[FOM] EXCLUDED MIDDLE AS LOGICAL LAW

[Laureano Luna]

After reading recent Avron's post I think I should add a short reflection.

How can one argue for a basic logical principle? One should rather argue 
from it... But in this case I feel that something can be achieved about the 
question.
Let me first introduce the distinction between the ontological principle of 
excluded middle and the logical principle of bivalence. The former asserts 
that for any well defined situation or state-of-affairs we have either that 
it is the case or that it is not; the latter affirms that every proposition 
is either true or false. If I think that the latter can be argued for it's 
only because I think that it rests on the former and the former is accepted 
by intuitionists too.

Let me define a proposition as the description of a well defined situation. 
If the described situation is the case, then the proposition is true; 
otherwise it is false. Take this as a definition too.
Consider Goldbach's conjecture. It depicts a situation that either is the 
case or is not; so it has to be either true or false. But intuitionists 
interpret (so Heyting) Goldbach's conjecture as stating that Godbach's 
conjecture has been proved and its negation as asserting that it has been 
proved that a proof for Goldbach's conjecture would imply an absurd.

The point in two final remarks: first, it's evident that intuitionists do 
not interpret the logical negation in the same way classical logic does, 
which as Avron argues is precisely the way in which they interpret it in 
ordinary everyday language; second, they do not interpret a proposition 
such as Goldbach's conjecture according to ordinary linguistic rules. So, 
when they use ordinary language to argue, in both cases they argue against 
the very position which they argue from.