Grigoris Antoniou and Frank van Harmelen 1. a well-defined synta 2. a well-defined semantics 3. efficient reasoning support 4. sufficient expressive power The importance of a well-defined syntan is clear, and known from the area of programming languages: it is a necessary condition for machine-processing of information. All the languages we have presented so far have a well-defined syntax. DAML+OIL and Ow build upon RDF and RDFS and have the same kind of syntax. Of course it is questionable whether the XML-based RDF syntax is very ser-friendly, there are alternatives better suitable for humans(for example, see the OIL syntax). However this drawback is not very significant, because ultimately users will be developing their ontologies using authoring tools, or more generally ontology development tools, instead of writing them directly in DAML+OIL or OWl. Formal semantics describes precisely the meaning of knowledge. " Pre- cisely" here means that the semantics does not refer to subjective intuitions nor is it open to different interpretations by different persons(or machines) The importance of formal semantics is well-established in the domain of math ematical logic, among others edge. For ontological knowledge we may reason about. ason about the knowl- One use of formal semantics is to allow humans to re Class membership: If r is an instance of a class C, and C is a subclass of D, then we can infer that r is an instance of d Equivalence of classes: If class A is equivalent to class B, and class B equivalent to class C, then A is equivalent to C, too Consistency: Suppose we have declared a to be an instance of the class A. Further suppose that A is a subclass of b∩C A is a subclass of D B and D are disjoint Then we have an inconsistency because A should be empty, but has the instance a. This is an indication of an error in the ontology Classification: If we have declared that certain property-value pairs are sufficient condition for membership of a class A, then if an individual a satisfies such conditions. we can conclude that r must be an instance of Semantics is a prerequisite for reasoning support: Derivations such as the above can be made mechanically, instead of being made by hand. Reasoning support is important because it allows one to check the consistency of the ontology and the knowledge2 Grigoris Antoniou and Frank van Harmelen 1. a well-defined syntax 2. a well-defined semantics 3. efficient reasoning support 4. sufficient expressive power 5. convenience of expression. The importance of a well-defined syntax is clear, and known from the area of programming languages; it is a necessary condition for machine-processing of information. All the languages we have presented so far have a well-defined syntax. DAML+OIL and OWL build upon RDF and RDFS and have the same kind of syntax. Of course it is questionable whether the XML-based RDF syntax is very user-friendly, there are alternatives better suitable for humans (for example, see the OIL syntax). However this drawback is not very significant, because ultimately users will be developing their ontologies using authoring tools, or more generally ontology development tools, instead of writing them directly in DAML+OIL or OWL. Formal semantics describes precisely the meaning of knowledge. “Pre￾cisely” here means that the semantics does not refer to subjective intuitions, nor is it open to different interpretations by different persons (or machines). The importance of formal semantics is well-established in the domain of math￾ematical logic, among others. One use of formal semantics is to allow humans to reason about the knowl￾edge. For ontological knowledge we may reason about: • Class membership: If x is an instance of a class C, and C is a subclass of D, then we can infer that x is an instance of D. • Equivalence of classes: If class A is equivalent to class B, and class B equivalent to class C, then A is equivalent to C, too. • Consistency: Suppose we have declared x to be an instance of the class A. Further suppose that - A is a subclass of B ∩ C - A is a subclass of D - B and D are disjoint Then we have an inconsistency because A should be empty, but has the instance x. This is an indication of an error in the ontology. • Classification: If we have declared that certain property-value pairs are sufficient condition for membership of a class A, then if an individual x satisfies such conditions, we can conclude that x must be an instance of A. Semantics is a prerequisite for reasoning support: Derivations such as the above can be made mechanically, instead of being made by hand. Reasoning support is important because it allows one to • check the consistency of the ontology and the knowledge;