Vocab AND Ontology?
bernard.vatant at mondeca.com
Mon Sep 24 09:06:21 PDT 2007
>> documents/data about "White Dwarf Stars", a SKOS concept "White Dwarf
>> Stars" is what you need, and the position of this concept in a Concept
>> Scheme hierarchy, and its association with other concepts such as
>> "Chandrasekhar Limit".
> I don't recall any concept linking proposed for vocabs apart from the
> narrower/broader relationships.
In the IAU thesaurus, like in most thesaurus, there is the notion of
"Related Terms" (RT) otherwise called "associative relationship". It is
non hierarchical. Basically it means : if you are interested in this,
you should also see that.
> What would you term this relationship
Related Terms (RT) in thesaurus standards. In IAU thesaurus you have this :
> and how would it be recorded in SKOS?
sv:Chandrasekhar_limit skos:related sv:Gravitational_Collapse
sv:Chandrasekhar_limit skos:related sv:White_Dwarf_Stars
> I assumed this sort of relationship was
> better modelled in an ontology.
Actually not at all. This is exactly the kind of relationship with fuzzy
semantics (even more fuzzy than broader/narrower) extremely useful for
human search and retrieval, but hard to encode in formal logic.
The meaning of such a relationship depending on what you want to achieve
with it ...
>> - If you want to classify objects, find them using logical inference,
> I'm not sure we do want to classify individual objects so much as classify
> datasets, queries, papers, people even?
What is the purpose of Ed's Star Ontology in this case, if not for
classifying individual stars? I mean here representing individual stars
as instance of a specific subclass of star. This is the functional
objective of such an ontology. Otherwise I don't understand what a
"star" class and its subclasses is good at. Certainly not at
classifying papers, data, observations about stars, people and labs
working on them, whatever - which would need other ontologies - an
ontology of documents, an ontology of data, of observations etc ...
>> But you don't find "Chandrasekhar Limit" at all in the star
>> ontology, although it's a very pertinent concept in stellar
> I know I'm exposing my lack of knowledge about ontologies, but that's what
> these lists are for - to learn something new from others. Why can we not add
> "Chandrasekhar Limit" as a concept in an ontology?
You certainly can, in (too) many ways. I'm curious to know how Ed would
fit this concept in, and what logical articulation with White Star class
he would propose.
> Does every concept have to be a class of physical things? Can things not have a relationship with
> this concept of either 'beyond' or 'within'?
What "things" do you speak about here? Individual stars? or Star
classes?. If you want to express formally something as : "A white star
has a mass beyond the Chandrasekhar limit", it's a necessary condition
you put on the "white star" class. And what kind of "thing" is
"Chandrasekhar Limit"? A concept? The value of a property? And if yes,
which property? It's a numerical value, right? If you look closely at
OWL, you will find it extremely difficult to express that kind of
relation, because it involves a specific numerical value, with numerical
constraints (mass greater than, etc ...) ... for which OWL, despite its
great expressivity, has no real satisfying representation (First Order
Logic doesn't like continuous quantities).
So actually, trying to express constraints like "The mass of a white
dwarf is within Chandrasekhar's limit" which is AFAIK a way to declare
in natural language the relationship between the two concepts (correct
me if I am wrong - this is certainly approximative and even wrong for
experts, but what they will propose instead will be even more difficult
to capture I'm afraid), there are two ways. The SKOS / Thesaurus fuzzy
way, saying those two concepts are closely related, and when one speaks
about theformer, he's likely to say something about the latter, or the
OWL hard way, and well, good luck.
> BTW - some of the things I've read before:
> 'A formal ontology is a controlled vocabulary expressed in an ontology
> representation language.'
> 'ontologies are specified in the form of definitions of representational
> 'map the relations between differing vocabularies, and pinpoint the location
> of properties within a larger ontological framework of interconnected
> knowledge, interconnected via the relationships established within an
I will go and read those in more details bandwidth permitting, but none
seems contradictory with what I say above. Vocabularies are indeed
generally the basis for ontology construction, concepts don't pop up out
of thin air, they are most of the time embedded in natural language, and
the ontology miner work is to try to explicit whenever possible their
implicit semantics. But there is a "whenever possible". It's not because
"white dwarf" and "Chandrasekar's limit" concepts occur in the same
vocabulary context that expliciting the semantics of this co-occurrence
is straightforward, or even possible in a given representation
framework. And having more expressivity does not necessarily help. Or it
helps to discover that you've better stick to fuzzy relationships ... :-)
> I'd be interested in your take on these documents (and many others beside)
> and why you think, either they've got it wrong or they're talking about
> something different than we are in this thread. I'm not trying to dispute
> your expert opinion but showing how I came to the conclusion that a
> vocabulary might be contained within an ontology.
I have no disagreement with this conclusion. But devil is in the
details. Some terms/concepts are straightforward to represent formally
as classes (Star, Planet), and some as properties (mass, radius) and
some as individuals (Sun, Moon, Messier 31). And some are less so.
Chandrasekhar's limit is one of those.
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