# Re: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG

```Hi Jean-Pierre!

Am 08.05.2012 09:25, schrieb Evain, Jean-Pierre:

> I understand the point which is being made being what is useful for reasoning or not.

Replace "useful for reasoning" by "required for the well-definedness of
the semantics" of OWL 2 or RIF!

It is a basic technical requirement for the specifications of these
languages that for every syntactically well-formed expression (aka an
OWL 2 ontology or a RIF rule set), the semantic meaning can be
determined by mathematical means. At a minimum, for OWL 2 and RIF, this
means that it can always be determined whether an input ontology is
satisfiable or not, or whether one given ontology entails another given
one or not. Only in the cases of OWL 2 DL and its profiles, it is an
additional requirement (by design) that there are reasoning procedures
that are able to do these determinations in an automated way for all
input, because these languages are required to be computationally
decidable. But having a well-defined semantics is always needed.
Clearly, if there are ontologies for which it cannot uniquely be deduced
(mathematically) whether they are satisfiable or not, a reasoner cannot
give the "right" reasoning result for them, because it cannot then be
determined whether it's answer is right or not, or just one correct

To illustrate this problem, take the case of xsd:duration in its
definition as of the time of finalizing OWL 2, where each literal of
xsd:duration would essentially denote a pair (m, s) consisting of a
certain number m of month plus a certain number s of seconds. Let there
be two such durations:

d1 := (2, 0)
d2 := (1, 30*24*60*60)

Now, depending on what is meant by "a month", these two durations can
represent either (i) the same value (if a month has 30 days), or (2) d1
can be greater than d2 (if a month has 31 days), or (3) d1 is smaller
than d2 (if a month has, say, 28 days = 4 weeks). I may well have missed
a precise definition of "a month" in the (newest version of the) XSD
spec, in which case the above example may be void. But if not, then it
is clear that any OWL 2 (+xsd:duration) ontology for which the question
of satisfiability depends on whether the above two durations are the
same or not, or which of them is greater, does not have a uniquely
defined semantic meaning.

An example for the need of being able to determine whether equality
between two duration values holds or not would be an ontology with data
enumerations consisting of duration values (denoted by "d1" and "d2", as
defined above, but in a real ontology one would use their correct
literal form, of course):

:D a rdfs:Datatype ;
owl:oneOf ( d1 ) .
:dp a owl:DatatypeProperty ;
rdfs:range :D .
:s :dp d2 .

This set of axioms should be satisfiable if and only if d2 = d1, because
only in this case, the object d2 of the property assertion (last
statement) would denote an instance of the singleton datatype :D = {d1}.
But if it cannot be determined whether d2 equals d1 or not, then it
cannot be determined whether the axiom set is satisfiable or not.

An example for the need of comparison of two durations (greater or
lesser than) could be constructed from the use of OWL 2 datatype
restrictions.

> But does that mean that all other information is garbage?

Everyone can say everything about everything. But an OWL 2 or RIF or
whatever language specification with a formal semantics at its core
would, if not well-defined, IMO count as just that: garbage. :-)

Best,
Michael

--
.........................................................
Dipl.-Inform. Michael Schneider
Research Scientist, IPE / WIM

FZI Forschungszentrum Informatik
Haid-und-Neu-Str. 10–14
76131 Karlsruhe, Germany
Tel.: +49 721 9654-726
Fax: +49 721 9654-727

michael.schneider@fzi.de
www.fzi.de

.........................................................
Forschungszentrum Informatik (FZI) an der Universität Karlsruhe
Stiftung des bürgerlichen Rechts
Stiftung Az: 14-0563.1 Regierungspräsidium Karlsruhe
Vorstand: Dipl. Wi.-Ing. Michael Flor, Prof. Dr. Ralf Reussner,
Prof. Dr. Rudi Studer, Prof. Dr.-Ing. J. Marius Zöllner
Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus
.........................................................
```

Received on Tuesday, 8 May 2012 08:39:45 UTC