Re: owl:sameAs - Harmful to provenance?

On 4 April 2013 11:58, David Booth <david@dbooth.org> wrote:

> On 04/02/2013 05:02 PM, Alan Ruttenberg wrote:
>
>> On Tuesday, April 2, 2013, David Booth wrote:
>>     On 03/27/2013 10:56 PM, Pat Hayes wrote:
>>         On Mar 27, 2013, at 7:32 PM, Jim McCusker wrote:
>>
>>             If only owl:sameAs were used correctly...
>>
>>         Well, I agree that is a problem, but don't draw the conclusion
>> that
>>         there is something wrong with sameAs, just because people keep
>> using
>>         it wrong.
>>
>>     Agreed.  And furthermore, don't draw the conclusion that someone has
>>     used owl:sameAs wrong just because you get garbage when you merge
>>     two graphs that individually worked just fine.  Those two graphs may
>>     have been written assuming different sets of interpretations.
>>
>> In that case I would certainly conclude that they have used it wrong.
>> Have you not been reading what Pat and I have been writing?
>>
>
> I've read lots of what you and Pat have written.  And I've learned a lot
> from it -- particularly in learning about ambiguity from Pat.  And I'm in
> full agreement that owl:sameAs is *often* misused.
>
> But I don't believe that getting garbage when merging two graphs that
> individually worked fine *necessarily* indicates that owl:sameAs was
> misused -- even when it appears on the surface to be causing the
> problem.  Here's a simple example to illustrate.
>
> Using the following prefixes throughout, for brevity:
>
>   @prefix :    <http://example/owen/> .
>   @prefix owl: <http://www.w3.org/2002/07/**owl#<http://www.w3.org/2002/07/owl#>>
> .
>
> Suppose that Owen is the URI owner of :x, :y and :z, and Owen
> defines them as follows:
>
>   # Owen's URI definition for :x, :y and :z
>   :x a :Something .
>   :y a :Something .
>   :z a :Something .
>
> That's all.  That's Owen's entire definition of those URIs.
> Obviously this definition is "ambiguous" in some sense.  But as
> we know, ambiguity is ultimately inescapable anyway, so I have
> merely chosen an example that makes the ambiguity obvious.
> As the RDF Semantics spec puts it: "It is usually impossible
> to assert enough in any language to completely constrain the
> interpretations to a single possible world".
>
> Arthur, an RDF author, publishes the following graph, G1,
> making certain assumptions about the interpretations that will
> be applied to it:
>
>   # G1
>   :x owl:sameAs :y .
>
> Aster, another RDF author, publishes the following graph, G2,
> making certain other assumptions about the interpretations
> that will be applied to it:
>
>   # G2
>   :x owl:differentFrom :z .
>
> Alfred, a third RDF author, publishes the following graph, G3,
> making still other assumptions about the interpretations that
> will be applied to it:
>
>   # G3
>   :y owl:differentFrom :z .
>
> Note that G1, G2 and G3 are all individually consistent with
> Owen's URI definition.  Furthermore, G1, G2 and G3 are all
> pair-wise consistent: there exists at least one satisfying
> interpretation for the merge of each pair.  But the merge
> of G1, G2 and G3 is not consistent: Arthur, Aster and Alfred
> made different assumptions about the set of interpretations
> that would be applied to their graphs, and the intersection
> of those sets was empty.
>
> Did Arthur misuse owl:sameAs?   What if Aster never
> published G2?  How could Aster's graph possibly affect the
> question of whether *Arthur* misused owl:sameAs?  It would
> be nonsensical to assume that it could.  What if Owen later
> said that Arthur was correct, that :x == :y ?  What if he
> later said the opposite?  Again, it would seem rather bizarre
> to say that the determination of whether Arthur had misused
> owl:sameAs could be changed -- long after Arthur had written
> G1 -- by Owen's later statements.
>
> One might claim that Arthur misused owl:sameAs because Owen
> had not specified whether :x was the same or different from
> :y or :z, and therefore Arthur had improperly *guessed* about
> the value of :x's owl:sameAs property.
>
> But by that logic, Arthur would not be able to assert *anything*
> new about :x.  I.e., Arthur would not be allowed to assert
> any property whose value was not already entailed by Owen's
> definition!  And that would render RDF rather pointless.
>
> Maybe someone can see a way to avoid this dilemma.  Maybe
> someone can figure out a way to distinguish between the
> "essential" properties that serve to identify a resource, and
> other "inessential" properties that the resource might have.
> If so, and the number of "essential" properties is finite,
> then indeed this problem could be avoided by requiring every
> URI owner to define all of the "essential" properties of the
> URI's denoted resource, or by prohibiting anyone but the URI
> owner from asserting any new "essential" properties of the
> resource (beyond those the URI owner had defined).  Or maybe
> there is another way around this dilemma.
>
> Unless some way around this dilemma is found, it seems
> unreasonably judgemental to accuse Arthur of misusing
> owl:sameAs in this case, since he didn't assert anything
> that was inconsistent with Owen's URI definition


+1

This is a great explanation of the issue.

Peter