[sc34wg3] TMDM doesn't specify what is reified?
Bernard Vatant
sc34wg3@isotopicmaps.org
Fri, 20 Aug 2004 00:13:38 +0200
Lars Marius, and Steve
Cool debate. Just a remark, hoping they will help.
I agree with the distinction needed between reification and representation, and I must
plead guilty for part of the confusion, being among those having pushed to bring back the
use of reification instead of representation in XTM 1.0 prose.
http://www.topicmaps.org/xtm/1.0/#desc-reification
But I detect some kind of inconsistency, or at least discrepancy, between two assertions
in your long answer below. Since they are quite apart from each other in the text, the
discrepancy does not show that much.
> A topic REPRESENTS its subject. The topic is an electronic symbol
> which stands in for, represents, the real-world thing that is the
> subject.
OK - except for the "real-world" reference, as you know :))
> 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).
This is slightly wrong, as detailed below. And in fact I'm not sure you mean what you
write here, since further on you write:
> | * When an information item has been reified by a topic information
> | item, what is the subject of the topic?
> 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 I agree with. Indeed, the "reifying" topic does not *represent* the topic map
construct it reifies, but the subject already represented by this construct. And that's
why we have to use a distinct verb "reify" here, and not "represent". If you want to
"represent" the topic map construct, you would need another topic.
So, I would rephrase your second paragraph above like the following :
"If the subject represented by a topic is already represented in the topic map
by a construct that isn't a 'fully privileged proxy', IOW by some representation which
is not a topic,
then this topic is said to reify the construct.
The construct and the topic which reifies it represent the same subject.
Reification just brings about a simplification of the representation."
Let me take an example, using an informal topic map syntax (on purpose). Suppose I write
in my informal topic map some construct like :
A : "Lars and Steve have a new debate about reification"
This sentence (this construct) represents a subject (what this sentence says) which could
have various and more formal representations, like an association of a specific type
between a topic representing Lars and another representing Steve. This subject can be
represented by a topic T, which therefore reifies A. T and A have the same subject, and
whatever can be asserted about the subject of T can be asserted about the subject of A,
like "this is good news" :).
But T does not represent A. If I want (in rare cases) to represent A (the construct)
itself to say something about it, like "A is a sentence in English, composed of nine
words" (or in a more formal TM : "A is an association involving two topics of class
Person"), I would need another topic T', of which subject would be A itself, and not the
subject that A represents. And I would say that T' represents A. But T' would not reify A,
and T and T' are definitely distinct.
In short : reification is not a special case of representation, it's simply a relationship
between two representations of the same subject, a complex one and a simplified one.
Cheers
Bernard
PS : Just read Murray's message before pushing the "send" button ... will certainly be
back :))
Bernard Vatant
Senior Consultant
Knowledge Engineering
Mondeca - www.mondeca.com
bernard.vatant@mondeca.com
> -----Message d'origine-----
> De : sc34wg3-admin@isotopicmaps.org
> [mailto:sc34wg3-admin@isotopicmaps.org]De la part de Lars Marius Garshol
> Envoye : jeudi 19 aout 2004 15:39
> A : sc34wg3@isotopicmaps.org
> Objet : Re: [sc34wg3] TMDM doesn't specify what is reified?
>
>
>
> 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.
>
> * Steven R. Newcomb
> |
> | What I'm missing, in the TMDM, are clear, explicit answers to the
> | question:
> |
> | "What are the *subjects* that are being reified?"
> |
> | By "reify" I mean "represent via a surrogate", and "provide with a
> | proxy".
>
> 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 >
>
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