A Description of Semantic Networks

In this section we shall describe in more detail the major features of semantic networks. Let us begin with the simple network figure 6.3. below. It is admittedly not much of a network, as it expresses only one simple relation, which is that the the colour of canaries is yellow. The network is made up of two nodes and a link between them, showing that the nodes are related in some way:


Figure 3

The nodes might be taken to represent concepts, or perhaps sets of individuals, or certain other kinds of entity; the differences between these are important, and we shall discuss them in section 6.2.4. For the time being, we shall informally interpret them as representing concepts.

The nature of the relation is specified by the label on the link and by the left-to-right direction of the arrow. A reading of the network in the direction of the arrow might be something like: "A canary has a colour property, which is that it is yellow". We might alternatively see the link as pointing out a particular kind of feature or attribute of canaries, and the node at the termination of the link as the value of that feature or attribute. If the arrow on the link were pointing the other way, of course, the meaning of the network would be "The colour of yellow is canary", which is nonsense. So it it quite important that we use directed links in our networks.

In addition to giving a link a simple word as a label, we may wish to show that the concept used to label the link also has some relation to other concepts, and is thus also a node. For example, we might wish to show that there are different ways in which a thing may have a colour: it may have that colour only on the surface, such as the yellow plumage of a canary, or it may have that colour all through, such as the yellowness of butter. The network below shows this distinction, by expanding the link between 'canary' and 'yellow' to show that a canary has feathers which in turn have a colour, and that the value of that colour is yellow. We also add information about a particular canary, Tweetie; note that here we use an INST link, to indicate that Tweetie is an individual canary, an instance of the class, (rather than a 'kind of' or 'subset of' canary):


Figure 4.

This network now represents the fact, not simply that canaries are yellow, but that Tweetie is a canary, and that canaries have yellow feathers. Importantly, it also allows us to distinguish, at a conceptual level, the difference in meaning between superficially similar statements such as "Canaries are yellow" and "Butter is yellow".