W3C home > Mailing lists > Public > semantic-web@w3.org > March 2011

Re: blank nodes (once again)

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 16 Mar 2011 11:34:34 -0500
Cc: semantic-web@w3.org
Message-Id: <5FE61E53-8F41-44B7-8919-71EFDBFCCDC9@ihmc.us>
To: Ivan Shmakov <oneingray@gmail.com>, Ivan Shmakov <ivan@main.uusia.org>

On Mar 14, 2011, at 11:56 AM, Ivan Shmakov wrote:

> 	I guess I'm close to become an annoyance, but I'm going to ask
> 	yet another blank node question anyway.
> 
> 	Suppose that I have the following graph:
> 
> :joe :has [ a :dog ]
> 
> 	And suppose that I also have the following one as well:
> 
> :joe :has [ a :dog ]
> 
> 	From the previous discussion I've learned that there're
> 	different opinions on whether to consider these graphs, which
> 	share exactly the same representation, same or different.

The *graphs* may be different, but what they *say* is the same. Each one simply repeats the content of the other one. So you learn nothing new by having both of them rather than one. 

However, that said, according to the current RDF specs, they are also in fact (perhaps two different serializations of) the same graph. Not that this really matters. 

>  In
> 	particular, there's a position that these graphs are different,
> 	unless they're named the same.  (Somehow, I feel that graph
> 	naming should be considered tangential to the knowledge it
> 	represents, but I've noted to myself that there's a different
> 	opinion.)
> 
> 	But my question is itself tangential to the equivalence of these
> 	graphs.  Instead, I wonder, if I've assimilated this above
> 	representation into an RDF store, and going to assimilate its
> 	exact twin again, does this later assimilation change the
> 	/knowledge/ contained within such a store, or not?

Not.

> 
> 	To speak it differently, I've never heard of Joe, and
> 	(unexpectedly) received a bit of information that speaks: Joe
> 	has a dog.  Now, I receive another bit, that says exactly the
> 	same.
> 
> 	I'm quite certain that after I've received the first bit I now
> 	have a bit more knowledge about the World.  However, I'm not so
> 	sure that the second bit gives me any more knowledge, since I
> 	still have no rational means to tell, whether the dog I'm told
> 	of this time is the same or different to the one about which
> 	I've already known.

Quite. BTW, this would also be true if you had two graphs with URIs instead of blank nodes, even if they were different URIs. 

:joe :has [:dog1 a :dog]
:joe :has [:dog2 a :dog]

You would still not know that the two different *names* for dogs did or did not name the same dog. Maybe (or maybe not) 

:dog1 owl:sameAs :dog2

Pat Hayes

> 
> 	TIA.
> 
> -- 
> FSF associate member #7257

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Received on Wednesday, 16 March 2011 16:35:52 UTC

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