Religions rely on miracles to demonstrate the authenticity of figures thought to have supernatural powers. Many people feel that key events in their lives were literally miracles. Many even claim to have witnessed miracles. But what counts as a miracle? Is it true, as Hume argued, that it is always more rational to disbelieve the testimony of a miracle than to believe in the miracle itself? John and Ken explore what miracles are, and what would constitute good reasons for believing in them, with Peter Graham from the University of California Riverside.