Blake's apparent preoccupation with satanic or magical nature in the period just described is so ambiguous that Yeats can hardly be blamed for pushing the interpretation a little too far, to suit his own theory and inclinations. Blake's third period begins with the prophetic book called America. Natural imagery tends more and more to turn into abstraction, human beings into superhuman types, even archetypes, psychological insight into mysticism. By grace or by choice Blake is treading the mystical way now, and nearing his goal. He has passed through the Dark Night and is on the verge of four-fold vision. Urizen, the God of Dogma, whom William Blake equates with the Old Testament Jehovah, is abstract dogmatic reason, and as such is the source of all evil. It is significant that it is Urizen and not Satan who fell from Eternity. Satan is a milder evil associated with nature, and is in fact much like the archetype of the enchanter.