[sc34wg3] RM4TM SLUO : Objective or Requirement?

Bernard Vatant sc34wg3@isotopicmaps.org
Fri, 22 Nov 2002 12:28:22 +0100

Some thoughts about SLUO ...

In the introduction:

"Many of the key advantages of the Topic Maps paradigm derive from the achievement of its
primary objective, the "Subject Location Uniqueness Objective", which is to make
everything known about every subject in a topic space accessible from a single location
within that space."

And further on:

3.4.1   One subject for each node

"In topic map graphs, only nodes can represent subjects, and every node represents a
single subject." [1]

Question en paasant : Why not use the word "topic" instead of "node" throughout? To what
extent is the above different of all the existing prose in ISO 13250, XTM 1.0, Published
Subjects TC ... "In a topic map, a topic is the formal representation of a single
subject". The notion of having nodes in the TMG representing "implicit" subjects that are
not topics in the corresponding topic map is IMO extremely confusing and hard to grasp.

Now a core issue ...

If I understand well the SLUO, out of RM prose and recent Steve Newcomb's comments,
SLUO is not expressed by 3.4.1 but by the reverse:

"In a topic map graph, every subject is represented by a single node" [2]

That I can't find anywhere explicitly expressed in the document - did I miss it?
I assume 3.4.1. means what it says. If it is intended to express also the SLUO, it's a bug
to be fixed.

OTOH if SLUO is only an Objective, it should be expressed by:

"In a topic map graph, every subject *should be* (as far as possible) represented by a
single node" [3]

But it seems that the SLUO is indeed a fundamental Requirement (in other words, an Axiom
of the model), if I understand well various Steve Newcomb's recent comments. If it is, it
has to be written clearly as such, and the various other requirements of the RM somehow
derived from or at least proven consistent with it.

Well, now my view on that:

[1] has to be taken as a pragmatic definition. A node, like a topic, is intended to
represent a single subject. But what this subject *is* no one can really tell. This is IMO
the common pragmatic approach in real-world TM applications so far.

For [2] ... in controlled environments where TM have been developed, the one-to-one
correspondence topic-subject is assumed, but people are well aware of the fact that
identifying the same subject from distributed sources is difficult to achieve, even if
those sources are ontologies from the same industry.
So [2] seems not only impossible to achieve in practice, but seems to express a
fundamentalist approach of subjects ... There is *no way* to make sure that two distinct
topics (nodes) do not *represent in fact the same subject* because of the above remark.
Subjects that are considered implicitly distinct in a given topic map, on the basis that
they are represented by distinct topics with distinct SIDPs, might be considered identical
by another topic map on the basis of new discovered properties ... This is a frequent
process in progress of knowledge that subjects considered as distinct at some point are
discovered as being the same later on. Think about various historical apparitions of
Halley's comet before Halley's discovery that they were the same one returning ...

So the RM has to ensure that merging does not split existing subjects, but it has to allow
merging of subjects considered previously as distinct, and admit that in many cases, the
same subject will be represented by different nodes, because the identity of subject for
those nodes has not yet been discovered ... In that spirit, SLUO should be considered only
as a pragmatic guideline, and not an absolute Requirement.

This would lead hopefully to relax a certain number of convoluted constraints discussed


Bernard Vatant
Consultant - Mondeca
Chair - OASIS TM PubSubj Technical Committee