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Re: [jena-dev] Re: Use cases for Reification in RDF Triplestores

From: Seth Russell <seth@robustai.net>
Date: Sat, 11 Jan 2003 10:42:46 -0800
Message-ID: <3E2065A6.20306@robustai.net>
To: Bob MacGregor <macgregor@ISI.EDU>
CC: pat hayes <phayes@ai.uwf.edu>, Dave Reynolds <der@hplb.hpl.hp.com>, www-rdf-interest@w3.org

Bob MacGregor wrote:

> (1) There are some fairly simple things that one would like
> to say in RDF that are not expressible.  Here are some examples:
>
> Suppose we start with a triple that states
> "Fred Martin is 52 (years old)".  Assume that 'S1' is that triple,
> in the abstract sense, not in the sense of a stating.
>
> Here are some things I would like to be able to say about S1:
>
> "The probability of S1 is P"
> "S1 is true at time T1"
> "I disagree with S1"
> "The object value of S1 is wrong.  The correct value is 53
>   (i.e., retract S1 and assert 'Fred Martin is 53')"
> "S1 is important (take note of it)" 


Yes I agree these are not expressable as pure RDF triples.  But they are 
expressable as Quads, which I am proposing as a natural extension to 
triples which would allow us to express things at the level of detail 
that you are requiring above.  

Please see:  <http://robustai.net/mentography/truthofstatement.jpg>

In this diagram I introduce a new property "truthOf".   The range of 
'truthOf' is a triple in some collection of triples (ie a Quad) and the 
domain is a truth value.   I think such an arrow is necessary to talk of 
propositional attitudes with labeled directed graphs.   Statements in 
FOPL languages like KIF talk only of the truthOf their nested 
components.  But, me thinks, truth is just one particular attitude that 
an agent can have twards a statement; your example of  "disagree with" 
is a case in point.  Since I want Quads to be able to express anything, 
we need to make any use of a truth function explicit, not implicit.  

> (2) Some of the WG members appear to be unaware of this defect.
>
> Here is a quote by Seth Russell to that effect (I imagine he is
> not alone): "But I really don't know any practical case where we
> would want to say something about a jena:Triple" (here I'm
> assuming that 'jena:Triple' means 'statement'). 


First let me clear up one thing:  I'm not a member of the WG.  

But I still don't see where you have provided an example of where 
someone would want to say something useful about a Platonic triple.  It 
seems to me that all the examples you gave above are of propositonal 
attitudes twards the stating of the age of John Martin in some context.  

> (4b) Allowing unfettered use of reified statements would
> make it very easy to introduce paradoxes.  Taking (4a)
> into account, if reified statements were constrained
> by the same semantics as nested statements, then I'm guessing
> that the possibility for self-reference, and therefore
> paradox, might be eliminated.  Has this been explored? 


I think that paradoxes can be avoided in Quads by one simple rule: 
 Never allow the truthOf a collection of statements to be discussed 
inside the collection of statements.   

> (5) Some of the WG members appear not to be aware that
> ordinary reification is not merely unsupported, but is
> actively discouraged.
>
> Brian McBride: "there are two concepts - statements and statings and 
> only one
> bit of vocabulary.  The WG, for reasons that Pat Hayes has explained 
> picked
> one for the existing vocabulary, and as Pat also mentioned,  that
> does not preclude you, or anyone else, from defining new vocabulary
> for the missing concept." 


I followed the WG's discussion of reification up to a point.  I made a 
mentograph of the discussion that you might find amusing, if not 
informative.

<http://robustai.net/mentography/reifyRDF_WG_decision.gif>

Seth Russell
http://robustai.net
Received on Saturday, 11 January 2003 13:43:26 GMT

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