We describe a knowledge representation and inference formalism, based on an intensional propositional semantic network, in which variables are structured terms consisting of quantifier, type, and other information. This has three important consequences for natural language processing. First, this leads to an extended, more "natural" formalism whose use and representations are consistent with the use of variables in natural language in two ways: the structure of representations mirrors the structure of the language and allows re-use phenomena such as pronouns and ellipsis. Second, the formalism allows the specification of description subsumption as a partial ordering on related concepts (variable nodes in a semantic network) that relates more general concepts to more specific instances of that concept, as is done in language. Finally, this structured variable representation simplifies the resolution of some representational difficulties with certain classes of natural language sentences...

One of the fundamental problems in the theory of knowledge representation is the difficulty of achieving both logical coherence and computational tractability. We present steps toward a theory of access-limited logic, in which access to assertions in the knowledgebase is constrained by semantic network style locality relations. Where a classical deductive method or logic programming language would retrieve all assertions that satisfy a given pattern, an access-limited logic retrieves all assertions reachable by following an available access path. The complexity of inference is thus independent of the size of the knowledge-base and depends only on its local connectivity. Access-Limited Logic, though incomplete, still has a well defined semantics and a weakened form of completeness (`Socratic Completeness') and is complete in some important special cases. 1 Introduction Access-Limited Logic (ALL) is a logic for knowledge representation which utilizes semantic network style access limit...

NUMERICAL -- REAL -- PHYSICAL -- NON-STATIONARY -- COLLECTIVE TEMPORAL -- RELATIONAL -- EVENT -- Table 1.

In this paper we describe a method of classifying facts (information) into categories or levels; where each level signi es a di erent degree of syntactic complexity related to a fact. Based on this classi cation mechanism, we also propose a method of evaluating a domain by assigning to it a \domain number" based on the levels of a set of standard facts present in the articles of that domain.

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

We define a knowledge representation and inference formalism that is well suited to natural language processing. In this formalism every subformula of a formula is closed. We motivate this by observing that any formal language with (potentially) open sentences is an inappropriate medium for the representation of natural language sentences. Open sentences in such languages are a consequence of the separation of variables from their quantifier and type constraints, typically in the antecedents of rules. This is inconsistent with the use of descriptions and noun phrases corresponding to variables in language. Variables in natural language are constructions that are typed and quantified as they are used. A consequence of this is that variables in natural language may be freely reused in dialog. This leads to the use of pronouns and discourse phenomena such as ellipsis involving reuse of entire subformulas. We present an augmentation to the representation of variables so that variables are ...

Access-Limited Logic (ALL) is a language for knowledge representation which formalizes the access limitations inherent in a network-structured knowledge base. Where a classical deductive method or logic programming language would retrieve all assertions that satisfy a given pattern, an access-limited logic retrieves all assertions reachable by following an available access path. The complexity of inference is thus independent of the size of the knowledge-base and depends only on its local connectivity. 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. This chapter presents an overview of ALL, and sketches the proofs of its Socratic Completeness and polynomial time complexity. 1 Introduction It has long been a guiding principle in work ...

ion The Kolmogorov complexity of a task, i.e. the size of the smallest Turing machine that achieves some required input-output behavior is not computable. This leaves us with approximations, i.e. machines we are able to construct. In building such machines, we choose an architecture. Even though many architectural styles are possible, in our case, with the experience of building NLP systems as knowledge based programs, it is natural to assume that they can be decomposed into the main control program and the knowledge base. Thus the complexity of the whole system is a function of the complexities of the two parts, the program and the knowledge base. Complexity Management Dialog systems are a good example of the NLP architecture in which the main task is decomposed into the control program and the knowledge base. The main algorithm of such programs is quite simple: at each step the following routine is called Parse one sentence: 1. Read sentence/string. 2. Parse sentence using (a)...

Inheritance networks are a type of semantic network which represent both strict (classical implication) and defeasible (non-classical) relationships among entities. We present an established approach to defeasible reasoning which defines inference in terms of the construction of paths through a network. Much of literature on inheritance is concerned with specifying the most "intuitive" system of path construction. However, when considering a fundamental feature of these approaches---the status accorded to redundant links---we find that topological considerations espoused in the literature are insufficient for determining the valid inferences of a network. This implies that the "intuitiveness" of a particular method depends upon the domain being represented. Though Touretzky has demonstrated that it is unsound in some cases, the path-preference algorithm known as shortest path reasoning, is actually the most intuitive algorithm to use when reasoning about the inheritance network which r...

One of the fundamental problems in the theory of knowledge representation is the difficulty of achieving both logical coherence and computational tractability. We present steps toward a theory of accesslimited logic, in which access to assertions in the knowledge-base is constrained by semantic network style locality relations. Where a classical deductive method or logic programming language would retrieve all assertions that satisfy a given pattern, an access-limited logic retrieves all assertions reachable by following an available access path. The complexity of inference is thus independent of the size of the knowledge-base and depends only on its local connectivity. Access-Limited Logic, though incomplete, still has a well defined semantics and a weakened form of completeness (`Socratic Completeness') and is complete in some important special cases.