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Re: [ontolog-forum] Logic, Datalog and SQL

To: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: "John F. Sowa" <sowa@xxxxxxxxxxx>
Date: Thu, 08 Feb 2007 17:47:45 -0500
Message-id: <45CBA891.50207@xxxxxxxxxxx>
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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