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proposal to close pan-01

From: pat hayes <phayes@ihmc.us>
Date: Tue, 1 Jul 2003 22:57:00 -0500
Message-Id: <p0600120cbb27f8f274a2@[10.0.100.7]>
To: w3c-rdfcore-wg@w3.org

Jeff,

with respect to your comment
http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0266.html
archived as pan-01, the working group has decided not to accept your comment.

This message is an attempt to summarize the primary reasons for this 
decision. A full response to your comment would amount to a research 
paper. We will also note a possible way to reconcile your position 
with the current RDF design.

Your message makes a number of points, but they can be summarized by 
the complaint that the RDF (and RDFS) model theory (MT) is 
"non-standard". While you do not define this exactly, I believe that 
I follow your point, and partly agree with you: there is a body of 
work and opinion, which is based on certain semantic and 
architectural norms, and relative to which the RDF design is somewhat 
unusual in certain respects.  Several other commentators have made 
similar points, speaking also from positions which accept these 
norms.  The norms in question vary in some details but they all 
accept the idea that 'normal' or 'standard' descriptive languages 
must be strictly layered in some sense; this is sometimes expressed 
as a sharp distinction between data and meta-data, or as a sharp 
distinction between first-order individuals and second-order sets or 
predicates. In order to avoid getting into details, let me summarize 
these various points of view as the 'layered' vision of a 
metamodelling architecture.

First, an observation. Proponents of a layered architecture sometimes 
assume that layering is somehow inevitable, or that non-layered 
architectures are inherently faulty. While this may be a defensible 
aesthetic judgement, it is important to note that it is not a 
technical one.  The most extreme non-layered theory yet developed is 
probably Aczel's non-well-founded set theory, in which layers are 
provably impossible; and has been shown that this set theory is 
relatively consistent with traditional ZF set theory; in other words, 
'layering' is not required for consistency, and a lack of layering 
has nothing to do with avoiding the classical set-theoretic 
paradoxes.  Aczel's results have been the source of much recent work 
in the foundations of mathematics and philosophy of language and 
meaning, by the way, and as he is a professor at Manchester 
http://www.maths.man.ac.uk/DeptWeb/Groups/Logic/LogicResearch.html 
you should find it easy to find out more: see for example 
http://www.cs.man.ac.uk/~petera/LogicWeb/settheory.html.  As a small 
contribution to this, recent work in the common logic initiative 
(included in the form of the Lbase language in the RDF MT appendix 
http://www.ihmc.us/users/phayes/RDF_Semant_Edit_Weak.html#Lbase) has 
shown that traditional first-order logic does not actually require 
the syntactic layering traditionally associated with FO languages, 
and that simply re-interpreting traditional Tarskian semantics 
relative to Aczel's set theory produces an 'unlayered' version of FO 
logic which maps transparently into the (now classical) 'holds/app' 
encoding of a "higher-order" logical syntax into traditional FO 
logic, a trick which as I expect you know has been widely used 
throughout applied KR work for about two decades or more. I mention 
all this only as background to the more essential point, however.

The key point about the RDF design is that it *allows* the use of 
layering but does not *require* it. This is one aspect of a critical 
design decision taken early in the RDF project: that RDF should 
impose as few restrictions as possible on what can be said in it or 
how it can be used. It is not intended that RDF should be an example 
of any particular 'style' of metamodelling or knowledge 
representation: as far as possible, it should be permissive rather 
than instructive.  This aim has been our guide in a number of design 
decisions, and we feel that to impose the 'layered' architecture that 
you espouse would be too restrictive, particularly when the current 
design does not prevent RDF users or applications from sticking to a 
layered discipline if they wish to do so (more on this point later) 
and since the technical work mentioned above has shown that there is 
no *technical* need or requirement to adopt the layering assumptions.

To turn to the particular points you raise in your message: all of 
the numbered problems you mention seem to refer to issues which have 
arisen in the layering of OWL onto RDF. As I expect you know, these 
issues have all been resolved by the Webont working group.  I confess 
myself unable to fully follow the critique of the Lbase proposal you 
offer in the message. If you feel there are any errors in the latest 
version of the Lbase translation of the RDF semantics, please point 
them out.

----

The discussions surrounding the relationship between OWL-DL (which is 
layered) and OWL-Full (which is not) have led to Jeremy Carroll 
formulating a set of detailed conditions on an OWL/RDF graph which 
suffice for it to be a legal OWL-DL document. These involve providing 
a labelling of the non-RDFS vocabulary into individual, class and 
property URIrefs and some conditions on the occurrence of blank nodes 
in various contexts. It has occurred to several of us that these same 
conditions could be applied to arbitrary RDF graphs and might provide 
a useful way to recognize that an RDF graph was 'layered' in the 
sense that you might accept as more "standard". We have not checked 
what relationship, if any , there might be between the Carroll 
algorithm and the design of RDFS(FA), but mention this work only to 
suggest that there might be a useful embedding of your preferred 
architectural style into the RDFS design; for if so, it would seem 
that RDF is able to serve its primary design aim of being a universal 
medium of data interchange even for users who prefer a layered 
architecture.

----------

Please reply to this message, copying www-rdf-comments@w3.org, 
indicating whether this response adequately addresses your comment.

Sincerely

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Received on Tuesday, 1 July 2003 23:57:03 EDT

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