RE: Some heresies (was Re: Comments on "SPARQL 1.1 Uniform HTTP Protocol for Managing RDF Graphs")

Greg,

I am a long time observer on this thread, and as far as I can gather many of your “heresies” are in fact uncontroversial,  and people like Pat Hayes said them loudly and often from the beginning. Others, Pat would disagree with I am sure,  especially number 4, but whatever.

“I think the fact that after all these years this stuff is still so ill-defined strongly suggests that the standard RDF metalanguage has failed and it's time for fresh approaches”

THANK YOU!!!!!   Thank goodness someone is saying this at last.  I have watched in astonished amazement as University professors bickered for over a decade on how to say “The cat sat on the mat”. This is a ludicrous waste of effort.  RDF is garbage and should be consigned to the dustbin of history.

My 2 cents worth  <;-P

Tim Glover.






From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org] On Behalf Of Gregg Reynolds
Sent: 22 March 2011 10:40
To: Kjetil Kjernsmo
Cc: SW-forum Web
Subject: Some heresies (was Re: Comments on "SPARQL 1.1 Uniform HTTP Protocol for Managing RDF Graphs")

On Fri, Mar 18, 2011 at 7:43 AM, Kjetil Kjernsmo <kjekje@ifi.uio.no<mailto:kjekje@ifi.uio.no>> wrote:
...
So, the key issue and the root of my confusion is the question: "What does the
URI of a information resource consisting of some RDF triples identify?"
...
So, my issue all boils down to whether the URI of the stuff that people have
been publishing for years identifies RDF Documents or RDF Graphs.
...
Now, I've possibly exposed myself as totally confused about core Semantic Web
concepts, but I do so with the confidence that I'm not a n00b, and if I'm
confused, I'm probably not alone, and the issue should be properly explained

Howdy Kjetil,

You are definitely not alone.  Personally, I think the fact that after all these years this stuff is still so ill-defined strongly suggests that the standard RDF metalanguage has failed and it's time for fresh approaches.  For what it's worth, here's my list of heresies:

1.  IRIs in isolation have neither denotations nor meanings.  Or rather, they denote themselves.  They are nonce symbols, with no meanings; another way to put this is to say that RDF (and the web) has a symbol set but not a lexicon.  Whatever meanings they may appear to have based on English-like fragments discernable in their internal structure is fictitious and irrelevant.  A human-readable IRI may serve as a mnemonic device for the human reader, but is just as meaningless as a random string to the machine.   ("Evening star" has meaning and denotes the planet Venus; "ex:EveningStar" is a meaningless symbol denoting itself.  "Unicorn" has meaning but no denotation; "ex:Unicorn" is a meaningless symbol denoting itself.)

2.  IRIs used in RDF graph expressions remain meaningless, but they are used to label (denote) vertices and/or edges.  This use endows them with denotation but not meaning.  They denote mathematical objects of a semantic domain (set-theoretic graphs, normally); they do not denote "abstract syntax". (ex:EveningStar may denote a graph node; it does not mean "Evening star".  ex:Unicorn may denote a graph node; it does not mean "unicorn".)

3.  RDF is a graph calculus, not a logic calculus.  RDF triple expressions do not assert propositions, they construct graphs.  Compare tuple expressions like (a,b,c) or set extension expressions like {1,2,3}, which function the same way:  they name (set-theoretic) values, not truth-values.  RDF allows one to construct triples; it does not allow one to assert propositions.

4.  Since RDF triples are not statements, they do not assert propositions, so truth-conditional semantics (model theory) is not only inappropriate but meaningless.

5.  It also follows that RDF inference is not about truth transfer (entailment) but about data construction.  The inference rules license the *construction* of data values derived from declared values by rule.  (cf proof-as-construction)

6.  RDFS-defined terms such as rdf:type and rdfs:Class, etc. do not carry the meanings of their English-language components.  Their meanings rest exclusively on the inference (construction) rules defined (implicitly) for them, which can be expressed succinctly as transitivity rules, or alternatively as arrow composition rules.  This is purely a matter of the graph calculus and has nothing to do with the meanings of the English-language terms "type", "class", "instance", etc. used in the (in)formal definition of RDF.  Such terms should be viewed as mnemonic devices, intended to help the user remember which inference rules apply where.  (cf. Gentzen "rules of use" as definitions of logical constants)

7.  "RDF Document" is ill-defined.  It seems to mean something like "file", but if RDF is a graph language it only makes sense if taken as a synonym for "RDF graph" or maybe "named RDF graph".  But it should probably be discarded.

8.  The logic of RDF is constructive, not classical.  The Law of the Excluded Middle is disallowed.  It follows that blank nodes cannot be defined in terms of existential quantification, and they /do/ exist in graphs, as unlabeled nodes and edges.  (They can be handled with an "rdf:This" term construction operator of one optional argument.)

9.  There is no such thing as "RDF knowledge".  The "knowledge representation" aspect of RDF is a pragmatic /use/ of the language that is parasitic on the base graph semantics.  It may be a good use of RDF, but the RDF /language/ itself is nothing more than a simple graph calculus.  Any use of RDF terms to "represent" knowledge is a matter of application pragmatics, not RDF semantics.

10.  It follows that debates about what IRIs "mean", IRI ambiguity, what "RDF knowledge" is, etc., while fun, are also irrelevant to the task of making software that works.  Machines do not understand meanings, they manipulate meaningless tokens according to rules whose meaning consists entirely in their "ruleyness".

11.  Debates about whether IRIs have only one meaning, whether it is stable, etc. are actually debates about policy recommendations, not language definition.

12.  "Resource" is a whole 'nother hairball that tends to provoke lots of philosophizing and hand-waving.  I think it can be rigorously and simply defined using the type/token distinction, category theory, and intensional sense, but that's another topic.

At the very least, the approaches outlined suggest fresh expository strategies, e.g. separating the graph stuff from the knowledge representation stuff, and avoiding altogether questions like "does louvre:MonaLisa identify a painting, or a cultural phenomenon, or a woman, or etc. etc." as beyond scope and unproductive.

Cheers,

Gregg Reynolds