[sc34wg3] TMDM doesn't specify what is reified?

Thu, 19 Aug 2004 15:39:01 +0200

I think this was a helpful reply that exposes a lot of the
misunderstandings between us, as well as some weaknesses of the TMDM
text.

I think we can identify one source of confusion here immediately.  ISO
13250:2000 and XTM 1.0 both use the term "reify" in a different sense
from the one it was coined for by the artificial intelligence
community, and I think you continue to use and understand it in that
sense. 

There are three problems with this usage of the term:

  a) it confuses people who know the original meaning of the term,

  b) it scares people, because "reify" and "reification" are
     difficult-sounding terms, and so most people go blank whenever
     these words are uttered, and

  c) it leaves us without a term for distinguishing "reification" (in
     the original sense of the word) from "reification" (in the ISO
     13250:2000 sense of the word), and we do need such a term.

So for all these reasons, and also because there appears to be no
reason to change the meaning of the term when the term
"representation" will do what we want,TMDM changes the meaning of the
term. This has been discussed at ISO meetings, and the current draft
uses and defines the terms carefully, but I suppose if you have gotten
used to your own meaning of the term for many years that isn't
necessarily enough to clear things up.

So, here's an attempt to do that.

  A topic REPRESENTS its subject. The topic is an electronic symbol
  which stands in for, represents, the real-world thing that is the
  subject.

  If the subject REPRESENTED by a topic is part of the topic map we
  have what the term reification was originally introduced to mean;
  REIFICATION is a special case of REPRESENTATION, where the thing
  represented is a construct in the same model that isn't a fully
  privileged proxy (to use TMRM terminology).

I think you smelled this, to judge from your comment:

| The TMDM's definition of "reify" is apparently a bit narrower
| than that, [...]

And that's precisely it. Reification, as used in TMDM and generally in
computer science, is a special case of representation, and the TMDM
defines this carefully as:

  The act of reification is the act of making a topic represent a
  subject that is a topic map construct. For example, creating a topic
  that represents an association is reification.

| but even if we use the TMDM's definition, thus narrowing the above
| question, it is still a valid question, and it, too, is not answered
| clearly by the draft TMDM (we'll look at this much more closely in a
| moment).

Actually, I think you are subtly wrong here. I think you need to
rephrase your original question in TMDM terms in order to be able to
find the answer in TMDM. I also think TMRM should start using the
normal meaning of the term "reification" and instead start calling it
"representation".

So your question becomes: 

  "What are the subjects that are being represented?"

which sounds a bit odd, so we should perhaps amend it to

  "What are the subjects that are being represented by explicit
  proxies?"

which IMHO is enormously much clearer than the original question,
since it does not depend on a private definition of "reified".

| Here are some related questions:
| 
| * Is every information item a representation of a subject? 

Notice how you fell back to "representation" here, when to be
consistent you should really have written "reification".

Anyway, I believe the answer is yes. Notice how for every information
item type the type is introduced by a sentence of the form

  "<typename> items represent <formal term>s."

as in

  "Locator items represent locators."
  "The topic map item represents the topic map."
  "Topic items represent topics."

Notice how TMDM explains things in terms of a trichotomy:

  subject world | topic world | TMDM world

where subjects in the real world are represented by abstract topics,
which again are represented by topic items.

|   Or do only topic information items represent subjects? 

TMDM keeps the original definition for "subject" word for word, since
it cannot be improved upon, and so in TMDM "subject" is explicitly
defined as "anything whatsoever". So any item that represents
something represents a subject, and they all represent something, so
they all represent subjects.

|   Or is the nature of TMDM such that there are no fixed
|   relationships between subjects and information items?  Or what?  I
|   don't think TMDM answers this question clearly -- for whatever
|   reasons -- but I think it can, and I think it should.

Here I am not sure what to say. I guess my answer is above; if that
needs further elaboration, let me know, and I will try.

| * When an information item has been reified (by creating a topic
|   information item that says it reifies the information item), does
|   the information item *then* represent a subject, or is the subject
|   represented exclusively by the reifying topic information item?  Or
|   do the two information items *collectively* represent the subject?
|   Or what?

Here I think we are back to the different meanings of "reify". An
information item is reified if a topic item has been created that
reifies it. An information item always *represents* something, which
is a completely different thing.

| * When an information item has been reified by a topic information
|   item, what is the subject of the topic?
|
| Lars, I suspect you believe TMDM answers this last question, at least,
| quite clearly.  But I don't think it does. 

I think you may be right here. We say that the topic represents the
topic map construct (topic name, occurrence, association, whatever),
but we don't say the last bit, which is that the topic then represents
the same thing that the topic map construct represents.

