Re: Guessing?

From: Brian Selzer <brian_at_selzer-software.com>
Date: Thu, 31 Jul 2008 07:53:08 -0400
Message-ID: <FIhkk.16852$mh5.1311_at_nlpi067.nbdc.sbc.com>


"David BL" <davidbl_at_iinet.net.au> wrote in message news:c89ae2a9-5880-4b96-bf7e-adf8f2a899e1_at_j22g2000hsf.googlegroups.com...
> On Jul 31, 10:01 am, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> > "David BL" <davi..._at_iinet.net.au> wrote in message
>
> > > Given relation r, let X(r) be the boolean valued characteristic
> > > function of r.
> >
> > > Consider the following definitions
> >
> > > 1. OnTheTeam_r : the relation value recorded by the DB
> > > 2. OnTheTeam_i : the internal predicate recorded by the DB
> > > 3. OnTheTeam_e : the external predicate meant to represent reality
> >
> > > Is CWA associated with saying:
> >
> > > a) OnTheTeam_i = X(OnTheTeam_r) or
> > > b) OnTheTeam_i = OnTheTeam_e?
> >
> > > You appear to suggest CWA implies both a) and b). Is that right?
> >
> > The closed world assumption involves what can be proved rather than what
> > something means; an external predicate involves what something means;
> > therefore, the closed world assumption is not associated with saying b).
> > On
> > the other hand, it is associated with saying:
> >
> > c) OnTheTeam_i --> OnTheTeam_e
> >
> > since whenever ~OnTheTeam_e, ~OnTheTeam_i.
>
> I think you have that arse about. c) is assumed under OWA or CWA.

You're right. I got it backwards:

OnTheTeam_e --> OnTheTeam_i
since whenever ~OnTheTeam_i, ~OnTheTeam_e

And when combined with

OnTheTeam_i --> OnTheTeam_e

becomes

OnTheTeam_i iff OnTheTeam_e

Which is not the case under the OWA.

> If anything the CWA means that a missing tuple in the DB implies the
> negation of the proposition in reality.
>

Since a database is a proposition under the closed world, domain closure and unique name assumptions, I prefer to refer to what a tuple corresponds to as a formula instead of a proposition, since it is just a small part of the whole.

> Also, you say CWA is concerned with what can be proved, and therefore
> isnít related to an external predicate (because it is informal) and
> yet c) refers to an external predicate.

The CWA does indeed involve what can be proved instead of what something means, but that doesn't mean that it isn't related to the external predicate. The internal predicate is related to the external predicate, and the CWA is related to the internal predicate; therefore the CWA is related to the external predicate. While the internal predicate is related to the external predicate, that doesn't mean that they are identical as is stated in b). '=' and 'iff' are different relations. Received on Thu Jul 31 2008 - 13:53:08 CEST

Original text of this message