Hello Planewalkers.
Do you know books which might have inspired the Planescape setting?
I read recently the "Pastel City" stories in "Viriconium" by M. John Harrison.
Parallels to Sigil are obvious. The atmosphere is similar to that in Planescape.
Sigil's geometry is surely lent from Larry Niven's "Ringworld".
Any further suggestions?
On that note, does anyone know offhand which came out first: Mage the Ascension or the PS boxed set? Were the releases close enough together that they had to both be in development simultaneously, or is it fairly certain that one influenced the other?
According to my research, the Planescape Campaign Setting Boxed Set was published in 1994. Mage: The Ascension was published in 1995. Planescape wins! 
The AD&D 2E Manual of the Planes came out in 1987.
Mage and Planescape were likely independent of each other. Mage was in many ways based off of Ars Magica, though taken into the modern World of Darkness.
The Baator is clearry inspired by the first book of the Divina Commedia (Inferno), while the seven skies of mount Celestia are from the second book (Purgatorio). Factions are inspired by different real worlds philosophies (i am less sure about those and i hope i won't become too obvious or rude):
Anarchists: inspired by real world anarchist party, a pre-first socialist international party against every form of government.
Xaosmen: maybe inspired by Dada movement?
Free League: there is always people against social discipline, and i hope there will always be.
Dustmen: i think they are inspired by Arthur Schopenhauer and his "Will" theory.
Godsmen: in old d&d (Mystara) players could actually become gods by winning some epic trials.
Ring Givers: in real world Saint Francis called himself "Jester of God".
Harmonium: every totalitarian ideology is based on their basic teaching, but also Kant's talk about perpetual peace inspired them.
Sign of One: I think they are loosely based on post-Kant ideas of solipsism.
Takers: the first who comes to my mind is Lorenz von Stein and his elaborated object idea, but many others have said things like that.
Well, these are just some... i hope i didn't get off-topic or too boring
Akenaton
There was a group of Imaskari whose philosophy was more or less identical to the Dustmen in the Horde boxed set (also by David "Zeb" Cook). Brian W. Aldiss (I think) wrote a short story with a future religion pretty much the same as the Dustmen, as well (with the added wrinkle that the sect's leader was a drag queen).
The Godsmen seem most similar to certain ideas in Hinduism, although I understand that Mormons have a similar idea.
The custom of ring-giving is an old one in Scandanavian cultures. The leader proved his right to rule through his generosity - often by giving literal gold rings. Odin had a magic golden ring that duplicated itself, so he could give nonmagical copies to his followers. This is why the Ring-Givers are associated with Ysgard.
I'm thinking Ayn Rand and the like.
The Takers remind me of Nietzsche: Everything has already been and will be again and, well, the Übermensch knows this and therefore gives a shit about anything but his own well-being since he cannot change anything in the great, endlessly repeating sheme of the world. It's basically like this I think, but proof me wrong.
The Übermensch beats up everybody who refuses to give him his pocket-money... 
Precisely!
Well, I think Nietzsche philosophy about helping others is a bit more complex, as the übermensch CAN be munificent, not like the "weak" philosophy of the slaves (you have to help others because it's good and it's your duty) but because he's the strong one and so he can knightly lend his help to weaker people (at least as I understood from Beyond good and evil). Also, his help would better be self-help (creating the conditions to others to help themselves). The darker side is his (in So Spoke Zarathustra) speech against weak ones and deformed, who should die.
Anyway i think comparing Nietzsche and takers is a good point.
Akenaton
I’d agree, roughly. I think one can see it as somewhat similar to the stream of thought incorporated into ‘The Closed Fist’ philosophy, from the computer rpg Jade Empire, more specifically by Smiling Mountain. As he points out that this view on life isn’t about mindlessly crushing or dismissing others for the benefit of oneself, one works for the greater good of the Jade Empire, maybe humanity/ the world can’t remember.. thus whether to help a person(s) in a given situation would depend on the specific context, it might be deemed more constructive to not give a hand and let the person(s) fend for themselves and by so doing let them grow stronger through their own effort , thus bringing more prosperity to the Empire in the long run. For example there is a quest in the game where you have to help a village, which have lost its main avenue of growth, its inlet has dried up; thereby cutting the village off from sea trade. Here you’ll eventually be given the choice to either, broadly, switch on a mechanism that will be bring water back or to destroy it and completely remove the possibility for sea trade.
