- From: Ian Horrocks <horrocks@cs.man.ac.uk>
- Date: Tue, 28 Jun 2005 13:08:27 +0100
- To: Jos de Bruijn <jos.debruijn@deri.org>
- Cc: Holger Wache <holger@cs.vu.nl>, Jim Hendler <hendler@cs.umd.edu>, dreer@fh-furtwangen.de, public-sws-ig@w3.org, Bijan Parsia <bparsia@isr.umd.edu>
Sorry to be late joining the party (I take a vacation and world war three breaks out!), but I feel compelled to add my "twopennyworth" (as we say). Much of this has been said before (by Jim, Bijan and Holger), but I will try to summarise... Regarding DLP under Horn (equivalently DL/FOL) and LP semantics, it is true that *under certain assumptions* the two semantics are indistinguishable, but this does *not* mean that they are equivalent. The assumption being made in this case (as Bijan has pointed out) is that only ground entailment will be considered. This is a rather strong assumption: it is common to compensate for weak representation languages (such as DLP) by providing much more expressive query languages (e.g., the case of SQL), and with more expressive languages we can easily distinguish two different semantics (as illustrated in [1]). This is particularly important in a layered architecture, where the increased expressive power of higher layers will naturally allow the two different semantics to be distinguished. This was the main point of [1], i.e., that we should not fool ourselves about the degree of interoperability that would be provided by the architecture proposed in the so-called "updated layer cake". Such interoperability only exists so long as neither tower extends above the DLP level *and* so long as we only allow for very weak query languages; DLP does not, therefore, provide a suitable foundation for a layered architecture, as higher layers (or more expressive query languages) might make false assumptions about the intended semantics of DLP ontologies. Regarding the existing layered architecture of OWL, and the proposed extension to SWRL, nobody (well not me anyway) claims that it is perfect, and there will (as Holger has pointed out) no doubt be some compatibility issues between different tools, but the situation is hardly comparable to the one that is being proposed in the "updated layer cake": as we have seen, even the very limited degree of interoperability suggested by this diagram turns out to be a chimera. In contrast, RDF, OWL and SWRL share a common semantic framework, and allow for a relatively clean layering: OWL-Lite, OWL-DL and SWRL are layered on a subset of RDF, but share the same semantics (and it can be syntactically determined when RDF ontologies are within this subset). Regarding the complexity of SWRL, yes it is a complex language, but as seen from presentations at the rules workshop (and as pointed out by Holger), some users *need* this level of complexity. For them, it is essential to have a more expressive language that layers on top of OWL. Should we thus forge ahead with the development of *two* fundamentally incompatible rules languages? Surely (as Jim has pointed out) we can do better than that, even if it requires a little more research effort! Regards, Ian [1] http://www.cs.man.ac.uk/~horrocks/Publications/download/2005/HPPH05.pdf On 23 Jun 2005, at 14:28, Bijan Parsia wrote: > > On Jun 23, 2005, at 3:06 AM, Jos de Bruijn wrote: > >> -----BEGIN PGP SIGNED MESSAGE----- >> Hash: SHA1 >> >> Bijan Parsia wrote: > [snip] >>> Key is the slipping in of "ground". >> >> No. For query answering only ground entailment is relevant.\ > > Interesting presumption. > >>>> entailments for both >>>> semantics are *equivalent* and thus the queries would return the >>>> same >>>> result. >>> >>> >>> Of course, RDF entailment includes existential generalization, so >>> that's not quite right. There seems to be more work that you need to >>> do to get what you wanted (e.g., you need to look at the semantics >>> of the query language; is the query "not" classical? how would that >>> classical not interact with the LP semantics?) >> >> The RDF language contains existentials and I'm not claiming that this >> can be done by a rule language. >> We are talking about Horn Logic and Horn Logic does not have >> existentials! I never claimed this! >> You claimed > > Please point to where I claimed this. > >> that a Horn formula under FOL semantics has other ground >> entailments than a Horn formula under LP semantics and this is simply >> not true. >> I think this can conclude our discussion on this topic. > [snip] > Oh, I *quite* agree. But perhaps not for the same reasons. > > Cheers, > Bijan. > >
Received on Tuesday, 28 June 2005 12:08:33 UTC