KRS-1(ID:8175/)Knowledge Representation Semanticsfor Knowledge Representation Semantics One KR language based on LISP by Brian Smith References: KRS-l (Smith, 1977), a formal system of Knowledge Representation Semantics, is an initial attempt to provide a coherent intellectual framework within which to understand systems of knowledge representation ? a way to answer -^ question "what does all this mechanism mear.7". The structure of KRS derives from a consideration of what the act of representation means within the context of building computational models of intelligence. Because it is being developed out of an interest not only in knowledge but also in active reasoning processes, the form of the semantic foundation that it provides differs from traditional theories of semantics. Instead of being interested in issues such as completeness, soundness, decidability, etc., KRS instead provides a formal structure in which to talk about such issues as memory chunking, locality of access, focus of description, abstractions, appropriateness of interpretive belief, etc. Specifically, KRS rejects the following three assumptions, that have traditionally been held as axioms of any formal theory of meaning: 1. The idea that statements or expressions by themselves have meaning. Instead KRS formalizes the idea that "meaning" is something which makes sense only in terms of an active process interpreting a system of symbols. 2. The notion that "truth" is appropriate as a primitive semantic concept. This is not at all to say that the concept of truth is not important, but instead to reject its formalisation as a binary and primitive notion, and also to reject the idea that deciding that truth of a sentence is the crucial aspect of uncovering its meaning. The truthfulness of a statement is instead seen to be a complex, subtle, contextual, and often useful description of that statement, which is neither primitive within the semantic theory, nor necessarily expressible in terms of the atomic symbols "TRUE" and "FALSE". 3. The assumption that it is in general possible or appropriate to say anything absolute or certain about the structure of the world being represented. What is considered to be important instead is what people or processes believe; KRS considers the only question that can be asked by a process to be what it believes, and also what it believes that another process believes. The overall framework of KRS-I is a formaliza?ion of 5 "levels" or viewpoints from which to understand a symbolic description, or piece of representational structure: 1. A "message" level, which deals with the words or communication string on its own, without reference to the structure of the process that sent or received it 2. An "intermediate" level, embodying what is traditionally thought of as the syntactic structure of a message. 3. A "memory" level, formalizing notions of organization and accessibility. 4. A "belief" level, dealing with the active conclusions and inferences that an interpretive process will come to, based both on its previous beliefs and goals, and also on the structure of the memory level. This can be thought of as short term memory, although part of KRS-i is an account of how the issues dealt with at this level differ substantially from those of the memory level. 5. An "external" level, capturing the notion that a representation is a representation of something ? this is the level which the interpretive process believes that the memory structures represent. In addition to these levels, KRS also gives a precise account of "layers of meta-description" to characterize the relationship that holds between two descriptions when one describes the other. The substance of KRS is a theory of the structure of, and the relationships between, these levels and layers, and of ?he role that an interpretive process can play within such a framework. (Note that "level" and "layer" are technical words naming t>vo orthogonal dimensions of the semantic framework.) KRS-l. the current version of the theory, deals only with the declarative structure of representational languages; although it identifies the place that an interpretive process must play in such a scheme (indeed formalizes the claim that you cannot understand the meaning of a symbol without understanding the processes that interpret that symbol), no attempt has been made to capture or describe the actual processing structures of an interpretive process. This is the direction towards which further work will be directed. KRS and Natural Language Semantics There is no doubt that the structure of natural language provides significant evidence of the structure of human thought. However KRS is not specifically designed to be a theory of English semantics, for two reasons: . _______________________ 1. KRS-l is an attempt to be a theory of the semantics of computational knowledge representation languages, not of knowledge representation in the abstract. Hence the current objects of study are languages such as KRL, OWL, semantic nets, etc., rather than English 2. As opposed to the philosophy of OWL, there is no effort in KRS to account for specifically linguistic behaviour. For example, the goal of KRS is to make clean and precise all distinctions which seem cogent in identifying and solving subtle problems in terms of reasoning. Just because such a. distinction is not apparent in the structure of English sentences will not be taken as any reason not to formalize the distinction. KRS and a Description of the Interpretive Process: One of the motivations for building a structured model of knowledge representation semantics is to provide a framework within which to describe the behaviour of an interpretive process. In traditional computer languages such as LISP, there is a well-defined and precise notion of evaluation which the interpreter is carefully-designed to implement. However as we build more complex description systems, this strict model of evaluation begins to break down. For example, consider a system such as MACSYMA (Moses. 1974); one of the powers of that system is an ability to reason with symbolic descriptions (such as the "integral of X") without evaluating them. KRS is designed to provide a model of an interpretive process; this model is the same as its characterization of any reasoning system. For example: suppose that an AI program is reasoning about a set of blocks on the top of a table. Suppose also that this program is "connected" to that table lop by a video camera and robotic arm. As the program goes about its business, it builds up internal memory representations about the state of the world on top of the table, develops hypotheses about possible actions that it might take, explores what it thinks the consequences of potential actions would be, etc. Every so often, when it decides that it actually wants to do something, it reaches out and moves a block, or in some way changes the world about which it is reasoning. One can draw a strong analogy between this program, and the interpreter that is running this program. In many ways their operations are very similar, except that where the domain of the blocks program is the table top. the domain of the interpreter is the program and representational structure of the blocks program. One can view it is a double-layered system, with the blocks program looking at the table and the interpreter looking at the blocks program. Where the blocks program reasons about a block partly by having a description of that block, so the interpreter can be seen to reason about the description of the block by having a description of of this description. In other words, the KRS characterization of intepretive process is of" a reasoning system working at the meta layer. in [Bobrow, Daniel G.] A Panel On Knowledge Representation chaired by Daniel G. Bobrow view details |