Re: An argument against modus ponens

On Sep 8, 3:25 pm, John Jones <jonescard...@xxxxxxx> wrote:
Implies is fine. If P implies Q and P is true then it follows that Q is
also true.

Yes, THAT "implies" is fine -- THAT implies is a purely rule-
driven, syntactic (and therefore LOGICAL) thing. It is YOUR
use of "implies", which is NOT that, that I was condemning,
when I said that nothing was ever implied around here.
You said that
your "exclude" implies R.
That was NOT a logical implication.

By R, here, you meant some third thing, other than P and If.
But P, BEFORE IT WAS A PREMISE, WAS a propositional variable.
At THAT point in time, BEFORE it had been asserted as a premise,
EITHER of P or ~P *could* have been adopted as a premise.

The "other things" you are talking about are models or interpretations
or possible worlds. Those were your R. But in 1st-order logic,
semantics is not absolutely necessary.

.

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