I can believe a miracle because I can raise my own arm. I can believe a miracle because I can remember. I can believe it because I can speak and be understood by you.
The genuine realist, if he is an unbeliever, will always find strength and ability to disbelieve in the miraculous, and if he is confronted with a miracle as an irrefutable fact he would rather disbelieve his own senses than admit the miraculous also.