summary of reification semantics issues (material for discussion).

Summary of the semantic alternatives for RDF reification.
---------------------------
A reification assertion such as

aaa rdf:subject bbb .

is saying that something is the subject of something else which is an 
RDF triple. There are at least two orthogonal dimensions on which one 
can choose alternatives about what exactly these 'somethings' are.

aaa  might refer to a triple as an abstract grammatical form, or it 
might refer to a particular occurrence of a triple in some actual 
document somewhere. This has been referred to as the 
'statement/stating' difference.

bbb might refer to a piece of syntax - the actual subject 'node', or 
subject token in a document - which occurs in the RDf graph, or it 
might refer to the thing that is considered to be the subject of the 
proposition expressed by that triple. This has been referred to as 
the 'de-dicto/de-re' difference.  This can be dramatized by asking: 
is the rdf:subject of the triple

ex:Mary ex:had ex:littleLamb .

a girl or a uriref  (respectively de dicto or de re)  ?

These two choices are independent, but each influences the 
significance of the other.

Currently, the semantics document recommends that one should 
interpret this vocabulary so that aaa is interpreted as a stating, ie 
referring to a concrete occurrence of a triple rather than an 
abstract form; and that bbb should be interpreted de re, ie as 
referring to Mary rather than to Mary's URIref.

The reasoning behind these choices are as follows.

Statings allows one to associate provenance information with triples 
in a document a document by using reification to talk about the 
triples. This would be impossible (meaningless) if we imposed the 
'statement' interpretation.  Such uses of reification form a large 
class of actual use cases. On the other hand, if someone wants to 
describe the abstract grammatical form of a statement, this can be 
done by using a particular instance of it as an exemplar for the 
abstraction, eg by associating a bnode with the reification by a 
property like  <ex:abstractformOf>.

The de re interpretation allows reifications to use the same 
conventions for  interpreting URIrefs as other triples, so that a 
URIref used as an actual subject node, and the same URIref used as 
the object of an rdf:subject assertion, have the same meaning. In 
contrast, the de dicto interpretation would require a reasoner to 
distinguish these two uses, using some extra-RDF convention such as 
quotation, or else require that subjects and objects of reified 
triples be given distinct URIrefs which are understood in a 'meta' 
sense to refer to the actual URIrefs in the subject and object 
position of the reified triple. Neither of these formal techniques 
have emerged in practice, suggesting that the de re interpretation in 
fact codifies existing intentions of users.

One objection to the de re interpretation is that it does not allow 
for the adequate representation of propositional attitudes such as 
belief. This is controversial (see the discussion of the Russellian 
theory in http://plato.stanford.edu/entries/prop-attitude-reports/) , 
but in any case there is ample experience which suggests that the de 
dicto interpretation would produce other problems with the 
representation of such ideas, and that an fully adequate 
representation of propositional attitudes is unobtainable using 
reification alone.

Pat

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Received on Tuesday, 11 March 2003 18:17:21 UTC