Re: Temporal database - no end date

On Jan 24, 1:51 am, "Marshall" <marshall.spi..._at_gmail.com> wrote:
> On Jan 22, 7:40 pm, "David" <davi..._at_iinet.net.au> wrote:
> > Marshall wrote:

> > > There is one area in particular that is of the utmost
> > > importance. The one overriding limitation on the logical
> > > level that the physical level applies is that it must exist.
> > > Any logical model that is not implementable is not worth
> > > much.
>
> > You need to qualify that, because as stated it implies that the reals
> > (as a logical model) is not worth much.
>
> > It is common for logical models to ignore physical limitations, making
> > them impossible to implement. Proofs of correctness often depend on
> > pure logical models. For example, a stack that never overflows is a
> > useful abstraction. The Turing machine with its infinite tape is
> > another useful abstraction.

> Hmmm. Well, that is not how I use the terminology, but perhaps
> this is my mistake. I wouldn't call the reals a logical model.
> (What are they a model *of*?) Rather I would describe them
> as an abstraction, but I suppose "abstraction" and "model"
> are similar concepts.

I agree that "model" should really only be used to describe something that tries to imitate something else. Perhaps we should indeed say: "a physical model of a (logical) abstraction".

I guess a DB can be regarded as a physical model of some logical model about entities that exist in the real world. In that case there really are (at least) three levels. The logical model would be free to use abstractions like the reals. A good logical model is parsimonious.

An interesting question is whether we can formalise what is reasonable in a logical model. It seems that asking whether it can be directly implemented is too restrictive. It seems to come down to whether there is a mapping to some physical model that successfully embodies its behavior.

[On a side note, as a Platonist I have wondered for many years whether our Universe in fact only exists in a Platonic sense - ie it exists mathematically simply because it can exist. This depends on whether the Universe (and in particular whether consciousness) is computable - ie there exists a Turing machine isomorphic to our Universe. If this is true and Turing machines are assumed to exist Platonically then by Occam's razor the need to postulate physical reality should be dropped. Some credibility for this bizarre viewpoint comes from its possible explanation of the first person non-determinism of Quantum Mechanics. Anyway, if this is indeed the case then strictly speaking "abstractions" are as real as anything else!]

[snip] Received on Wed Jan 24 2007 - 02:32:41 CET