- From: Bob MacGregor <macgregor@ISI.EDU>
- Date: Fri, 10 Jan 2003 19:22:13 -0800
- To: pat hayes <phayes@ai.uwf.edu>
- Cc: seth@robustai.net, Dave Reynolds <der@hplb.hpl.hp.com>, www-rdf-interest@w3.org
Some conclusions are in order. I certainly have a better understanding of some of the issues in RDF regarding reification and nested statements. Below, I've listed a number of conclusions that summarize my interpretation of the current state of affairs. Within that list are a couple of technical questions for which I still lack answers. I get the feeling that only a portion of the WG has understood the full import of some of the decisions that have been made. However, the net result is disappointing. It seems like the WG Charter may be impeding progress on RDF (and the Semantic Web), but that's just a guess. (1) There are some fairly simple things that one would like to say in RDF that are not expressible. Here are some examples: Suppose we start with a triple that states "Fred Martin is 52 (years old)". Assume that 'S1' is that triple, in the abstract sense, not in the sense of a stating. Here are some things I would like to be able to say about S1: "The probability of S1 is P" "S1 is true at time T1" "I disagree with S1" "The object value of S1 is wrong. The correct value is 53 (i.e., retract S1 and assert 'Fred Martin is 53')" "S1 is important (take note of it)" (2) Some of the WG members appear to be unaware of this defect. Here is a quote by Seth Russell to that effect (I imagine he is not alone): "But I really don't know any practical case where we would want to say something about a jena:Triple" (here I'm assuming that 'jena:Triple' means 'statement'). (3a) One means for alleviating the problem in (1) with regard to examples I've listed would be to permit statements to be nested. Nested statements are explicitly forbidden (apparently by the WG charter). (3b) A second means for alleviating the problem in (1) would be to admit reified statements (using the traditional sense of that phrase) into RDF. Although that appears to have been sanctioned in the original RDF, it has now been forbidden. (4a) KIF provides an existence proof of a logic language that explicitly allows nested statements and does not introduce paradoxes. Question: Is the Charter the only reason for forbidding nested statements, or are there technical reasons as well? (4b) Allowing unfettered use of reified statements would make it very easy to introduce paradoxes. Taking (4a) into account, if reified statements were constrained by the same semantics as nested statements, then I'm guessing that the possibility for self-reference, and therefore paradox, might be eliminated. Has this been explored? (5) Some of the WG members appear not to be aware that ordinary reification is not merely unsupported, but is actively discouraged. Brian McBride: "there are two concepts - statements and statings and only one bit of vocabulary. The WG, for reasons that Pat Hayes has explained picked one for the existing vocabulary, and as Pat also mentioned, that does not preclude you, or anyone else, from defining new vocabulary for the missing concept." My guess is that Pat explicitly does not want me or anyone else to define vocabulary for the missing concept, i.e., for statements, because "its too dangerous" (which I translate as "it might allow for paradoxes"). (6) Its very easy to write statements about statements and never introduce paradoxes (freeways are dangerous, but its easy to walk beside them without crossing them). Users have the option to pretend that statings are in fact statements, and then use the RDF statings machinery to make assertions about them. In my case, we appear to have no choice; our application requires the ability make assertions like those listed in the beginning, and no alternatives have yet been offered up. I'm not worried about the so-called 'danger', but its disappointing that no legitimate alternative has been suggested. (7) The RDF Semantics Document http://www.w3.org/TR/2002/WD-rdf-mt-20021112/ contains a section on Reification that is quite difficult to read. This is not a criticism per se, because the subject itself is relatively difficult. However, it is not reasonable to think that many readers could make a pass through it and know what they had just read. On my first pass through, I noted that the term 'token' is used repeatedly but never defined. That made things tougher. The term 'stating' never occurs. It took several readings to determine that what was said did in fact seem to jibe with the notion of stating that has appeared in the discussion groups. That also made things more difficult. (8) Parts of the OWL document http://www.w3.org/TR/2002/WD-owl-semantics-20021108/ are even more abstruse (c.f., the very short section on OWL-FULL). I can't tell from scanning it whether any of my examples can be expressed in OWL. If OWL indeed is the solution to some of our problems, I would like to see some examples. (9) The triple stores that have emerged in conjuction with RDF could in principle solve all of my current problems. However, the restriction that they conform to RDF semantics means that they can't handle the examples I've listed. Since Jena currently allows one to include statements as arguments to other statements and RDF does not, for my purposes one of Pat's claims is exactly backwards: Pat Hayes: But the fact is that triple stores are a limited tool, and RDF is committed to the use of this very restricted tool. Jena 1.6 already gives me what I need except for some tiny tweeks to the API. My guess is that Jena's current representation (ignoring unbridled reification, which I don't use) can be embedded as a subset of KIF, thereby making it logically sound. However, owing to its commitment to RDF, Jena 2.0 is scheduled eliminate the statements-as-arguments feature (replacing it with statings as arguments?). This is, to say the least, disappointing. Cheers, Bob
Received on Friday, 10 January 2003 22:30:08 UTC