RE: URI Declarations [Usage scenarios 4 and 5]

> From: Pat Hayes []
> [ . . . ]
> BUt take this (actual) example: I recently found
> an entry on a site which listed interesting facts
> about various numbers:
> 30.48  ....  By convention, the number of inches in a foot.
> Hmm. Seemed to me like a typo (which it was, of
> course.) But following the logic used by David,
> what I should have done was assume that the site
> didn't mean what it seemed to mean by the numeral
> "30.48", and in fact it meant that to denote 12.
> Or maybe, it meant "foot" to denote a yard, or a
> meter; or, maybe, that it meant "inch" to denote
> centimeter. None of which seem like sensible
> strategies to me.

No, that's not the logic I'm suggesting.  Let me make your example more concrete to better illustrate what I'm suggesting.

SCENARIO 4: Gary publishes a lumber ontology at http://gary.example/lumber.  The ontology includes the concept of a standard 8-foot "2x4", which is denoted by the URI http://gary.example/lumber#2x4x8.  Pat is writing a set of assertions to describe the house he plans to build, and he is interested in Gary's #2x4x8 URI, but during testing he notices that Gary's ontology (erroneously) asserts that "30.49 inches = 1 foot".  For perhaps obvious reasons, Pat does NOT wish to accept that assertion.

Question: Should Pat use http://gary.example/lumber#2x4x8 to denote a standard 8-foot "2x4" anyway?

My answer: No.  Pat should either find a different ontology to use, or mint a new URI and indicate its relationship to Gary's URI.  For example, Pat could make a copy of Gary's ontology (with Gary's permission, of course), delete the offending assertion, change every occurrence of "http://gary.example/lumber" to "http://pat.example/lumber" and publish the new ontology at http://pat.example/lumber.    The assertions for his house should then use http://pat.example/lumber#2x4x8 instead of http://gary.example/lumber#2x4x8.

SCENARIO 5: Helen is writing some assertions about the color of concrete, and is also interested in using a URI from Gary's ontology to denote concrete: http://gary.example/lumber#concrete.  Helen's application makes no use whatsoever of linear dimensions, so although she is aware of the erroneous assertion that Pat discovered, it does not interfere with her application.

Question: Should Helen use http://gary.example/lumber#concrete to make assertions about concrete?

My answer: She may, but she probably shouldn't, because it would severely restrict the reuse of her assertions.   Helen's assertions presume that the "core assertions" in Gary's URI declaration at http://gary.example/lumber have been accepted -- including the erroneous assertion that "30.49 inches = 1 foot".  So if an application using Helen's assertions makes use of linear dimensions, and it recursively pulls in Gary's ontology, then Gary's erroneous assertion about the number of inches to a foot is likely to cause havoc.  If the application is lucky enough to have other information indicating that "12 inches = 1 foot" then it may detect a logical inconsistency.  If it is unlucky, it may silently produce absurd conclusions, such as "David Booth is over 180 inches tall".

SCENARIO 6: Helen is lazy and publishes her color assertions at http://helen.example/colors anyway, using http://gary.example/lumber#concrete to denote concrete.  Ian finds Helen's assertions and wishes to use them, but he notices that Gary's ontology at http://gary.example/lumber erroneously asserts that "30.49 inches = 1 foot", and Ian's application cannot withstand that erroneous assertion.  Ian is aware that Pat has published an ontology at http://pat.example/lumber that is equivalent to Gary's ontology except that it does not contain this erroneous assertion.

Question: What should Ian do?

My answer: Ian should effectively rewrite Helen's assertions to use  http://pat.example/lumber#concrete instead of http://gary.example/lumber#concrete throughout.  He can either do this by modifying a copy of Helen's assertions, or by reference, using special expressions to indicate proper URI substitution in Helen's graph.  (I intend to describe this further at .)

David Booth, Ph.D.
HP Software
+1 617 629 8881 office  |

Opinions expressed herein are those of the author and do not represent the official views of HP unless explicitly stated otherwise.

Received on Monday, 3 March 2008 00:39:55 UTC