summary: acyclic class hierarchy

From: Frank van Harmelen (
Date: 08/28/01

For what it's worth, here is my summary/understanding of what we could say to the RDF Core group in response to their proposal to enforce acyclicity of subClassOf.


1. in DAML+OIL, subclass-relations can be inferred even if they are not explicitly stated (note that this is an important difference from RDF-S, where A is only a subclass of B if >*and only if*< there is an explicit statement to that effect). 

2. as a result of point 1, acyclicity of the subclass-relation in DAML+OIL cannot always be detected by purely syntactic means, since computationally expensive inference may be needed to detect implicit ("implied") subclass-relations

3. because of point 2, DAML+OIL cannot require acyclicity of the subclass-relationship, since that requirement could only be enforced at great computational expense.

4. Point 3 means that if RDF-S will enforce acyclicity of rdfs:subClassOf, then DAML+OIL can no longer use rdfs:subClassOf. In other words, DAML+OIL will be forced to introduce daml:subClassOf

5. Point 4 would mean that much (all?) backward compatability between RDF-S and DAML+OIL would disappear: an RDF-S processor would be unable to catch any of the semantics of a DAML+OIL ontology (whereas currently, all the explicitly stated subclass relationships in a DAML+OIL ontology are accessible to an RDF-S agent, since DAML+OIL uses rdfs:subClassof

6. An important design rationale behind DAML+OIL (and in fact much other stuff on the Semantic Web so far) has been a layered approach, where languages are stacked on top of each other, with as much partial interpretation between the layers as possible. Tim BL has even argued in [1], [2], [3] and many other places that such "partial understanding is an essential design principle of the Web in general, Semantic or not. 

The decision to make rdfs:subClassOf acyclic will force DAML+OIL to introduce daml:subClassOf, and will therefore lead to an almost total loss of partial understanding between these two closely related ontology languages... 


[1] Evovability, WWW7 keynote speach, 
[2] "Web Architecture from 50,000 feet"
[3] "Web Architecture: Extensible languages"

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