- From: William Waites <wwaites@tardis.ed.ac.uk>
- Date: Thu, 29 Nov 2018 13:19:30 +0000
- To: David Booth <david@dbooth.org>
- Cc: semantic-web@w3.org, Henry Story <henry.story@bblfish.net>
> Do joe and monica have the *same* address? If we know that those attributes form > a composite key, then the answer is yes. Otherwise, the answer is no. I don’t think that blank nodes are actually at issue here. Consider: A a ComplexNumber; real 1; imag 4. B a ComplexNumber; real 1; imag 4. This is much simpler and better defined than street addresses, doesn’t use blank nodes, and yet still we would want to conclude that A = B. They are equal in that their meanings are the same but they are not the same in that they have different names. Now what about: C a Length; value 10; unit cm. D a Length; value 3.94; unit in. Those also have the same meaning but now the graph labels are different so you can’t say “graph isomorphism” or (possibly better) “bisimulation”. This is kind of the complement to Hugh's Colliding Kardashians. Here you can synthetically construct alternative URIs for C and D from the properties and get something distinct. It still doesn’t help the problem of deciding if they are equal or if they are the same. I was in America recently and had brought some small art prints from Europe. The picture was 25cm x 25cm. I stood by helplessly as a slightly eccentric picture framer measured them with an inch ruler and muttered to himself about complicated and unusual fractions of an inch. He did a good job in the end, but it would have been easier using the right units. Being easier is a relevant difference so maybe C and D aren’t the same after all, despite being equal. Maybe sameness depends on the query context at least as much as the data context. Things can be the same — relevantly similar — for one purpose and different for another. For answering queries, you need to pick which notions of equality you want to use. And at query time you have to close the world because you can only make answers from the information that you have. It seems to me that the appropriate notion of equality can be different according to the type of the entity. This must simply be defined by the person who defines the type. It would be nice to provide a sensible default equality for when we don’t have a more specialised one. What do you think? -w
Received on Thursday, 29 November 2018 13:19:58 UTC