This is kind of unfortunate, because there is a difference between
representing the relationship between me and Ontopia, and representing
the association between the two.

I think reification (TMDM sense!) must be taken to mean the former,
since otherwise it is not of any use whatsoever. (There has been quite
a bit of discussion on this, but I'm not sure how many people have
been in on it. I'll expand if anyone's curious.)

| > 3.8 information item
| > abstract representations of topic map constructs
| 
| From the above definition, I understand that "topic map constructs"
| are not the same things as "information items", and "information
| items" are representatives of "topic map constructs".  
| 
| Unfortunately, I'm not able to tell, for sure, what "topic map
| construct" means; this term is not formally defined in the TMDM.

That's a very good point. I believe Graham and I discussed whether or
not to define this term, and decided against it. I guess we were
wrong.

The definition would read something like this

  3.x topic map construct
  topic maps, topics, associations, topic names, variant names,
  occurrences, and association roles are topic map constructs

which implies that nothing else is, of course.

Does that help? Does anyone think we should add this definition to the
text?

| However, TMDM does say, about the source locators property of
| information items,
| 
|   > ...source locators are created that point back to the syntactical
|   > constructs that gave rise to the information items in the data
|   > model instance.
| 
| (I found this by searching for "construct".  N.B. This doesn't say
| "topic map construct", but I'm hoping "construct" implies "topic map
| construct".  I'll take any help I can get.)

Actually, it doesn't. I suppose this could be clearer, too. Any
proposals for replacements for "syntactic constructs" that would
retain the original meaning of "syntactic element of intuitively or
formally apparent extent" without using the loaded term "construct"?
"Element" leads one in the direction of XML, which isn't good.

| So, is a "topic map construct" an element, or part of an element, or
| a group of elements, in an XTM instance?  

No. It is abstract. Those are all "syntactic constructs", which is
something else.

| In other words, is the subject that is being represented by an
| information item always a piece of information (as opposed to an
| idea, tangible thing, etc.)?

No, the other way around.

| If a "topic map construct" is a syntactic phenomenon, then the
| definition of "merging" is very confusing: [...]

True. Thankfully, it isn't a syntactic phenomenon. :-)

|   > 3.19 reification
|   > making a topic represent a subject that is a topic map construct
| 
| I don't understand what's being reified, when we're reifying "topic
| map constructs", because, as already noted, TMDM doesn't define
| "topic map construct", and what TMDM does say about "topic map
| construct" leads me to draw conflicting conclusions about what this
| term means.

Yep. I hope I cleared this up above.

| [source locators]
|
| I don't know how to understand this.  Are we assigning identifiers
| to topic map constructs, or to their representations?

Topic map constructs are already representations, are they not? A
topic is a representation of a subject, right?

| In the first sentence of the above paragraph, we're assigning
| identifiers to topic map constructs.  In the second sentence, we're
| avoiding saying when or how these (same?) identifiers are assigned
| to information items.

Correct. It's outside the scope of the TMDM. XTM handles assignment of
source locators when reading XTM files. LTM does the same for LTM
files, and so on.

| Now, if you tell me that source locator items are proxies of
| subjects that are syntactic constructs in, for example, the XTM
| source from which the TMDM instance was constructed, then I'm happy,
| [...]

Well, first of all, there are no source locator items, only locator
items. These represent URIs, or HyTime locators, or some so far
undefined locator syntax.

The other item types all represent some topic map construct.

The [source locators] property is used to say that the locator refers
to the syntactic construct that the topic map construct represents.

| because then I know what's going on -- I know what subjects certain
| parts of a TMDM instance are representing.  

Hopefully you do now. :-)

| Once again, it sounds like topic map constructs are syntactic
| phenomena, and that information items are the surrogates for them.
| But, if that's true, and if information items only represent topic
| map constructs, then how can a "topic information item" represent
| any subject other than some instance of a syntactic phenomenon?

I can see how the TMDM prose led you to this conclusion, but hopefully
you'll now see that this is not the case.

| We really need for TMDM to be extremely explicit and clear about how
| TMDM instances invoke, represent, surrogate, proxify, reify,
| correspond to, or whatever, SUBJECTS.

We do. I fully agree with this. I'm hoping that with your help we can
make sure that this is the case. I think we're pretty close and that
the causes of this are tiny pieces of missing text in the TMDM draft,
as well as confusion over the meaning of the term "reification".

I hope this brought us at least another step forward.

-- 
Lars Marius Garshol, Ontopian         <URL: http://www.ontopia.net >
GSM: +47 98 21 55 50                  <URL: http://www.garshol.priv.no >