notes from today's JC telecon on requirements and basic approach to layering rules on top of OWL

From: Benjamin Grosof (bgrosof@MIT.EDU)
Date: 07/29/03

  • Next message: Harold Boley: "Re: updated Requirements"
    Hi folks,
    Here are my notes from today's (dramatically productive!) telecon, inline'd 
    and attached too for convenience.
    Mike:  wrt drafting next iteration of document describing our proposed 
    approach, see esp. the part beginning "***consensus".
    % notes from JC telecon 7/29/2003
    % by Benjamin Grosof
    Mike Dean
    Benjamin Grosof
    Ian Horrocks
    Sandro Hawke
    Pat Hayes
    Harold Boley
    Said Tabet
    Deborah McGuinness
    Peter Patel-Schneider
    (? perhaps one or two others)
    agenda topic:  requirements and basic approach to layering rules on top of OWL
    seeded by msg sent out by Mike Dean earlier today
    initial comments by Mike Dean reviewing that
    Mike:  propose restricting rules to what stacks nicely on top of OWL,
    and thus keep it to binary unordered relations and all names being URI's;
    this will help serve goals of DAML and OWL users
    Pat:  yes, it's non-trivial to do n-ary on top of OWL
    Benj: rules are more mature in deployment than OWL or RDF, it's easy
    to do n-ary in Horn/Datalog, so let's not keep it restricted to only
    using OWL and RDF; rather, we can have rules help pull users to adopt
    OWL and RDF
    Deb and Benj:  hopefully we can do both relatively soon, in V1 of Rules
    Ian:  i.e., have Lite (binary, on top of OWL) and Full (with n-ary, not
    necessarily on top of OWL) versions
    Pat: we're dealing with intersection of rule and ontology languages
    and DAML program objectives -- let's try to keep things smaller to make
    it more doable
    Harold:  n-ary unordered, with slots/roles, is very close to RDF binary;
    also can use collections
    Pat:  one potential requirement is to transmit rulesets as RDF graphs
    Deb and Pat:  can view all this as baby step that's in part political
    and is outreach to the DAML community
    Benj:  an important concern is not to make later versions more difficult,
    i.e., anticipate moving later to n-ary, nonmon, procedural attachments, etc.
    Benj: there will also be issues for layering on OWL that are more hard
    to work out, that will have to be deferred as well
    Mike:  yes, I do anticipate some issues at semantic level
    e.g., unique names assumption, explicit equality and how it's treated in
    Ian and Mike:  rules with property chaining on top of OWL-DL immediately
    loses decidability
    all:  need more than DLP, e.g., want rules to do property chaining
    discussion around LP (least Herbrand) vs. FOL semantics for Horn, to
    add on top of OWL-DL
    - LP is a weakening of the conclusions (rather than the premises)
    - concern:
    do we need to weaken the conclusions of the OWL-DL subset of a
    ontology+rules language
    Benj:  it's not well studied what are the expressiveness/semantics of
    - Horn FOL + full DL (tho' this is known to be a subset of FOL)
    - Horn LP  + full DL
    - (tho' we know Horn LP + DLP is just LP)
    but the former is better understood from previous research
    Mike:  what are the advantages of restricting to LP semantics?
    Benj: primarily further extensibility to nonmon and procedural attachments;
    also congruence with existing LP rule (and DB) systems
    Harold:  also some aspects of recursion
    Mike:  we don't want to break OWL content
    discussion around incompleteness of practical systems,
    and expectations about that
    1st step:
    OWL-DL + binary Horn FOL + warning label
    warning:  (phrased by Benj): do not rely on the diff between FOL - LP 
    (i.e., entailments)
    if you want (as is common in most practical rule applications)
    to later extend to nonmon or (some kinds of) procedural attachments
    warning is to developers and also to users (tho' users are less likely to
    read the warning)
    Benj:  there's a radical issue lurking however, which is that it's not
    well understood how to coherently restrict all of (OWL-)DL to LP semantics
    when adding Horn LP (rather than Horn FOL) to DL
    Benj:  wrt warning label, we
    will need to give some explanation that some kinds of conclusions
    and some kinds of premises will likely not be supported when extending to
    more fully featured rule language that has nonmon and
    (more kinds of) procedural attachments
    - Mike:  it will help to give users examples (for exposition)
    Mike:  we can have different warning labels for different audiences
    discussion:  can view a lot of this in terms of incompleteness
    - background:  Horn LP semantics is a weakening of Horn FOL semantics.
    I.e., it sanctions as conclusions
    a subset of what FOL sanctions as conclusions; that subset is simply the
    set of ground atomic facts.
    some discussion about whether LP semantics is "syntactic" from viewpoint of
    FOL semantics
    - Pat: dislike calling LP semantics "semantics" since it is not based upon
    model theory in the same style that FOL semantics is
    - Benj: observe that LP is a coherent KR in itself, and its use of
    "semantics" there is standard terminology in its literature.
    In particular, I just dislike how LP literature defines "model"; rather,
    I prefer personally to speak of the "conclusion set" that is entailed
    from a given set of premises
    Sandro:  the incompleteness issue means we could be talking about OWL-Full
    not just OWL-DL, relative to adding Horn rules
    Plan:  Mike and Benj will draft a next version, send it out and
    solicit more comments from those committee members not on today's telecon,
    then we'll discuss this as part of the next telecon.
    Prof. Benjamin Grosof
    Web Technologies for E-Commerce, Business Policies, E-Contracting, Rules, 
    XML, Agents, Semantic Web Services
    MIT Sloan School of Management, Information Technology group or

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