Ed Summers writes:
 > On Wed, Apr 1, 2009 at 6:14 AM, Mike Taylor <m...@indexdata.com> wrote:
 > > As usual, an ounce of example is worth a ton of exposition, so:
 > >
 > > Suppose I always keep a PDF of my latest paper at
 > >        http://www.miketaylor.org.uk/latest.pdf
 > > for the benefit of people who want to keep an eye on my research.
 > > (Hey, it might happen!)  Today, I have a PDF there of a paper with the
 > > DOI 10.1111/j.1475-4983.2007.00728.x.  Tomorrow, my new paper comes
 > > out, and I replace the old one with a PDF of that new paper whose DOI
 > > is 10.abcdefghij.  I move the PDF of the old paper to
 > >        http://www.miketaylor.org.uk/previous.pdf
 > >
 > > Now, then -- the DOIs are identifiers: they are not in themsleves
 > > dereferencable (although of course they can be used as keys for some
 > > mechanism that knows how to dereference them).  Each DOI always
 > > identifies the same Thing.  The URLs are locations: they are
 > > dereferencable, but they do not give you any guarantee about what you
 > > will find at that location.  Two different days, two different papers.
 > > Note that a single location (latest.pdf) contains at different times
 > > two different Things.  And note that a single Thing (the older of the
 > > two papers) can be found at different times in two different
 > > locations.  In contrast, the same identifier always identifies the
 > > same Thing, irrespective of what location it's at.
 > Hoorah for examples!

Yes, they're good, aren't they?  :-)

 > Assuming a world where you cannot de-reference this DOI what is it
 > good for?

It wouldn't be good for much if you couldn't dereference it at all.
The point is that (I argue) the identifier shouldn't tie itself to a
particular dereferencing mechanism (such as dx.doi.org, or amazon.com)
but should be dereferenced by software that knows what's the most
appropriate dereferencing mechanism _for you_ in your situation, with
your subscriptions, at particular distances from specific libraries,

(Even aside from the, identifiers are still good for telling whether
or not two Thing references areboth reference the same Thing, by
simple blind comparison-for-equality.  Of course that only works
reliably when both identifiers are from the same vocabulary, but a
well-formed identifier URI will let you know whether or not that
condition holds.)

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor    <m...@indexdata.com>    http://www.miketaylor.org.uk
)_v__/\  "For the 1995-96 season lint output is forecast at 257,000 tonnes
         compared with 254,000 tonnes last season and the bumper crop of
         416,000 tonnes in 1993-94" -- Financial Times, 15th December 1995

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