Hector J. Levesque

Knowledge level interfaces to information systems

The knowledge level view advocates treating knowledge bases (KB) roughly as abstract data types: what is required of a KB is to be completely specified functionally, without regard to how it is implemented. We elaborate on this view, here, by providing detailed knowledge level accounts of several different types of languages for specifying KB. These accounts are expressed mainly in terms of two operations applicable to KB: TELL, which allows a system to tell a KB something about its application domain; and ASK, which allows the system to ask the KB questions about the domain. We also consider some of the important things that can be shown about an information system (a DBMS or a knowledge-base system) once the Knowledge Level description is in hand.

Chapter 11 – Defaults

Publisher Summary This chapter talks about default reasoning in detail and in logical terms and also examines four different logical formalisms forit. . Although each of them does the job in many cases, each formalism has drawbacks of one sort or another. Getting a logical account of default reasoning that is simple and applicable in broad term, and intuitively correct remains an open problem. In fact, because so much of what is to be known involves default reasoning, it is perhaps the open problem in the whole area of knowledge representation. Default Reasoning is based on the assumption that is in general terms known as the closed-world assumption (CWA). Ordinary deductive reasoning is monotonic, that is to say that new facts can only produce additional beliefs. In contrast, default reasoning is nonmonotonic. The simple formalization of default reasoning is considered in the chapter.

Chapter 10 – Inheritance

Publisher Summary This chapter reduces the frames to simple nodes that appear in inheritance networks and expressed by a figure and as a part of discussion it also treats objectlike concepts like Elephant, and properties, like Gray, equivalently as nodes. The simple form of inheritance is the kind used in description logics and other systems based on classical logic named strict inheritance. In a strict inheritance network, conclusions are produced by the complete transitive closures of all paths in the network. Any traversal procedure for computing the transitive closure is expected to do for determining the supported conclusions. In a tree-structured strict inheritance network, inheritance is very simple. In a strict inheritance network that is a directed acyclic graph (DAG), the results are the same as for trees. Inheritance in directed acyclic networks is often called multiple inheritance when a node has more than one parent node; in such cases, because of the meaning of the edges, the node must inherit from all of its parents.

Chapter 3 – Expressing Knowledge

Publisher Summary This chapter outlines the basic principles of knowledge representation and decides an initial representation language. The task of the chapter is to create a knowledge base (KB) that contains appropriate entailments in first-order logic (FOL). In creating a KB it is a good idea to start with the set of domain-dependent predicates and functions that provide the basis for the statement of facts about the KBs domain. The most obvious place to start is with the named individuals who are the actors in human drama. In FOL, these would be represented by constant symbols, like mary Jones, johnQSmith, and so on. The FOL language gives the basic tools for representing facts in a domain, but in many cases there is a great deal of flexibility that can be exercised in mapping objects in that domain onto predicates and functions. Another class of named individuals would be the legal entities that have their own identities, such as corporations (faultylnsuranceCompany), governments (evilvilleTownCouncil), and restaurants (theRackAndRollRestaurant.

Chapter 4 – Resolution

Publisher Summary This chapter examines how to automate a deductive reasoning procedure. At the knowledge level, the specification for an idealized deductive procedure is optimized. The reasoning procedure that is considered in this chapter works on logical formulas in a special restricted form. The procedure to convert any propositional formula to conjunctive normal form (CNF) is also presented in the chapter. It is convenient to use a shorthand representation for CNF. A clausal formula is a finite set of clauses, where a clause is a finite set of literals. The interpretation of clausal formulas is precisely as formulas in CNF. A clausal formula is understood as the conjunction of its clauses, where each clause is understood as the disjunction of its literals and is understood normally. To discuss reasoning at the symbol level, it is common to posit what are called rules of inference, which are statements of what formulas can be inferred from other formulas. The chapter uses a single rule of inference called (binary) Resolution.

Chapter 9 – Structured Descriptions

Publisher Summary This chapter deals with the representation techniques that look more directly towards the aspects of objects and categories than frames did. During focusing on the more declarative aspects of an object-oriented representation the chapter analyzes the concepts such as predicates and entailment from First-order logic (FOL). Traditional first-order logic does not provide any tools for dealing with compound predicates of this sort. In a sense, the only noun phrases in FOL are the nouns. But given the prominence and naturalness of such constructs in natural language, it is worthwhile to consider knowledge representation machinery that does provide such tools. Because a logic that allows to manipulate complex predicates is expected be working mainly with descriptions, that is known as system based on the ideas of description logic (DL). There are four types of legal syntactic expressions in DL including constants, roles (both seen earlier), concepts, and sentences. These expressions are further described in much detail in the chapter.