

Access-Limited Logic (ALL) is a language for knowledge representation which formalizes the access limitations inherent in a network structured knowledge-base. Where a deductive method such as resolution would retrieve all assertions that satisfy a given pattern, an access-limited logic retrieves all assertions reachable by following an available access path. The time complexity of inference is thus a polynomial function of the size of the accessible portion of the knowledge-base, rather than the size of the entire knowledge-base. Access-Limited Logic, though incomplete, still has a well defined semantics and a weakened form of completeness, Socratic Completeness, which guarantees that for any query which is a logical consequence of the knowledge-base, there exists a series of queries after which the original query will succeed. We have implemented ALL in Lisp and it has been used to build several non-trivial systems, including versions of Qualitative Process Theory and Pearl's probability networks. ALL is a step toward providing the properties-- clean semantics, efficient inference, expressive power-- which will be necessary to build large, effective knowledge

. An anytime family of propositional reasoners is a sequence R 0 ; R 1 ; : : : of inference relations such that each R i is sound, tractable, and makes more inferences than R i\Gamma1 , and each theory has a complete reasoner in the family. Anytime families are useful for resource-bounded reasoning in knowledge representation systems. Although several anytime families have been proposed in the literature, only one of them has been provided with a model-theoretic semantics. We present model-theoretic semantics for a new anytime family of reasonersthat are basedon Boolean Constraint Propagation. For proving soundness and completeness results, we develop a new knowledge compilation technique called vivification. This allows us to obtain logically equivalent theories for which Boolean Constraint Propagation efficiently makes all logical inferences. 1 INTRODUCTION Since deductive reasoning is intractable for propositional knowledge representation systems, several tractable approaches for ...

The Spatial Semantic Hierarchy and its predecessor the TOUR model are theories of robot and human commonsense knowledge of large-scale space: the cognitive map. The focus of these theories is on how spatial knowledge is acquired from experience in the environment, and how it can be used effectively in spite of being incomplete and sometimes incorrect. This essay is a personal reflection on the evolution of these ideas since their beginning early in 1973 while I was a graduate student at the MIT AI Lab. I attempt to describe how, and due to what influences, my understanding of commonsense knowledge of space has changed over the years since then. 1 Prehistory I entered MIT intending to study pure mathematics. I was generally steeped in the ideology of pure mathematics, and I had every intention of staying completely away from practical applications in favor of abstract beauty and elegance. However, on a whim, in Spring of 1973 I took Minsky and Papert’s graduate introduction to Artificial Intelligence. I was immediately hooked. I had always been fascinated by the idea

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