But there were stories that out there, wherever that was, there existed a shadowy half-man, simultaneously alive and dead, murderer and victim. No one knew what he looked like, except that he had only one arm – and no one could agree which one he had lost, or how it had happened. His name was Grandfather Paradox, and if you were naughty he would ﬁnd you and use you and destroy you, as part of his labyrinthine schemes against the Time Lords. The Doctor had taken the story very, very seriously as a child. For months, perhaps even years, afterwards he had worried that Grandfather Paradox was under his bed, or lurking beneath the table in the refectory, or making the noises he could hear outside at night.
– Lance Parkin, The Gallifrey Chronicles