The justifications, ranking from best to worst are:
- The Spirit Worlds are a backdrop for a wild science-fantasy setting, where the Earth lies at the center of a network of alternate worlds, each with their own inhabitants and natural laws, and which are the prize in an epic war between supernatural factions.
- The Spirit Worlds are metaphors for the human experience, abstractions made real so that mages who explore the outer realms wind up learning more about their inner selves.
- The Spirit Worlds are a source of mad lootz
- The Spirit Worlds exist to give the Spirit Sphere something to do
- The Spirit Worlds are a bridge between gamelines and are the way they are so that Mage, Werewolf, Changeling, and Wraith can all exist in the same universe.
- White Wolf had a bunch of spare jargon just lying around and they had to use it somewhere.
On the other hand, the first justification lets you play a goth witch who feeds insects to a dinosaur in the lost jungles of the Hollow Earth (as depicted in the rather remarkable art on pg. 95). And, honestly, I don't know what game that comic-book nonsense belongs to, but whatever it is, it's so much better than Mage: the Ascension that it's no wonder The Book of Worlds completely derailed 2nd edition.
Yes, this is the origin of the "wars for the moons of Jupiter" meme. It's only about a quarter of the solar system chapter, but it's a very memorable quarter (plus, most of the rest of the chapter has similarly pulp-inspired stuff like jungles on Venus or an ancient race of Martians). With a selective enough reading, you can base an entire campaign off of Etherspace and it will be an eccentric, over-the-top space opera that is a top tier game setting in its own right. The only thing holding it back is its vestigial connection to Mage: the Ascension.
Like, seriously, the cosmology of outer space is needlessly convoluted. There's the physical planets, but then Mars and Venus have their own spiritual shadows in the Penumbra, and thus they're inside the Horizon, which is physically in the asteroid belt and spiritually at the border of the Deep Umbra, but the Deep Umbra is basically just space, but not really because it's psycho-reactive and the aliens who live there are spirits, but not really and in any event, beyond the Horizon, the physical world and the Umbra are one, so some regions of space are magic and some aren't, but mages can use magic to survive in the nonmagic regions of space, which are only inhabited in the spirit world, which is magical. Also, each of the planets has an astrological association with one of the nine spheres of magic, which doesn't really affect them noticeably, but does mean they are called the "shard realms" of the various spheres and they are connected in some nebulous way to the "shade realms" of their respective spheres which are, with no exceptions, the absolute fucking worst (well, maybe the shade realm of matter being the Hollow Earth wouldn't be so bad except the shard realm of matter is fucking Jupiter).
And . . . and . . . ugh.
Although, to be entirely fair, this pattern of overburdened alternate worlds is the rule, rather than the exception. Earth has the physical world and three different Umbras . . . plus border areas, zones, realms, vistas, and epiphanies, many of which exist as a form of niche protection for werewolves, changelings, and wraiths. It's just especially noticeable in etherspace because the only thing any of it needs to be is exactly what it appears to be. Your pitch is that the setting's version of Mars is "astronauts vs witches in latter-day Barsoom" and then you feel compelled to make that share a billing with the Shade Realm of Forces, which is basically AD&D's elemental plane of fire, but somehow less gameable. It's frustrating as hell, but probably just a victim of M:tA's general failure to engage with materialism as a philosophical concept.
The Technocracy is here, and they're the unambiguous villains, but the book's problems with them are the most abstract we've yet seen. There's the standard jab that they've "lost their wonder," but that's presented as more or less synonymous with "believing only what can be seen or touched." And at one point, the narrator, a Son of Ether, says, "Looking for a definitive answer to everything is the Technocracy's gameplan, not ours," which is a hell of a thing to admit without realizing how bad it makes you look.
Imagine me shaking the Mage corebook and shouting, "what the hell did epistemology ever do to you? Why'd you leave it beaten half to death and bleeding out in a grime-crusted alley? Why? Tell me!"
They do try to tie the Technocracy to actual atrocities, so as to not entirely dilute their villain cred, but those atrocities are undermined by the clumsy way the book attempts to make them an inevitable consequence of the Technocracy's metaphysics. They genocided all the life on physical Venus and Mars!
