Folks,  (01)
I completely support Pat's discussion of closed worlds
and open worlds. This is not a "debate", but a discussion
of how to handle two kinds of databases, both of which
are important for various applications:  (02)
 1. Open worlds, such as information derived from
 observation, in which the absence of an entry
 does not imply its negation.  (03)
 2. Closed worlds, such as information derived from
 an exhaustive enumeration or a declaration by fiat;
 i.e., anyone whose reservation has not been entered
 in the database does not have a reservation.  (04)
Both kinds arise in many applications, and both must
be supported. I also agree with Pat that the term
"negation as failure" is misleading and that "negation
by inference" is a better term.  (05)
Once you adopt that term, you can begin to ask what
kinds of inferences should be supported. Failure
to prove (or observe) is a common basis for the
inference, but there can be more subtle variations.  (06)
For example, the fact that I haven't observed an
elephant in my living room allows me to infer with
high probability that no elephant is lurking there.
But my failure to observe any bacteria in the living
room does not imply their absence.  (07)
Having thrown in this bit of support, I'd like to make
a brief comment on our argument about models and reality:  (08)
 1. Pat prefers to identify the model with the thing
 that is modeled, except in circumstances when the
 thing modeled may not exist or may be difficult
 to observe in sufficient detail.  (09)
 2. I prefer to make a distinction between the two in
 all cases, but allow the option of saying that they
 may sometimes be identical or at least isomorphic.  (010)
This reduces the debate to a question of relative importance
or frequency.  (011)
But there is one important reason for making the distinction
between the model and the thing modeled: it allows the
possibility of discussing and comparing different models and
deciding which one(s) have a better correspondence with
reality.  (012)
John  (013)
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