Re: More on identifiers
Date: Fri, 05 Jun 2009 10:25:50 GMT
"David BL" <davidbl_at_iinet.net.au> wrote in message
> Informally I think of abstract identifiers as "internal glue" within a
> relational database. A bit more formally, they are characterised as
> identifiers that could be mapped bijectively to different values
> throughout the database without changing the recorded information.
> One would therefore hope that they aren't visible to end users.
> I have wondered for some time whether abstract identifiers are only
> needed within the confines of a flat relational model. The following
> hypothetical example is meant to cast some light on this question.
> I'm hoping you'll see the underlying matters of principle and see how
> it raises some interesting questions and ideas:
> Let a cardboard box of items land on your desk and your task is to
> record information about the items in a database. In theory each item
> is uniquely identified by its (x,y,z) position at any given epoch.
> However, the idea is that the items are mixed up in the box and their
> positions are irrelevant to the information that needs to be recorded.
> The items are not labelled. The idea is to uniquely identify them
> (only) by their observable properties. This is indeed assumed to be
> an important integrity constraint to be enforced by the DBMS. Note as
> well that it would be upsetting (and potentially very costly or
> impractical) if the database system forces items to be labelled when
> there shouldn't be a need to.
Consider two electrons. They both have the same mass, and they have the same charge. They might have opposite spins. But the minute we add a third electron, the spin of two of them is going to be identical. It seems that, on the surface at least, electrons do not have enough properties to carry identity. As you descend into lower level particles like quarks, things get even more this way. Particles seem more and more interchangeable. Subatomic particles might not have much to do with your objects in a box, but it seems to me that any theory of reality and identity that falls apart at the subatomic level should at least take that into account. Received on Fri Jun 05 2009 - 12:25:50 CEST