Re: Tautologies Then and Now

From: Chris Menzel (cmenzel_at_remove-this.tamu.edu)
Date: 12/13/04 

Date: 13 Dec 2004 10:46:29 GMT

On Mon, 13 Dec 2004 00:44:52 GMT, Stephen Harris said:
> ...
> I could have used Modal logic as another counter example; where the
> term tautology is applied outside of propositional logic.

Hm, nothing I saw in what you quoted supports this claim. Did I miss a
quote where the validities of propositional modal logic are referred to
as "tautologies" by someone?

As you note, there is a limited use for truth tables -- of a sort -- in
the case of the propositional modal logic S5, but this is directly
analogous to their limited use in Monadic Predicate Logic (indeed, in a
certain sense, it's identical). But because the modal operators
function semantically as quantifiers over "possible worlds", a general
truth table style semantics for propositional modal logic is not
possible.

Chris Menzel