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

Danny Frederick danny.frederick at btinternet.com
Sat Jan 1 14:13:36 EST 2011

Hi Roger and Steve,

Yes, Steve about sums it up. Kripke, of course, uses that argument in
'Identity and Necessity' (and no doubt elsewhere). But he also argues
informally, along the following lines.

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).

Given the informal nature of the argument, the necessity/impossibility
involved is taken to be ordinary, everyday necessity/impossibility. But, as
Steve's argument shows, it doesn't matter what kind of
necessity/impossibility is involved so long as (in the requisite sense of
'impossible') it is impossible that Hesperus is not Hesperus.

I explain the meaning of necessary as follows: it is necessary that p =df it
is impossible that not p. Of course, this explains one modal notion in terms
on another; but there can be no reductive definition of modal notions.

Ultimately, the only ground for believing a claim of necessity is its
intuitive appeal. In some cases we may be able to argue that one thing is
necessary given that some other things are; but then those others are either
intuitively necessary or themselves derived from other necessities. We have
ultimately to rest on intuition if we are to rest at all. But all our
intuitions are fallible and revisable. So, any claim that a proposition is a
necessary truth can be only tentative.


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