On 7/17/2012 3:35 PM, Chris Partridge wrote:
> It looks to me as if one can distinguish between (a) terms/classes whose
> definitions (membership conditions) change over time but whose extension
> remains the same across time and (b) terms whose extensions change over
> time, so are probably not classes in any set-theoretic sense. (01)
For each class, the defining relation is fundamental. The set
of instances can change at different times, places, and possible
(or actual) worlds. (02)
> for a standards body at any particular time there is an official definition
> of a metre and so the extension of the class metre at any particular time
> is fixed by the official definition at that time. (03)
The old standard was the distance between two marks on a bar made
of a platinum-iridium alloy when measured at the melting point of ice. (04)
The current standard is equal to 1,650,763.73 wavelengths of the
orange-red emission line in the spectrum of a krypton-86 atom
in a vacuum. (05)
In either case, there is no "instance" of a meter. The length
of something is always determined by a relation (some procedure
that carries out some kind of comparison). (06)
Basic principle: the *relation* is fundamental. (07)
John (08)
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