1. Resource Identity and Semantic Extensions: Making Sense of Ambiguity David Booth, Ph.D. Cleveland Clinic (contractor) Semantic Technology Conference 25-June-2010 Latest version of these slides: http://dbooth.org/2010/ambiguity/ http://dbooth.org/2010/ambiguity/ Also available: Companion paperCompanion paper

2. Outline Part 0: Myths about resource identity Part 1: RDF semantics and ambiguity – Interpretations – Interpretations of a URI Part 2: Constraining ambiguity through URI declarations – Bounding ambiguity – Ambiguity and owl:sameAs Part 3: Determining resource identity – Semantic extensions

3. PART 0: Myths About Resource Identity This section makes some observations that will be further explained in later sections.

4. 4 URIs as names for resources URI http://example/#apple Resource In the semantic web, URIs are used as names for resources – Separate from a URI's use as a locator “Resource” == “Thing” == Universal class – E.g., people, proteins, medications, concepts, etc. But which resource does a URI name? ?

5. Resource identity Which resource does a given URI denote? Because a URI can denote any resource, this question is central to RDF semantics This is the question of resource identity – the determination of which resource a URI denotes

6. Myth 1: A URI denotes only one resource Myth: “By design a URI identifies one resource” – W3C Architecture of the World Wide WebArchitecture of the World Wide Web Reality: True as an ideal True for one interpretation of one RDF graph, but... Different interpretations of the same graph may map the same URI to different resources Different graphs may permit different interpretations

7. Myth 2: RDF semantics are global Myth (a variation of #1): There is only one giant graph, with global semantics E.g., “owl:sameAs makes a very strong statement” – The implication is that it must hold universally Reality: RDF semantics are defined for a given graph There are many graphs The “meaning” of a URI depends on the graph – A URI may denote different resources in different graphs

8. Myth 3: Resource ambiguity is due to sloppiness Myth: A URI's resource identity can be uniquely defined if you are precise enough Reality: Ambiguity is unavoidable –... with vanishingly few exceptions Always possible to make ever finer resource distinctions See examples in In Defense of Ambiguity by Pat Hayes and Harry HalpinIn Defense of Ambiguity

9. Myth 4: Truth is absolute Myth: “If your RDF models the world as flat, then it is wrong” Reality: “Truth” is irrelevant; what matters is usefulness Different apps have different needs – Flat world model may be best for street navigation: Precise enough, and simpler than round world model Different apps need different models

10. PART 1: RDF Semantics and Ambiguity This section examines some consequences of standard RDF semantics.

11. 11 “Interpretations” in RDF Semantics An interpretation maps URIs to resources http://example/#plum http://example/#apple http://example/#pear http://example/#banana http://example/#orange InterpretationURIsResources

12. 12 An interpretation applied to a single URI An interpretation maps that URI to one resource – Associates the name with a particular resource http://example/#apple InterpretationURIResource

13. 13 Multiple interpretations RDF semantics does not constrain a graph to a unique interpretation Different interpretations may map the same URI to different resources http://example/#apple InterpretationsURIsResources i3 i2 i1

14. 14 Many interpretations There may be many interpretations – Potentially infinite http://example/#apple

15. 15 RDF semantics constrains the possible interpretations for a given graph For a given graph, RDF semantics constrains the possible interpretations http://example/#apple

16. 16 Adding assertions reduces the set of possible interpretations By merging RDF graphs, constraints of both graphs must be satisfied http://example/#apple

17. 17 “Interpretations of a URI” For a given graph, “Interpretations of a URI” == The set of resources from applying all possible interpretations to that URI http://example/#apple

18. 18 Resource ambiguity For a given RDF graph, a URI's resource is ambiguous if there exists more than one possible interpretation for that URI – I.e., the possible interpretations map that URI to more than one resource Referent of a URI is almost always ambiguous! – But that's okay – it's just life http://example/#apple

19. 19 Interpretations of different URIs may overlap URIs X and Y may map to some of the same resources Interpretations of XInterpretations of Y

20. 20 Effect of owl:sameAs X owl:sameAs Y Limits the interpretations for X and Y to the intersection X owl:sameAs Y Interpretations of XInterpretations of Y

21. PART 2: Constraining Ambiguity through URI Declarations This section proposes a standard way to constrain resource ambiguity.

