  Almost forty years ago, Phil Anderson introduced the negative U center to
account for the fact that most glasses and amorphous semiconductors are
diamagnetic. This paper has been highly influential, but certainly does not
rank among Phil's most famous works. However, focusing on it enables us to
reacquaint a younger generation with another of Phil's contributions, and to
use this as a springboard to discuss some forward looking extensions that
continue to fascinate us.
