What you do is you take as many fairy-tales and myths and other stories as you can — Campbell is extraordinarily good at collecting them — then squint and trace out the patterns. Us humans are very good at finding patterns where none exist (just ask Percival Lowell), so it is no wonder that we end up with an overarching story, albeit disjointed, which is — of course it is — steeped in New Age monism.
Plot twist: unlike similar attempts in other arts, this one becomes wildly successful, serving as a template for other stories that end up following it more closely than any of the tales of old ever did (see Kevin Garvey’s literal and metaphorical travails for the most recent example). I like The Leftovers, so I would say The Hero… is a net benefit for the civilization. It just wasn’t for me.
Full disclosure: I stopped reading the book near the end of the first (of two) sections, the one about The Hero’s Journey. I therefore never got to the Cosmogonic cycle, and cannot comment. Do let me know if there was a surprise ending.
Written by Joseph Campbell, 2008