Characteristics of Literal Symbols

Literal symbols in the ontology are used to stop unending decompositions of meanings. These symbols are used to fill certain slots (namely LITERAL-ATTRIBUTEs) but are not defined in any way in the ontology. Some characteristics of literal symbols worth noting include:

• Literals symbols are used in our representations in much the same way as the qualitative values used in qualitative physics and other areas of artificial intelligence (AI) that deal with modeling and design of physical artifacts and systems (de Kleer and Brown, 1984; Goel, 1992).
• Literal symbols are either binary or constitute approximate positions along an implied scale. For binary values, it is often preferable to use attribute-specific literal symbols rather than a generic pair (such as ``yes'' and ``no'' or ``on'' and ``off''). The primary benefit of doing this is being able to map the lexical entries of corresponding words in a language to these literal symbols.
• Literal symbols are often used when there is no numerical scale in common use in physical or social models of the concerned part of the world. For example, OFFICIAL-ATTRIBUTE has in its range the literals ``official'' and ``unofficial.'' Although one can talk about an event or a document being more official than another, there is no obvious scale that in use in the world for this attribute. Thus the two literals seem to serve well in the range of this attribute.
• It is not always true that scalar and literal attributes correspond to the existence or not of a numerical scale in the physical or social world. A classical example of this is ``color.'' Although several well-defined numerical scales exist in models of physics (such as the frequency spectrum, hue and intensity scales, etc.) such a scale does not serve our purposes well at all. First of all it would make our TMRs more or less unreadable if it has a frequency in MHz in place of literal such as ``red'' or ``green.'' Moreover, it makes lexicon acquisition more expensive; lexicographers will have to consult a physics reference to find out the semantic mapping for the word ``red'' instead of quickly using their own intuitive understanding of its meaning.
• Often values of literal attributes nevertheless will need to be quantified to express the meanings of phrases such as ``light red.'' It is not practical to introduce more and more literals (such as ``light-red'') to solve this problem. As a solution to this, we have introduced two general relations in the ontology called GREATER-THAN and LESS-THAN. Using this along with a reification operation called attribute raising, we can say LESS-THAN ``red'' to indicate a color that is somewhere between red and the previous literal in the list of literals in the range of COLOR.