> reading: Chapter 4 "Incorporating Semantics", Semantic Web Programming, J. Hebeler et al.
Open World and Closed World Assumption
Open world assumption (OWA) means that information that is not present in a certain domain is not considered to be false. Just because it's not there, does not mean it's wrong.
Closed world assumption (CWA) is obviously the opposite. When there is a piece of information missing, it means it is wrong.
For example: "John works with Paul".
With CWA we couldn't say whether John and Paul know each other. We would have to assume that they don't. There are just working together. We would have to answer "no" to the question, whether they know each other.
With OWA the information about whether they know each other or not is not stated. So we just can not say anything about it, whether it's true that they're working together or not. But we couldn't say it is false either.
No unique Names Assumption
In the WWW we can not assume that information is always identified by an unique name. When two web sites referencing to a single information, we can not assume both are using the same URIs to reference to this information. Unless the two are explicitly stating that the information they are talking about are the same (by adding additional attributes) ....... wait something is confusing! need to think about that again
Ontology Elements
- Header
- represents the ontology itself
- contains comments, labels, version and other imported ontologies
- Class
- special kind of resource that represents a set of resources
- Individual
- member of a class
- can be member directly or indirectly
- Property
- predicate to describe individuals
- object properties link individuals to other individuals
- data properties link individuals to literal values
- Annotation
- basically like a Property
- has no associated semantics
- commonly used for label or comment
- Datatype
- well data type