If someone argues against obvious facts, Epictetus said, it's hard to find arguments that will change their mind. This isn't because the person is strong or the teacher is weak. When someone has been proven wrong but stays stubborn as a rock, how can you reach them with logic?
If a man, said Epictetus, opposes evident truths, it is not easy to find arguments by which we shall make him change his opinion. But this does not arise either from the man's strength or the teacher's weakness; for when the man, though he has been confuted, is hardened like a stone, how shall we then be able to deal with him by argument?