W3C home > Mailing lists > Public > semantic-web@w3.org > December 2012

Re: Well Behaved RDF - Taming Blank Nodes, etc.

From: David Booth <david@dbooth.org>
Date: Wed, 19 Dec 2012 15:34:58 -0500
To: Henry Story <henry.story@bblfish.net>
Cc: Lee Feigenbaum <lee@thefigtrees.net>, Steve Harris <steve.harris@garlik.com>, Pat Hayes <phayes@ihmc.us>, semantic-web <semantic-web@w3.org>
Message-ID: <1355949298.2229.47949.camel@dbooth-laptop>
Hi Henry,

On Wed, 2012-12-19 at 20:05 +0100, Henry Story wrote:
[ . . . ]
> Now one could go further and see what are the fundamental differences
> between URLs and blank nodes. Perhaps that would be useful.
> 
> 1. With bnodes there is no intension associated with the name.
> 2. with well functioning URIs there is an intension associated with
> it, such that you can GET their meaning in the document in which they
> are defined - and there relative URIs play an important role!

That is an interesting philosophical view on the role of blank nodes,
and it goes pretty deep.  But I fundamentally disagree with it, as I
think it is too limiting.  I'll explain why in a moment, but first an
unrelated (non-philosophical) comment on point 2.

I think point 2 unnecessarily bundles the use of "cool",
dereferenceable, follow-your-nose http: URIs with the use of URIs in
general, which could simply be URNs whose definition you may not be able
to easily determine (by dereferencing), but which nonetheless still
avoid the problems that bnodes present of being unstable across
serializations, difficulties querying, non-lean graphs, etc.  I think it
is better to separate the bnode issue from the issue of whether an RDF
consumer can easily find an appropriate definition for a given URI.

Now turning to your philosophical point criterion for minting URIs:

> 1. With bnodes there is no intension associated with the name.

which you also described in:
http://lists.w3.org/Archives/Public/semantic-web/2012Dec/0097.html 

> - bnodes: you know there is something but you can only refer to 
>   it by description
> - relative URIs: you can speak of something but you don't yet 
    know the full publication context
> - full URIs: there is agreement between different people one 
    how to resolve the meaning of it.

In the semantic web, nearly *everything* is identified by description --
in essence, RDF properties.  We have almost no other way to define
things, except by listing their properties.  So if the criterion for
minting a URI instead of a bnode is that you must have some means beyond
a description -- ostension? -- for identifying that thing, then I think
that sets the bar for minting a URI far too high, because in a practical
sense descriptions are often all that we have.  We would be allowed VERY
few URIs if the bar were so high.  

Even if you only know a few properties of something, it can still be
very useful to mint a URI for that thing, as I illustrated in the
previous temperature measurement example:
http://lists.w3.org/Archives/Public/semantic-web/2012Dec/0107.html

On Wed, 2012-12-19 at 20:54 +0100, Henry Story wrote:
> I think a document that summarises some of the different pragmatic
> uses of bnodes, URLs relative urls still needs to be written.

Yes, good point!


-- 
David Booth, Ph.D.
http://dbooth.org/

Opinions expressed herein are those of the author and do not necessarily
reflect those of his employer.
Received on Wednesday, 19 December 2012 20:35:26 UTC

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