[hist-analytic] Knowing that I know a Necessary Truth

Roger Bishop Jones rbj at rbjones.com
Sat Jan 1 15:51:28 EST 2011


I don't doubt that it is possible to devise a formal system
in which something like the proof which Steve offered
would be legitimate.
However, the relevance of this either to ordinary or
technical notions of necessity would have to be established.

On Saturday 01 Jan 2011 19:13, Danny Frederick wrote:

> Hesperus is necessarily Hesperus (provided Hesperus
> exists). Why? Because it is impossible that Hesperus
> exists and is not Hesperus. But Hesperus is Phosphorus. 
> Since it is impossible that Hesperus exists and is not
> Hesperus, it is impossible that Hesperus exists and is
> not Phosphorus (given that Phosphorus simply is
> Hesperus). So Hesperus is necessarily Phosphorus
> (provided it exists).

So why should we not accept the following argument?

> 9 is necessarily 9 (provided 9 exists).
> Why? Because it is impossible that 9
> exists and is not 9. But 9 is the number of planets. 
> Since it is impossible that the number of planets exists
> and is not the number of planets, it is impossible
> that 9 exists and is
> not the number of planets (given that the number of
> planets simply is 9). So 9 is necessarily the number of
> planets (provided it exists).

and what is wrong with this argument?

Hesperus names the evening star, and Phosophorus names
the morning star.  We have discovered that these two are
both in fact the planet venus and hence Hesperus is the same 
object as Phosphorous, but this might not have been the 
case, so it is not a necessary truth that Hesperus is 
Phosphorus.

Roger Jones




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