4.2 Relation type
Definition and examples of relations
A relation type is simply a name which we give to the relation. It tells us what kind of relation we are dealing with. The type also determines the valence of the relation and its signature. That is, the type of the relation determines the number of arcs that belong to it, and the types of the concepts that are attached to those arcs.
For example, all of these are relation types:
A very important relation is "Agnt" or "agent". It is used again and again when we draw conceptual graphs. It relates an act (such as "Sing") and an animate being (such as "Bird") which performs the act. We will use this relation in the next example.
Example using a conceptual graph
Consider this conceptual graph:
This conceptual graph says:"There is a bird which is the agent of Sing. This same bird is in a sycamore tree".
Or, put into better English:"A bird is singing in a sycamore tree".
This graph has two relations, "Agnt" (agent) and "In" (in). The relation type of "Agnt" is "Agnt". Its valence is 2, and it relates an act with an animate being. The relation type of "In" is "In". Its valence is 2, and it relates two physical entities in a spatial relationship.
Note on tense and aspect informationThe tense and aspect of the verbal concept "Sing" are unspecified in the graph. It is a generally accepted idea that graphs are atemporal -- i.e., they do not say anything about tense and aspect, unless explicitly specified. The default interpretation is to interpret a graph as active indicative present tense unless otherwise specified.
There are techniques for specifying both tense and aspect, but we will not be looking at them in this course. The only exception is that we will note that to specify past, we can use the "Past" relation:
(Past)->[Situation: [Sing]->(Agnt)->[Bird]->(In)->[SycamoreTree] ]
This graph has a sub-graph that is embedded in a concept (the graph that is the referent of the concept "Situation"). This structure will be explained later.
To summarize: A relation is defined by its relation type. The relation type not only gives a name to the relation, it also determines the other two ideas associated with relations, valence and signature. We discuss valence next.
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Next: 4.3 Valence