Paired Insulators and High Temperature Superconductors

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.