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All,
I am sharing this thread from the Schema.org forum so at to shed
light on confusion that tends to arise in regards to RDF (language
for creating structured data with varying degrees human and
machine comprehensible entity relations semantics) and OWL.
John Sowa:
When I make reference (for instance) to practical applications
(e.g., inference and reasoning in regards to heterogeneous data
integration etc.) of the Semantic Web via the Linked Open Data
cloud, its very much in line with the worldview expressed by
Holger.
Please read on...
On 5/8/14 1:31 AM, Holger Knublauch wrote:
Simon,
On 5/7/2014 6:07, Simon Spero wrote:
There are a number of reasons why the
vocabularies at http://qudt.org/ are
not legal OWL or RDF.
They are perfectly legal RDF, but indeed not legal OWL DL. As
stated elsewhere, not being OWL DL compliant is not necessarily a
bad thing.
Issue
1: You can’t use user defined datatypes as the
type of a literal
Yes in OWL, but in RDF this is perfectly valid, see
http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#dfn-recognized-datatype-iris
"Semantic extensions of RDF might choose to recognize other
datatype IRIs..."
In the case of QUDT I would even argue that it is of great value
to create literals such as "4.2"^^qudt:Meter that are a much more
meaningful information exchange format than "4.2"^^xsd:decimal. Of
course OWL DL would prohibit this, but OWL DL tools can probably
run such literals through a normalizer algorithm that also
converts feet to meters etc. Oh but I forgot... OWL cannot even be
used to check if a > b or a + b so nevermind.
There are no reasoners for OWL 2 Full.
Not true. For example the TopSPIN engine (part of TopBraid
Composer Free Edition) can run rule on any RDF graph, even with
user-defined datatypes etc. It's just a matter of writing the
right rules. Unless of course you mean "Reasoner" = "OWL DL
Reasoner", which is not my understanding. On a general discussion,
see
http://composing-the-semantic-web.blogspot.com.au/2010/04/where-owl-fails.html
Corollary 3:
The QUDT ontologies cannot be edited or
processed using OWL tooling.
Not true, they were edited with TopBraid Composer, which is an OWL
tool (and also an RDF tool).
Why
this matters
The
RDF to OWL parsers in OWLAPI (used by
tools like Protegé) will try their best
Proper RDF based tools based on APIs such as Jena or Sesame have
no problems with these files. You keep coming back to OWL DL
compliance, and I keep coming back to the fact that it was an
(unfortunate) design decision by the OWL 2 community and the OWL
API to abandon the RDF roots. If these design decisions now fire
back into RDF and OWL people want to enforce their world view upon
RDF graphs only because their tools cannot handle RDF, then we
need to decisively make clear what the source of the issue is.
And to be very clear: the OWL DL community has way too long
dominated ontology development. There is already sufficient value
in the basic principles of RDF such as URIs and the ability to
link anything with everything, and the adoption of these simple
principles is hindered by an overly complex layer cake of features
and constraints that few people really need. A generation of
graduates leaves their universities with the impression that the
semantic web is OWL DL, and early adopters of this technology in
industry are overwhelmed by the complexity of the whole stack. RDF
and SPARQL is really all that is needed to know, and with SPARQL's
CONSTRUCT you even get a rule language for free (see for example http://spinrdf.org).
(3) The RDF Semantics specification
(hereafter RDF-MT) §7 states that:
"[a]n
ill-typed literal is one whose datatype
IRI is recognized, but
whose character string is
assigned no value by the
lexical-to-value mapping for
that datatype."
"any triple,
and hence any graph, containing an
ill-typed literal will be
D-unsatisfiable, i.e. false in every
D-interpretation. This applies only
to literals typed with recognized
datatype IRIs in D".
(7) vaem:dateUnion a
rdfs:Datatype .
(10) The IRI vaem:dateUnion is
declared as a datatype and hence is
known(1,2a,7).
(11) Any RDF literal
with the datatype IRI vaem:dateUnion is
ill typed (1,3,10).
I do not believe these statements are correct. "2011-04-20"^^vaem:dateUnion
is a perfectly valid RDF literal.
If a problem falls within the class of problems
that OWL 2 DL can handle, it is good to stay within the
constraints of DL. Going to OWL Full should be made on a
carefully considered basis of costs vs. benefits.
This is a theory, not a fact. One could argue the other way
around: if a problem can be solved in RDF Schema, then why use
OWL?
Thirdly: The vocabularies being discussed were
inconsistent at every level of the RDF, RDFS, and OWL chain, and
contained basic type errors.
I don't see where they were invalid RDF, please clarify.
I do not know what tools were used to create them,
but whatever tool it was could not do basic consistency
checking.
They were created with TopBraid and TopBraid uses Jena which has
no problems with those files either. Only because they are not
valid OWL DL does not mean they are inconsistent.
Holger
--
Regards,
Kingsley Idehen
Founder & CEO
OpenLink Software
Company Web: http://www.openlinksw.com
Personal Weblog: http://www.openlinksw.com/blog/~kidehen
Twitter Profile: https://twitter.com/kidehen
Google+ Profile: https://plus.google.com/+KingsleyIdehen/about
LinkedIn Profile: http://www.linkedin.com/in/kidehen
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