[topicmapmail] Two Models of Facets

[Murray Altheim]

Dan,

First, to keep things clear in light of the many labels we're facing,
I'm quoting from:

   Two Models of Facets, Murray Altheim, 18 Nov 2003
   http://www.infoloom.com/pipermail/topicmapmail/2003q4/005406.html

with the "Model A" facet:

 > Model A: The Metadata Model
 > ---------------------------
 >
 > The first model of facets might be considered as the RDF model,
 > where we're essentially adding property-value pairs to a resource.

and the "Model T" facet:

 > Model T: The Faceted Classification Model
 > -----------------------------------------
 >
 > Faceted Classification (FC) is a very powerful concept coming from
 > library science. It's how modern classification is often done. Even
 > when classification is not done quite according to FC, FC has had an
 > enormous influence on most library classification schemes. In a
 > nutshell, FC defines its subjects not by creating a set of canonical
 > categories, but by collections of "facets". [...] Anyway, this model
 > of defining subjects as collections of facets is all about subject
 > identity, a topic near and dear to our hearts. [...]

For purposes of discussion I'll delineate two kinds of Topics, class
Topics and instance Topics. And we'll call (for purposes of discussion)
"properties" the things attached to instance Topics, and  "facets" as
the things assigned to class Topics (we'll see if this pans out; if
not I'll drop it). I'll also try to remember to capitalize "topic" when
I mean Topic Map Topics. I'll use "characteristic" when I am being
deliberately ambiguous about whether something is a facet or a property.

Now, one aspect of this I don't like necessarily is the implied rule
that one wouldn't assign a facet to an instance, or a property to a
class. But I am meaning to say that there are things that are
characteristics of all members of a class, and there are characteristics
that are meant to be assigned to a single individual. Now, without going
too far down the extensional/intensional waterslide, I'll use an
example.

We could say that Jim has blue eyes. Are the colour of Jim's eyes a facet
or a property? If in our model it's important we'd be better off creating
a "proper" Topic for eye color and creating a "proper" Association with
it to its faceted Topic. We're assuming there's some value in dealing
with this as facets or properties, namely the value of Faceted Classifi-
cation in the former, the ability to add scalar properties to a Topic
in the latter (basically what RDF does).

We could classify people by their eye colour, hence we'd have created
a topic of "blue eyes", just as we could create a class of "people who
are 5'7" tall. But *normally*, we think of eye color or height as a
characteristic of an individual. This will always to some degree come
down to the purpose of the characterization, as should any modeling or
ontology.

If we assume that for the purposes, "having two eyes" is a property of
Jim as a human, and "having blue eyes" or "being 5'7" tall" is
properties of Jim and not all humans (a simple rule of thumb, then I'd
consider the former a facet and the latter as a property. Facets are
used to delineate subjects, properties provide meta-information about
a specific individual (as a subject, since everything we're talking
about here is a subject-reified-as-a-Topic). Like I said, all cautions
here apply.

Dan Corwin wrote:
> * Murray
> 
>  > I didn't follow the argument you are making. Since Models A and T
>  > are quite different, how is it that your idea of PSI trees states
>  > some kind of equivalence?
> 
> 1) Relative to model-A, a psi-tree seems to define an enumerated data
> type, one node of which (its PSI) can be selected as the characteristic
> of some topic - lets call it X - that you want to model.

So basically, you're talking about a property-value pair being added to
an instance topic (as opposed to a class topic), and that the property
type is part of a hierarchy.

[I wouldn't necessarily think the data type had to be enumerated,
unless by that you mean that the type is itself defined by a selection
of types from up and down a hierarchy, which I think is a bit strange,
so I'm guessing you don't mean that -- an example of this strangeness
might be "Jim has _____" where the slot filled in is "two eyes" and the
hierarchy has something to do with body parts.]

> 2) You can expand the psi-tree into an equivalent structure of topics
> and associations, one node of which (topic for its PSI) can be used
> in the same way to model X.

I don't want to bog us down in PSIs. They're just stand-ins for stable
Topics. So the tree you're talking about is a hierarchy of Topics from
which one is plucked to act as a property for a Topic, which I've been
calling the "faceted" Topic, as opposed to the "facet" Topic.

> I do not think (1) and (2) are equivalent, but they do seem very
> similar from a functional view (especially as seen by X).
> 
> Meanwhile (2) is not equivalent to your Model-T characteristic of
> X, but it also seems very similar.  

I think this is where I'm losing you a bit. If we ignore PSIs for a
moment and consider only that any property is likely part of a larger
hierarchy of characteristic, then okay, I agree. But "very similar"
here in what light? Any assignment of characteristic might be considered
similar, but Models A and T are (I hope) fairly clearly different. The
former is (to use today's parlance) a property of an instance Topic (or
individual), the latter is a facet of a class Topic.

> So maybe there is a spectrum
> of similar things, whose differences at each stage we might try
> to further clarify:
> 
>    Model-A
>    Model-1 = Model-A with psi-tree doc
>    Model-2 = Model-1 with topics added
>    Model-T
> 
> To me, the biggest changes lie between model-A and model-1: the
> PSIs which declare the occurrence to be an enumerated data type.

Well, I have to kinda translate what you're saying, removing "PSI"
and inserting "Topic". A PSI is a URI label for a Topic. So we have
a tree/hierarchy of Topics that may be used as types for properties.
By "enumerated" I'm assuming you mean to also include "scalar"?

> But I am still fairly confused about model-T.  If you added psis,
> how do you see the result differing from model 2?

Because Faceted Classification (in the formal sense) relies on a
hierarchy of facets (or facet Topics, in our case), both the
A/property/instance/RDF and T/facet/class/FC models may have a
complete hierarchy of Topics behind the scenes providing an
inheritance chain of properties and facets, so from an entity
"human" we can transitively infer (via its facets) "mammal",
"animal", "living thing", "temporal thing", etc.  This applies
whether we're talking about properties or facets. So yes, they
do share this idea of a hierarchy. I think both semantically
and epistemologically this must always be the case.

> And does the spectrum above make sense to you as a prop that can
> clarify how model-A and model-T differ?

I think we can continue to discuss this and perhaps glean a better
understanding than I think at least I have right now. I'm a bit
surprised we don't have more participation from the Topic Map
community in a discussion about facets in Topic Maps, but I'm
assuming everyone is busy, or maybe this discussion is happening
in the ISO list or has already been decided, or maybe people don't
consider conversations here binding or important so they're staying
with the ISO discussions. To me, it's still a hanging thread for
Topic Maps, and something I'd like to clear up with the help of
the whole community. I'm at the moment trying to implement something
reasonable (both in terms of human and machine reason), and I hate
to go it alone on this one. There's plenty of confusion about facets
and properties, and a lot of overlapping terminology and
understanding/misunderstanding.

Thanks,

Murray

......................................................................
Murray Altheim                    http://kmi.open.ac.uk/people/murray/
Knowledge Media Institute
The Open University, Milton Keynes, Bucks, MK7 6AA, UK               .

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