Meta-Level Knowledge

This chapter explores a number of issues involving representation and use of what we term meta-level knowledge, or knowledge about knowledge.1 It begins by defining the term, then exploring a few of its varieties and considering the range of capabilities it makes possible. Four specific examples of meta-level knowledge are described, and a demonstration given of their application to a number of problems, including interactive transfer of expertise and the "intelligent" use of knowledge. Finally, we consider the long-term implications of the concept and its likely impact on the design of large programs. The context of this work is the TEIRESIAS program discussed in Chapter 9. In the earlier chapter we focused on the use of TEIRESIAS for knowledge acquisition. Here we focus on the classification and types of knowledge used by TEIRESIAS. In the most general terms, meta-level knowledge is knowledge about knowledge. Its primary use here is to enable a program to "know what it knows," and to make multiple uses of its knowledge. As mentioned in Chapter 9, the program is not only able to use its knowledge directly, but may also be able to examine it, abstract it, reason about it, or direct its application. This chapter discusses examples of meta-level knowledge classified along two dimensions: (i) specificity character (representation-specific vs. domain-specific), and (ii) source (user-supplied vs. derived). Representation-specific meta-level knowledge involves supplying a program with a store of knowledge dealing with the form of its representations, in particular, their design and organization. Traditionally, this design and organization infor-This chapter is an expanded and edited version of a paper originally appearing in Proceedings of the Fifth IJCAL 1977, pp. Used by permission of International Joint Conferences on Artificial Intelligence, Inc.; copies of the Proceedings are available from William Kaufmann, Inc., 95 First Street, Los Altos, CA 94022. IFollowing standard usage, knowledge about objects and relations in a particular domain will be referred to as object-level knowledge. Type declarations are a small step toward more explicit specification of this information, especially as they are used in extended data types and record structures. As we discuss below, this sort of information, along with a range of other facts about representation design, can be employed quite usefully if it is made explicit and made available to the system.