With their long, rich and varied history demons can tell us about both nature and thinking itself, says Robert P Crease Meet the family Demons come in four types: one-off, law-governed, instructional and place-holder. (iStock/ringo sono) Demons might be mythical, but for hundreds of years these imaginary creatures have been fantastically useful for thinking about science and the humanities. For physicists, the most famous demon was dreamed up by James Clerk Maxwell in 1867.