22. 22 URI Declarations A URI declaration provides a definition for a resource denoted by a URI – See “URI Declaration in Semantic Web Architecture” Definition is provided by a set of core assertions Core assertions constrain the possible interpretations for the URI URI declaration should be provided via the URI's follow-your-nose location – See “Cool URIs for the Semantic Web”

23. Why URI Declarations? Easy to know what definition to use – Dereference the URI to find its URI declaration (usually) Permits all users of the URI share the same definition – Stablizes meaning / Avoids semantic drift Resource ambiguity is precisely bounded – Interpretations can still vary within bounds

24. 24 Bounding the interpretations of a URI X URI declaration bounds the interpretations of URI X Use of X in graph A further limits the possible interpretations Interp. of X in graph A Interpretations of X consistent with X's URI declaration

25. 25 Interpretations of a URI X in different graphs Same URI may have different possible interpretations in different graphs – E.g., URI X is used in graphs A and B All are within the bounds of the X's URI declaration When graphs are merged, the possible interpretations for X are limited to the intersection In graph A In graph B In A+B In URI declaration

26. 26 In X's URI declaration Inconsistent combined graphs URI X is used in graphs A, B and C – Graph A+B is consistent – Graph B+C is consistent Graph A+C (or A+B+C) is inconsistent: no possible interpretations In A In B In C In A+BIn B+C

27. 27 In X's URI declaration Splitting identities Use of A+C (or A+B+C) together requires splitting X's identity, e.g.: – Mint new URI Xab to replace X in graph A to make A' – Mint new URI Xbc to replace X in graph C to make C' – Then merge graph A' with graph C' See http://dbooth.org/2007/splitting/ X in A X in B X in C Xab in A+B Xbc in B+C

28. 28 Trade-off: precision versus reusability Broader URI declaration: – Permits the URI to be used in more applications – Causes more down-stream contradictions, when the URI is re-used in other graphs and those graphs are later combined Narrower URI declaration: – Restricts the URI to few applications – Reduces likelihood of downstream contradictions Recommendation: – Choose the degree of precision that will best attract the community of applications that you wish to attract – See also discussion of “clumping” in http://dbooth.org/2007/uri-decl/20100615.htm#clumping http://dbooth.org/2007/uri-decl/20100615.htm#clumping

29. PART 3: Determining Resource Identity This section proposes a standard process for determining resource identity.

30. 30 Determining resource identity 1. Select assertions – what graph? 1.a. Recursively merge ontologies and URI declarations – Ontologies and URI declarations should be cached! 2. Apply RDF semantics Constrains the possible interpretations for each URI 3. Select an interpretation RDF Semantics only defines step 2!

31. 31 3. Select an interpretation Resource identity with RDF semantics 1. Select assertions 2. Apply RDF semantics Available assertions Possible interpretations e.g.... Informal assertions e.g. rdf:comment "... ". Formal assertions 1.a. Get ontologies & URI declarations

32. 32 Semantic extensions Define additional entailment rules and constraints – E.g., OWL or FruitOnt Must be monotonic – All previous entailments still hold Further limit the set of possible interpretations Typically triggered by a predicate URI

33. 33 Resource identity under semantic extensions 1. Select assertions – what graph? 1.a. Recursively merge ontologies and URI declarations – Ontologies and URI declarations should be cached! 2. Apply RDF semantics + semantic extensions Predicate URI triggers the use of semantic extensions: – Opaque plug-in, or – Set of rules 3. Select an interpretation

34. 34 Resource identity under semantic extensions 1. Select assertions 3. Select an interpretation 2. Apply RDF+extension semantics Available assertions Possible interpretations e.g.... Informal assertions e.g. rdf:comment "... ". Formal assertions Semantic extensions e.g. OWL, FruitOnt 1.a. Get ontologies & URI declarations

35. Summary Part 0: Myths about resource identity Part 1: RDF semantics and ambiguity – Interpretations – Interpretations of a URI Part 2: Constraining ambiguity through URI declarations – Bounding ambiguity – Ambiguity and owl:sameAs Part 3: Determining resource identity – Semantic extensions

36. 36 Questions?

37. 37 BACKUP SLIDES

38. 38 Splitting URI X resource identity What if you really want to combine graphs A+B+C? URI X may be split into two URIs, e.g.: In graph AB = A+B, change all X to X1 In graph BC = B+C, change all X to X2 In graph A In graph B In graph C In A+BIn B+C

39. 39 X owl:sameAs Y Interpretations of XInterpretations of Y

40. 40 X owl:sameAs Y Interpretations of XInterpretations of Y

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43. 43 Effect of owl:sameAs X owl:sameAs Y Each URI has a set of possible interpretations owl:sameAs limits this set to the intersection Interpretations for X Interpretations for Y

44. Brief Description This presentation shows how ambiguity fits within standard RDF semantics, explains how it relates to owl:sameAs, and proposes a standard operational sequence for determining the referent of a URI.

45. Abstract What does a URI denote? How should its referent be determined, even in the presence of semantic extensions that affect the interpretation of an RDF graph? How should ambiguity be viewed? One view is that a given URI has no fixed referent, but may denote different things in different contexts. Another is that each URI should have a URI declaration that precisely delimits its interpretation. Some suggest reusing existing URIs in new contexts, while others prefer to mint new URIs and then allow owl:sameAs assertions to indicate that two URIs denote the same thing. This presentation sheds light on these issues by explaining how ambiguity of a URI's referent fits within standard RDF semantics, how this ambiguity applies to the use of owl:sameAs, and proposes a standard operational sequence for determining the intended referent of a URI, even in the the presence of semantic extensions.