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[LC Response] To Jonathan Rees Re: Problems with sameness and namedness of individuals

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 19 Feb 2009 11:04:13 -0500 (EST)
Message-Id: <20090219.110413.268298777.pfps@research.bell-labs.com>
To: jar@creativecommons.org
Cc: public-owl-comments@w3.org
Dear Jonathan,

Thank you for your message

on the OWL 2 Web Ontology Language last call drafts.

It is quite often common to be a bit sloppy about the distinction
between names and whatever they denote or mean, consider, for example,
the common use of "the variable x" instead of "the variable named x".
Often the loss in precision is gained back in readability.  In the
interests of gaining this readability, the OWL Functional Syntax
generally does not use markers in its syntactic categories to indicate,
for example, "ClassName" or "ClassID", instead using just "Class".

Of necessity, this breaks down for individuals.  The overall syntactic
category in the OWL Functional Syntax is "Individual", which is then
divided into "NamedIndividual" and "AnonymousIndividual".  The
alternative "IndividualName" and "IndividualAnonymousMarker" would have
been a reasonable alternative, but would have somewhat conflicted with
the usage for other syntactic categories.

As you point out, there are some parts of the document that should be
revised so as to not be so confusing.

The WG has decided to make the following modifications in response to
your comments:

Section 3.4:

	The axiom closure of an ontology O is the smallest set that
	contains all the axioms from each ontology O' in the import
	closure of O with all anonymous individuals *standardized* apart — that
	is, the anonymous individuals from different ontologies in the
	import closure of O are treated as being different; see Section
	5.6.2 for further details.  

Section 5.6:

	Individuals *in the OWL 2 syntax* represent actual objects
	*(semantic individuals)* from the domain being modeled. There
	are two types of individuals in *the syntax of* OWL 2. Named
	individuals are given an explicit name that can be used in any
	ontology ** to refer to the same *semantic*
	individual. Anonymous individuals *do not have this global name
	and thus* are local to the ontology they are contained in.

Section 5.6.2:

	Special treatment is required in case anonymous individuals with
	the same node ID occur in two different ontologies. In
	particular, these two individuals are structurally equivalent
	(because they have the same node ID); however, they are *not* treated
	as *identical* in the semantics of OWL 2 (because
	anonymous individuals are local to an ontology they are used
	in). The latter is achieved by *standardizing* anonymous individuals
	apart when constructing the axiom closure of an ontology O: if
	anonymous individuals with the same node ID occur in two
	different ontologies in the import closure of O, then one of
	these individuals MUST be replaced in the axiom closure of O
	with a fresh anonymous individual (i.e., with an anonymous
	individual having a globally unique node ID).  

Section 5.6.2 Example 2:

	In order to ensure that these individuals are treated
	differently by the semantics they are *standardized* apart when
	computing the axiom closure of O1 — either _:a5 in O1 is
	replaced with a fresh anonymous individual, or this is done for
	_:a5 in O2.  

Section 9.5:

	OWL 2 supports a rich set of axioms for stating assertions —
	axioms about individuals that are often also called facts. For
	clarity, different types of assertions are shown in three
	separate figures, Figure 18, 19, and 20. The SameIndividual
	assertion allows one to state that several individuals are all
	equal to each other *(more precisely, the several different
	individuals in the syntax denote the same semantic individual)*,
	while the DifferentIndividuals assertion allows for the opposite
	— that is, to state that several individuals are all different
	from each other. *(More precisely, that the several different
	individuals in the syntax are also semantically different.)* The
	ClassAssertion axiom allows one to state that an individual is
	an instance of a particular class.

Section 11:

	The axiom closure Ax (with anonymous individuals *standardized* apart
	as explained in Section 5.6.2) of each OWL 2 ontology O MUST
	satisfy the global restrictions defined in this section. As
	explained in the literature [SROIQ], this restriction is
	necessary in order to obtain a decidable language. The formal
	definition of these conditions is rather technical, so it is
	split into two parts. Section 11.1 first introduces the notions
	of a property hierarchy and of simple object property
	expressions. These notions are then used in Section 11.2 to
	define the actual conditions on Ax.  

The diffs can be found at

The Direct Semantics document mentions standardizing of anonymous
individuals, and has been appropriately edited.
The diffs can be found at

The WG considers these to be editorial changes.

Please acknowledge receipt of this email to
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are satisfied with the working group's response to your comment. 

Peter F. Patel-Schneider
on behalf of the W3C OWL Working Group 
Received on Thursday, 19 February 2009 16:04:27 UTC

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