This is a question directed at Roman Catholics, or anyone familiar with the history of the Roman Catholic church.
Every Doctor of the Church is a saint. I was curious if this is perhaps a requirement to be recognized as a Doctor, or if it is merely a "coincidence" (i.e. unintentional by the church) that all Doctors also happen be saints?
I've also noticed that some Doctors of the Church are also Early Church Fathers. The opposite is not true though - not all Fathers of the church are necessarily Doctors (see Origen, for example).
What exactly are the requirements to be appointed a Doctor of the Church?