If I know that proposition A is true, can I perform a reductio ad absurdum this way (I know it sounds redundant but humour me for a moment):
This leads to the contradiction ~A & A, because we know A is true.
Therefore, conclude the negation of the assumption: A
I have a specific case in mind. Let A = "God exists." Let B = "It's possible that God exists."
Assume ~A. Contradiction: ~A & B (because B is true (we are not hard atheists here)). Therefore A.
In fact it seems anything that is unknown can be proven this way, so there must be a problem with it.
I'm thinking that the propositions used in the contradiction must arise from calculations derived from the assumption. They cannot arise from anywhere external. Do you reckon that's right?