In many programs using semantic nets, unfortunately, the meaning of nodes and links has been interpreted intuitively rather than described rigorously and systematically. If we are using a semantic network to represent an area of knowledge, we ought to be clear about what kinds of things we are depicting in the nodes of our networks and what kinds of relations we are specifying in the links between nodes. That is, we ought to specify the semantics of our formalism. We will consider just two interpretations of nodes and links -- though if you are interested, and are prepared for more advanced reading, you could look at Brachman (1983).
One interpretation of generic nodes (ones that refer to a general class or concept) is as sets. Thus the relation holding between a node at one level of a hierarchy and that at a higher level is that of subset to superset. If, for example, we indicate in our network that a canary is a bird, what we are in effect stating is that the set of all canaries is a subset of the set of all birds This is shown diagramatically in figure 6.4. Everything that lies within circle B is a bird; that is, circle B is the set of all birds. Within circle B lies circle C, which is the set of all canaries. Thus a, c, d, e, and f are all birds, since c and e, while contained by circle C, are also contained by the larger circle. But c and e are also canaries, while a, d, and f, because they lie outside that circle, are not. Whatever b is, on the other hand, it is not a bird, as it lies outside the larger circle.
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Figure 6.4: A set representation of a concept hierarchy.
If the generic node is interpreted as a set, then the relation of nodes representing individuals to generic nodes in the network is that of set membership. Thus, the meaning of the network below
is represented in figure 6.4 by the relationship of c or e to the circle C.
A second interpretation of generic nodes is as prototypes, and of instance nodes as individuals for whom the generic nodes provide a prototypic description. Rather than describing an individual or picking out a class of individuals, a prototype describes what the typical exemplar of the class is like, in terms of physical characteristics, behaviour, diet, habitat, and so on. Individuals will then be more or less like the prototype. Thus, for example, mammals are prototypically thought of as land-animals which give birth to live young; a cow or a rabbit is a typical mammal in these respects, though a dugong is atypical on the first count, a platypus on the second. Canaries prototypically are yellow and can sing, so we recognize Tweetie, who is yellow and can sing, as a fairly typical instance of what we imagine canaries to be. A detailed model of human concept organization, based on prototypes, has been proposed by Rosch (1983).
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