The Birth of Purgatory

I am interested in the question of where the idea of purgatory came from.   Roughly speaking purgatory is a kind of third place, between heaven and hell.   The abject sinners (or those who reject Christ, or whoever you think is destined for punishment) go to hell; the righteous saints go to heaven.  But what about those who will ultimately be saved but who have not lived a good (enough) life?  They go to purgatory.   This has been the standard teaching of the Catholic church since the 12th or...