Answer
A theorem can be cited as a "reason" for a proof as long as it is not this specific theorem that you are presently trying to prove.
Work Step by Step
A theorem can be cited as a "reason" for a proof as long as it is not this specific theorem that you are presently trying to prove. That is, if you are trying to prove a theorem, we can not cite that theorem as a "reason" for its own proof.
