[FOM] Is mathematical realism compatible with classical reasoning?

[tchow]

Andre writes:
> The crux of the argument in my FOM posting is that if
> classical reasoning is valid for this truth predicate also, then it
> appears to have all the features of a mathematical predicate.

Appearances can be deceiving.  In this case, we have a mathematical
proof that the truth predicate is not a mathematical predicate.  What
stronger reason could we have for rejecting the hypothesis that this
truth predicate---"appearances" notwithstanding---than a mathematical
proof?

> I agree that mathematical realism does not require that there be a
> uniform sense in which a sentence may be true or false. Truth may be
> treated as a trivial operator, so that the truth of each sentence is
> taken to be equivalent to that sentence. In this variant of
> mathematical realism, the validity of classical reasoning for the
> mathematical universe cannot be expressed as a general principle,
> mathematically or otherwise.

Why can it not be expressed as a non-mathematical general principle?

Tim