Message 12303 of the SUO list Subject: Re: W3C approves RDF and OWL as recommendations (original subject header for the discussion but inappropriate here) Date: Mon, 16 Feb 2004 19:10:24 +1000 From: Philippe Martin In-reply-to: msg12300 by Danny Ayers (in answer to msg12299 by Asha) Follow-up: msg12321 by Adam Pease (answered in msg12331 by Philippe Martin) > > Asha: > > 'Being a parent' is merely a relational property, while 'parentage' > > (parenthood, but not the act or process of parenting) is a relation > > of parent to child. Or, more generally, the relational property of > > being a cause is just a monadic reduction of causality, the relation > > of cause to effect. So, instead of classes and relational properties, > > we have rather to speak of classes and their relationships as two > > sorts of distinct entities. > > Danny: > Within OWL DL, yes. Within OWL Full (which encompasses 'generic' RDF) > properties are classes too. ... I do not think that Asha's main point (if I understood it) has been answered. Given the additional precisions in http://www.eis.com.cy/ I think that Asha is referring to the fact that each "role-related notions" such as "parent" (not the parenting process) may be represented by a relation type (e.g. parent) and/or a concept type (e.g. Parent), and that these two categories are exclusive (e.g. in the SUMO, parent would be abstract while Parent would be physical), thus leading to (i) an increase of the complexity of knowledge entering/management (since the notions have to be represented twice and then connected by definitions to permit inference engines to compare the representations using the relation types with the representations using the concept types), and/or (ii) a decrease in knowledge sharing/retrieval possibilities (this is actually the most likely outcome). As a partial solution, in Ontoseek and WebKB-2, certain role-representing concept types may be used in relation nodes, as in the following example in CGLF: [Person:Tom]->(Parent)->[Person:Mary]. In WebKB-2, the constraints are that Parent is a subtype of pm#thing_that_can_be_seen_as_a_relation and that Mary has been declared as instance of Parent. In Ontoseek, if I remember correctly, I think that Mary may simply be instance of Person if Parent has been declared has a role type and is a direct subtype of Person. Assuming that no relation type name parent exists in the ontology, to export the above representation into KIF, such a relation type should be generated and in its signature, the destination type should be Parent or Person. Then, the KIF representation could be: (parent Tom Mary). An extension of this approach that I will probably implement in the medium-term is to permit a signature to be associated to a concept type, e.g. specifying that if Parent is used as a relation, it must connect an instance of Animal to another instance of Animal. E.g. in the FT notation: Parent (Animal,Animal) < Animal; could be converted into the following in KIF+SUMO: (subclass Parent Animal) (instance parent binaryRelation) (domain parent 1 Animal) (domain parent 2 Animal) (=> (parent ?A1 ?A2) (Parent ?A2)) (=> (Parent ?A2) (exists ((?A1 Animal)) (parent ?A1 ?A2))) Such a shortcut can also be used in KIF, e.g. by defining the relation roleTypeDomain in such a way that the following 3 formulas are equivalent to the 6 previous ones: (subclass Parent Animal) (roleTypeDomain Parent 1 Animal) (roleTypeDomain Parent 2 Animal) Yes, that would involve string manipulation but KIF can do that. A relation such as roleTypeDomain cannot be "defined" in RDF+OWL but it could be added as a primitive to OWL. That would address one of the points raised by Asha. However, in my opinion, that would not be sufficient for RDF+OWL to be an adequate language for knowledge representation and sharing since (i) there are still many important constructs (related to contexts, sets, and numerical quantifiers) that can only be represented in (very) ad-hoc ways or not at all (e.g. see the 2nd half of http://www.webkb.org/doc/model/comparisons.html i.e. starting from the section "contextualization"), and as John Sowa pointed out, it is not the role of a "knowledge sharing" language to reduce the expressivity, it is up to each inference engine to select the (parts of the) representations that it can use, and warn about the fact that it ignored some information (or advertize its capabilities, as in the OKBC protocol), (ii) the model and the adopted conventions are low-level (i.e. they lead users to represent the same thing in many different ways, many of which are uncomparable or very hard to compare automatically), prevent very important checks (e.g. subsumption link cycles are allowed because the used subtype links are not "proper subtype" links), and make the conversion/presentation of the RDF representations into high-level languages and controlled Englishs quite awkward, (iii) its XML syntax makes the detection of errors difficult (this syntax is not supposed to be seen but in practice it is, if only by researchers and those who develop RDF-related tools). Philippe