To describe relationships among resources, we need to understand what the relations mean. This semantic perspective is the essence of relationships and explains why the resources are related, relying on information that is not directly available from perceiving the resources. In our Simpson family example, we noted that Homer and Marge are related by marriage, and also by their relationship as parents of Bart, Lisa, and Maggie, and none of these relationships are directly perceivable. This means that “Homer is married to Marge” is a semantic assertion, but “Homer is standing next to Marge” is not.
Semantic relationships are commonly expressed with a predicate with one or more arguments. A predicate is a verb phrase template for specifying properties of objects or a relationship among objects. In many relationships the predicate is an action or association that involves multiple participants that must be of particular types, and the arguments define the different roles of the participants.
We can express the relationship between Homer and Marge Simpson using a predicate(argument(s)) syntax as follows:
is-married-to (Homer Simpson, Marge Simpson)
The sequence, type, and role of the arguments are an essential part of the relationship expression. The sequence and role are explicitly distinguished when predicates that take two arguments are expressed using a subject-predicate-object syntax that is often called a triple because of its three parts: