- From: Seth Russell <seth@robustai.net>
- Date: Fri, 18 May 2001 11:38:56 -0700
- To: "Ziv Hellman" <ziv@unicorn.com>, "pat hayes" <phayes@ai.uwf.edu>
- Cc: <www-rdf-logic@w3.org>
RE: What do the ontologists wantZiv, I think that if you start composing your representations at the node level rather than the arc level,what you percieve to be clumsiness will disappear and you will see that triples gives you more flexiablility than fixed position ntuples. Let's suppose I compose a CycL kind of predicate like this {f, Afrom, Bto, Csomething} and define the meaning of each position in the tuple, and accumulate a bunch of instances of the tuples in my graph, and then subsequently find that I was missing a ~Dwiggy~ part of the concept. With the CycL technique I must redraw all of my predicates according to the new knowledge ... not so with a node based slot system as proposed in [1]. With such a slot based system I can very easily write into the inference engine the idea that we may or may not have complete information about the concept. The class\model nodes of instances tells us what we should anticipate and\or seek out. I call that more flexiable and more attuned to the way real world models evolve. [1] http://robustai.net/mentography/conceptualDependency.gif Seth ---in response to--- From: Ziv Hellman To: pat hayes Cc: www-rdf-logic@w3.org Sent: Friday, May 18, 2001 10:58 AM Subject: RE: What do the ontologists want > >I have no objection to binary predicates; I could > >even live with all predicates being binary if it would allow me to > >speak for lots of ontologists. :) > > The restriction to binary (plus unary, ie at-most-binary) predicates > is mildly inconvenient but quite live-with-able, I agree. That's two > ontologists on the list. > > Pat At the risk of being on the receiving end of a hailstorm of flames from the regulars on this list, I will toss a spanner into the works here and question the use of triples. As correctly pointed out above, using triples is essentially reducing everything to binary predicates. Now it is certainly provably true that every multi-ary relation can indeed be reduced to a collection of binary predicates, and this has been known for a very long time. The RDF spec even notes this and provides examples for doing so. The question is whether too high a price is paid in certain cases. On the one hand, essentially reducing the world to binary predicates is what the OO and XML communities have done for a long time, with the attributes assigned to objects really being binary predicates. This viewpoint can be understood as stemming from looking at most relations as functional, in the sense that, as the canonical RDF example puts it, if one asks "who is the creator of this resource?" and the answer is "Ora Lassila", then one is working with a binary predicate associating a specific resource with a specific person. So far so good. On the other hand, standard mathematics and logic, KIF, the relational data-base world, and even full-power UML, all permit the use of multi-ary relations and do not limit themselves to binary predicates. Why? I think the reason has to do with the fact that although it appears at first that one is gaining simplicity by using only binary predicates, or encoded triples, in practice when one is forced to exchange a straightforward n-ary predicate with a clumsy collection of binaries, the simplicity one has seemingly gained is more than lost in the translation. If we really are going to create a world-wide web of semantic meanings for a plethora of daily needs, this issue may need to be addressed again down the road. Take as simple an example as requesting a bank balance. This requires a relation that is at least 3-ary: at minimum one needs the account number and the date&time. The balance cannot be assigned as a simple attribute of the account, because its value changes with time, and it certainly is not an "attribute" of the date&time alone. For another, more complicated example that is a canonical one I use, consider a travel agent asked by a customer the flight seating he/she has been assigned. The travel agent will respond that in order to answer the question, one needs to know at minimum the quadruple of {name of the customer, the date of the flight, the airline carrier, the flight number} -- because the seating of a particular person on a particular flight is not an attribute of any one element in that list, but an attribute of the full quadruple. Again, I know that these examples can be reduced to encoded triples -- but is the resulting clumsiness worth it compared to the straightforward multi-ary statement? And perhaps more to the point, consider that in order to really take off, the SW will eventually have to come into contact with the data the world has stored in relational data-bases, which routinely make use of reams of tables representing very large multi-ary relations. If the industrial world is told that uploading/downloading this data through the SW will require painfully chopping up the tables into an explosion of triples, waiting for the transmission traffic to complete and then reconstituting from them the tables at the other end, one may fear that it will recoil in horror. Cheers, Ziv
Received on Friday, 18 May 2001 14:42:59 UTC