Hallorann and his grandmother, who would 'shine' together by having 'conversations without ever opening our mouths', were a pair - like the twins. Danny is 'the only one.' This is partially true, but Danny has solved this problem by having Tony. Later in the movie Danny will pass through the mirror and Tony will take over his body for him. Danny is alone, but he is also a twin. Halloran also explains, refering to Danny's parents, that some people don't know or don't believe they 'shine'. This explains why the ghosts can lead Jack to murder, he does not know himself - he does not know he can shine. He mistakes the illusions for reality.

HALLORANN Do you know how I knew your name was Doc? You know what I'm talking about, don't you? I can remember when I was a little boy, my grandmother and I could hold conversations entirely without ever opening our mouths.