On Wed, Jul 27, 2011 at 12:52 AM, Steve Baskauf steve.baskauf@vanderbilt.edu wrote:
[...] Well you could say this about any non-disjoint classes. Why not combine the classes Teacher and ElectedOfficial if a person can be an instance of both? We don't do that because there are instances where persons are Teachers and not ElectedOfficials and other instance where people are ElectedOfficials and not Teachers. Instances of the class Teacher share properties like numberOfStudents and schoolOfEmployment, while instances of ElectedOfficial share properties like votesReceived and yearElected.
OWL supports intersection and union. In OWL the set of things that are both a Teacher and an Elected Official would form a class. It would perhaps have fewer individuals in it than either class, but in OWL it is nevertheless a class. Similarly the set of things that are either a Teacher or an ElectedOfficial, though not necessarily both, is an OWL class. Note that http://code.google.com/p/darwin-sw/source/browse/trunk/dsw.owl is at this writing not an OWL ontology, as you will see if you try to put it into the Manchester OWL validator. Much of that failure is minor syntactic botches, but when you fix them, some issues remain about dsw's secondary goal to "(hopefully) making design choices that do not constrain full SW reasoning downstream".
RDFS does not guarantee that the intersection of two classes is a class, but allows you to avow that in particular cases if you wish, whereas OWL always requires it. More to the point, RDFS allows you to conceptually think of classes and properties as having the same kind of structure, so that you can effectively talk about intersecting properties, e.g. define a property that is true for those individuals for whom two particular properties are true. This is discussed on p. 135 of [1], though you won't like the word "infer" used there. Chapter 8 of [1] introduces a subset RDFS-Plus of OWL that is largely motivated by being content with the expressiveness of SPARQL, which I guess fits in your comfort zone, especially as to "AND". My understanding of LOD is that its community is indeed content with the expressiveness of SPARQL for now, and dsw probably corresponds to the approach of Chapter 8 in a way that meets your primary goals for dsw.
In summary, I suspect that you are struggling about modeling individuals because: a. You believe in your heart that there is a fundamental difference between properties and classes but b. RDFS does not require that of one's models and c. dsw is expressed in RDFS and not in a modeling language that requires you to abandon a.
[1] Dean Alemang and Jim Hendler "Semantic Web for the Working Ontologist", 2nd Edition 2011
Bob