Semantic network

The main goal of the UNL is to represent, in a machine-tractable format, natural language meaning, i.e., the information conveyed by natural language documents. In the UNL framework, this information is represented by a semantic network, a network which represents semantic relations between concepts. This semantic network, or UNL graph, is made of three different types of discrete semantic entities: Universal Words, Universal Relations and Universal Attributes. Universal Words, or simply UW's, are the nodes in the semantic network; Universal Relations are arcs linking UW's; and Universal Attributes are used to delimit the use of UW's. This three-layered representation model is the cornerstone of the UNL, and a distinctive feature over other semantic networks, which normally propose only two levels: edges and vertices.

However, this three-layered representation poses several problems to the UNLization as the distinction between what is supposed to be represented by each unit is not always clear. One difficulty concerns what is to be represented as a UW (i.e., as a node in the UNL graph) and what is to be represented as a relation between UW's. How many UW's are there, for instance, in the sentence "Charles Dickens was the author of Oliver Twist"? Should "author" be represented as a UW or as a relation between "Charles Dickens" and "Oliver Twist"? Should the verb "to be" be represented as a UW or as a relation between "Charles Dickens" and "author"? Should the preposition "of" be represented as a UW or as a relation between "author" and "Oliver Twist"?

Given the difficulty to clearly define the concept of "concept", the UNL assumes the following principles:

1. If the information can be conveyed, in any language, by inflectional affixes, it is represented by attributes;
2. Otherwise, if the information can be conveyed, in any language, by open lexical categories (nouns, adjectives, adverbs and verbs), it is represented by UW's;
3. Otherwise, the information is represented by relations.

Let's consider, for instance, the case of "Charles Dik