. . . by observing that they were lifeless rocks. And I guess this is supposed to have been an intentional act where they so rapidly and thoroughly convinced a large enough portion of Earth's population that consensual reality made it a fact, but that doesn't address any of the numerous problems with such a theory (If a change that dramatic can happen so suddenly, doesn't that mean that all life everywhere is at the mercy of the whims of fashion? Don't those planets have their own penumbras, meaning that they aren't actually subordinate to the consensus on Earth? Is modern science education actually that persuasive?).
If you want to know where the "heroic Technocracy" came from, this here is a big part of the story. By making "believing it's possible to figure shit out" into an intrinsically villainous trait, you wind up less "crafting a memorable, complex villain" and more "making your designated heroic narrator sound really unreliable." There's a lot of people reading the book (myself included) thinking there's nothing extraordinarily wrong about the Technocracy's paradigm, its problem is that it's led by a bunch of corrupt imperialists. Like, it is so cruel and selfish that, politically, it's unsalvageable as an organization, but even in a post-Technocracy world, inventing technology usable by the masses and placing reasoned understanding ahead of uncritical "wonder" are going to be admirable goals. Reproducibility isn't a sign of inferior imagination, but of superior craftsmanship.
Of course, you could probably write whole volumes exploring the connections between modernism and colonialism - was colonialism a logical outgrowth of modernist thinking, or did modernism arise, post hoc, to justify colonialism? Can the degree to which it was used to justify the era's worst atrocities be weighed against the ways it served to mitigate its greatest excesses and pave the way for the more compassionate philosophies that followed? It's a discussion that's way above my pay grade (unless, of course, you guys start paying me - 40k for tuition at a small liberal arts college ought to do it). But to reference a great modernist philosopher, the ruling ideas of any age are the ideas of its ruling class. Modernism couldn't stop slavery any more than Christianity could hold back warring barons or post-modernism could silence the alt-right. Power is much too slippery for that, and usually winds up co-opting the most effective forms of criticism. The postmodern right is a big reason I think of Mage as a 90s game. How can we possibly take the Traditions seriously as a rebel faction when the real-life equivalent of the Technocracy starts deriding "the reality-based community?"
Getting back on topic, The Book of Worlds is a beautiful mess. It's got a ton of great ideas, but they are so many that they start to stumble over each other. I didn't like the parts of the setting that were basically like a velvet rope for the other gamelines, the issue with the space opera I already covered, but the purer "journey to the center of the primordial mind" style of spirit worlds also takes a hit. There are Verbena growing a garden in the astral junkyard where spiritual echoes of new inventions pile up like snowdrifts. You can visit heaven and hell on the astral plane and it's pretty clear that those are only reflections of humanity's ideas about heaven and hell, but also there's an actual afterlife in the Shadowlands, and you can go there too (sort of) and it's also pretty explicit that the heavens and hells there are just as ersatz.
Nothing is real, but also there are islands of pure, carnal reality in places that should be allegorical. There are realms that are populated by essentially mundane creatures, living essentially mundane lives, and also realms where everything is a living spirit pantomiming the rhythms of a life, a society, or an entire world. Often, the former are more fantastic than the latter.
It's all too much. Wheels within wheels, and no answers to any question (or rather, so many answers that truth is a distant dream). I think, philosophically, mage comes down on the position that wanting answers is a form of oppression (this book refers to exploring the deep umbra with the metaphor, "once you snatch your butterfly from the air, it dies"), but honestly, the book, purely as a form of fiction, would have benefited from a little more focused curation.
I don't want to be too much of a curmudgeon about this, though. There is something to be said for the spirit world being total pandemonium. I just with the text didn't take it for granted that uncertainty was a virtue.
Ukss Contribution: The Summer Grove of the Verbena is populated by all manner of animals. These animals are just normal creatures, but each species is protected by its own guardian spirit. I like that idea - divine animal spirits that are drawn to overhunted herds and exploited groves.