Chris
On 18 May 2007, at 18:10, Chris Mungall wrote:
I'm afraid I'm unclear how to state the OWL n-ary relation
pattern(http://www.w3.org/TR/swbp-n-aryRelations) where I really
need it. In all the examples given, the "lifted"[*] n-ary relation
was never truly a relation in the first place and always better
modeled as a class. It's kind of cheating. What if my n-ary
relation is transitive or if the 3rd argument is a temporal
interval over which the relation holds?
I think the former is doable with property role chains. Updating
the n-ary relations note with this - and all the other omitted
details, such as how to re-represent domain/range, functional
properties, n-ary relations in restrictions etc - would take a lot
of work and would make it utterly terrifying to the naive user.
Nevertheless the results are clunky and will need special tool
support[**] to avoid going insane.
I'd love to see DLR or similar means worked into future versions of
OWL or other standards, although I am not the one to comment on the
logical/complexity issues. I certainly agree that re-expresssing
relations as properties carries a modest penalty by being more
verbose, but it is manageable.
And for the record, it has long and very respectable history. It was
first noticed by CS Peirce in the late 1800s, and he himself regarded
it as one of his important contributions to logic: it has been
re-invented since then by several notable logicians, including WVO
Quine, and it has been widely used in formal linguistics and KR work
under the terminology of 'roles'. In many ways it corresponds to the
tendency in natural language to treat predications as verbs denoting
an 'event' or 'fact' to which the relational participants have named
binary semantic relations (properties in OWLspeak) corresponding to
grammatical cases like subject, object, manner, time, place, etc.. In
that latter context, it has the notable advantage of allowing any
number of arguments to be used, and for some of them to remain
unspecified, as indeed they often are in normal NL usage. So for
example if time is an argument, then omitting time information is a
syntactic error; but using the property encoding, one can simply omit
the time property assertion.
To take the example in question for some relation R, let's take
temperature as an example. I shall use the subrelations
"has_feature" / "has_state" to minimise arguments over what is, and
is not a "quality" - an issue not germane to this discussion. Also I
will use "has_state" as the property name so we don't have both a
property "has_value" and a keyword VALUE.
In the binary relation form in manchester simplfied syntax in OWL 1.0 we have:
Organism has_feature SOME (Temperature_Feature THAT
has_temporal_extent VALUE temporal_extent_1 AND
has_state SOME (has_magnitude VALUE 37 AND has_units VALUE degrees_C))
where temporal_extent_1 is an individual which has facts
has_start_time VALUE n AND has_end_time VALUE m.
has_magnitude is a functional datatype property and has_units
is a functional property.
where n,m are date-time expressions, for simplicity let us assume
integers representing milliseconds since some reference point.
Inn OWL 1.1 we can do quite a bit better - although again there is a
need for improved tools to make it easier.
* An organism has a given temperature at some point in an interval
anOrganism -->
has_feature SOME (Temperature_feature THAT
has_time_point SOME (has_coordinate SOME int[>=n, <m])
has_state...
Don't you also need that the temperature is a functional value of the
time-point? BTW, this 'manchester syntax' is quite unreadable :-)
* An organism has a given temperature throughout an interval.
(This has to be expressed as "Any temperature feature of the
individual anOrganism in the time interval has the given state"
Temperature_feature THAT
is_had_by VALUE anOrganism AND
has_time_point (Some has_coordinate SOME int[>=n, <m]) -->
has_state...
where is_time_point_of: inverse has_time_point
has_time_point: functional
Axiom: (Feature THAT has_time_point SOME Time_point)
has_value Max 1 State.
has_coordinate is used here with int since I am
assuming it is measured in "ticks since basepoint", but could
equally well be a float
Nevertheless the results are clunky and will need special tool
support[**] to avoid going insane. In general I am wary of design
pattern type things - they are usually a sign that the language
lacks the constructs required to express things unambiguously and
concisely.
Separate "unambiguously" and "concisely". Whether or not there is
something ambiguous about a design pattern depends on the case. In
this case I think there is no ambiguity. "Concisely" is a matter
for tools and layered "higher level languages".
The history of computing is the history of "design patterns" at one
level that eventually get built into "higher level languages" at the
next level of abstraction up. No one would argue against layoring
more convenient languages on top of OWL ( or its successors). The
patterns are a first step towards this end, just as they were in the
early days of programming languages. Neither would anyone argue
against more expressive languages.
While I agree with the general sentiments here, that last sentence is
rather misleading. In fact, people have argued against more
expressive languages, in fact have argued with great force and
vehemence, which is why we are saddled with such a baroque DL-based
standard as OWL. There is no shortage of thoroughly understood and
semantically coherent more expressive languages.
But I would argue that building on known, tested, and proven
semantics and computational methods is preferable to inventing new
ones.
Again, I agree: but there are other known, proven semantics than the
OWL-DL design. That represents one choice in a spectrum of
possibilities. It is not necessary to invent new formalisms (which I
agree is not something to be done lightly or carelessly) in order to
find something well-understood, standardized and supported by a
community. ISO Common Logic qualifies on all these counts, for
example, and is vastly more expressive than OWL (which it contains as
a subset). N-ary relations are trivial in CL: one simply writes
relations with as many arguments as one wishes. It need not be the
same in every case, as variadic relations are possible.
I am not meaning to suggest that OWL be abandoned; there is no doubt
that it is the de facto Web standard at present, and receives a lot
of attention and support. But to presume that it represents the only
possibility, or that more expressive, semantically coherent,
well-understood, notations are an open research problem, is to
mis-state the case.
Pat Hayes
--
---------------------------------------------------------------------
IHMC (850)434 8903 or (650)494 3973 home
40 South Alcaniz St. (850)202 4416 office
Pensacola (850)202 4440 fax
FL 32502 (850)291 0667 cell
phayesAT-SIGNihmc.us http://www.ihmc.us/users/phayes
