Stack of two towers

Hi, following some leads in the blogsophere I came across a paper  
entitles "Semantic Web Architecture: Stack of two Towers?"[1].

I have a not had time to look at this in depth yet, and probably  
don't understand a few of the pieces of the puzzle. They are basing  
their argument on the difference between the open and closed world  
assumptions, and how this leads us to conclude different things from  
the same facts.

[[
Given an ontology containing only a single RDF triple:

     <#pat> <#knows> <#joe>

the answer to a query asking if pat knows exactly one person would be  
“no” under
RDF’s open world semantics, but “yes” under the closed world  
semantics of Datalog.
]]

Now is this really a problem or is this one of these puzzles that can  
be unraveled by
just noting a distinction?

My thought on the following is that perhaps this is a problem that  
disappears as soon
as one allows as N3 does, to speak of named graphs also known as  
labeled formulae - one
needs not triples but 4-tuples.

First to avoid ambiguity, let's change the example to something that  
does not itself involve
named graphs. Knowledge, belief, desire, etc are all known in  
philosophy as propositional
attitudes: ie they relate a knower to a statement or set of  
statements (named graphs). So to
have an example that uses those terms, though perfectly reasonable,  
is just going to make
it confusing to make the points that follow.

So let me take the following statement that limits its elements to  
the world of objects
(things that can be expressed clearly with triples and only triples)

     S: <#pen>  <#is_in> <#pocket>

Now the closed world assumption is a logic limiting itself to the  
deductions that follow from a graph, as if the graph contained all  
the facts in the world. So making deductions in a closed
world always makes reference (sometimes implicity) to the graph in  
which the question was posed.
We therefore don't really have one question which has two answers,  
one given a open world assumption, and one given a closed world  
assumption, rather we have two questions:

   OW) Is there more than one pen in the pocket?
   CW) Does the graph S contain information about there being more  
than one pen in the pocket?

clearly each of these questions have different answers. I suppose to  
OW one would answer don't
know, there could be more, whereas to CW one would answer no, there  
is no more information there.
Both of these questions could clearly use the same ontology, one  
allowing more than one pen to be
in a pocket.

Henry Story









[1] http://morenews.blogspot.com/2005/06/two-semantic-webs.html

Received on Thursday, 16 June 2005 10:16:09 UTC