Introduction to semantic primitives
by Beate Grawemeyer
School of Cognitive and Computing Sciences
The University of Sussex
The first step in developing a Knowledge Based System is the selection of a proper representation formalism. The next important design question examines the concepts which are to be defined, such as the definition of predicates, functions and constants use and meaning, in a logical knowledge representation. For example in the application of frames, individual frames and their attributes have to be specified.
Many of the concepts to be defined, can be reduced to other concepts already defined. 'Running' for example, might be defined as a 'quick movement of a person by foot'. Semantic primitives are the most basic concepts, which can not be reduced to other concepts.
During the development of a knowledge base for a particular domain a set of such knowledge units must be stated. A default set of basic knowledge units, the semantic primitives, are used to build more complex units, which are independent of one-other.
The selection of primitive elements for a presentation of knowledge
in a particular domain might be seen as a basic problem, which is independent
of the representation formalism (for example, predicate logic, frames,
semantic networks, production rules or procedures). The selection of primitives
determines the expressiveness and the problem solving abilities of the
There exists several approaches (Wilks, Schank), in the area of the development of natural language processing systems, which have tried to use a basic set of primitives, to represent the meaning of any natural language sentence.
Wilks states a set of around 80 concepts as semantic primitives as a basis for the development of a mechanical translation system. He divided these concepts into five classes. Some examples of such primitives are:
The use of such semantic primitives concerning the representation of the meaning of natural language predicates does not solve the problem of ambiguity and uncertainty in natural language.
There also does not exist a 'correct' set of primitives for a particular language or domain. The proof of the adequacy of such a primitive set can only be done operational or experimental. The adequacy of a primitive set will be shown, if it is possible to present the knowledge in terms of the primitives, in a simple and meaningful way, and to solve the range of possible problems which the task may involve.
Roger Schank, who also deals with the development of natural language processing systems, tried to identify a small number of primitives, which would make it possible to represent every possible action. While Wilks work concentrated on the area of language translation, Schanks aim was to develop a domain and task independent basis for knowledge representation. Schank views this approach as an intuitive theory of human language processing.
Schanks aim was to produce a knowledge representation formalism, which when interpreted, was free of any ambiguity and uncertainty. To achieve this aim Schank defined the following eleven primitive actions (ACTs):
In addition to the primitive actions (ACTs) of the CD-Theory there are other concept categories, which allow other dependency structures to be built:
For example the sentence: ' I gave the man a book '
The Symbols of this example have the following meaning:
A disadvantage of the CD-Theory might be seen in the fact, that the
whole knowledge has to be slit into several small units. This is often
very inefficient and sometimes impossible.
When selecting knowledge representation formalism, the primitive concepts (lowest level) must be stated, as these are responsible for building higher level concepts. The selection of primitives determines the abstraction level, the detail facility and the problem solving ability. Through the selection of a primitive it will be stated how detailed knowledge will be described, is influenced by the kind of problems that has to be solved.
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|© 1999 b.grawemeyer|