Anyway my point about Nietzsche and ‘The Closed Fist’ philosophy is that they both work in the service for something bigger than themselves. IMO it’s glaringly obvious that Nietzsche worked for the greater good of humanity as a whole, one may characterise him as notoriously elitist, but he still, in a accordance with the values he treasured, devoted a large potion of his life to the betterment of humankind as a whole. The fated only think of themselves as far as I’ve understood.
I think its important to emphasis that the Factions/sects are inspired by real life things, but are quite different if by nothing else then by the virtue of the fact that the Planscape universe and our own are different.
No, not necessarily. Many of the Fated are good (including their factol Rowan Darkwood for most of his career before the time loop). To be good, you have to be compassionate and think of others. They won't give charity, but they'll still work to help others help themselves, sacrificing much for the benefit of all.
@ ripvanwormer
Interesting ^^.. read more Planescape I should, but so little time and so many things to do ... 
On Nietzsche: pretty much. The Ubermensch would see himself as a supreme good of humanity. His will justifies existence. The Takers by the way seem a lot more capitalistic than Nietzsche himself, who didn't actually have an American flag on his car.
You can find aspects of the Godsmen and Dustmen both in Hinduism. Although I guess (forms of) Buddhism might fit the Dustmen better.
I don't think Nietzsche praises any other political system the way he praises the castes of India. They may need merchants, but the warriors and philosophers come closer to his goal (ignoring the whole problem of religion.)
Kay wrote :
I take it you refer to Nietzsche’s notion of eternal recurrence (ewige Wiederkunft). Though there is controversy surrounding the meaning of this concept, I for one, find a literal understanding of it quite far out – indeed, if one considers Nietzsche’s train of thought in its context. I’d be keener to the interpretation done by the contemporary American philosophers’ Robert C. Solomon and Kathleen Higgins, for instance, that it is a test; to see if you truly love life the way you live it at the moment; do you really live in the most optimal way according to yourself. Exemplified by the hypothesis, by Nietzsche, of a demon showing up and asking the question of whether one would accept to live one’s life, the exact same life, again and again and again for all eternity, what would be one’s answer in such a circumstance? … To me, and Solomon and Higgins this is not to be taken literally. But meh.
Apart from the philosophical topics in this discussion I would like to mention Fritz Leiber’s stories set in “Lankhmar”, which used to be licensed by TSR as an AD&D setting. I find the stories really entertaining and close to some aspects of Planescape. “Bazaar of the Bizarre” is a good example for that.
The modrons might be descendants of the flatlanders in Abbott’s “Flatland: A Romance of Many Dimensions”. They somehow managed to become three dimensional.
The Planescape inspiration site contains very interesting material. (Thanks to Ripvanwormer for the tip.) Unfortunately the last update was made in 1998. Maybe somebody would ask the author to contribute some of it to Planewalker.com?
PS.: The Xaositects remind me of Diogenes the Cynic. He is the one who lived in a tub. (‘Cynic’ is derived from the Greek word for ‘dog’. The “starved dogs barking” might hint in that direction, too.)
PPS.: Nietzsche is difficult to pinpoint and echos of his teachings can be found in the Athar (“God is dead.”) as well as in the Sensates with their decadent artistry. He mentions several (somewhat imperfect) role models for his concept of the uebermensch, including Napoleon Bonaparte and Leonardo da Vinci. It is more about virtuosos and egoists.
Just read about the work on wikipedia, neat way to criticize one's society! ^^
In what way decadent artistry? Curious...
I shall try to make myself clearer. Which one do you find unfitting in this context – the Sensates or the Nietzsche?
‘Decadence’ may be a harsh judgement on the Society of Senses, although they were invented according to a certain cliché, of course. I understand them to be Bohemians with a susceptibility to decadence, as was said before. Consider the “Brothel of Slating Intellectual Lusts”. As to artistry, a poet has defined art as “the expression of the world within us by means of the world surrounding us”. That sounds like a Sensate’s motto to me.
Nietzsche blames himself in the preface of “The Wagner Case”: “I am like Wagner a child of these times, which means to be a décadent…”, but what I rather have in mind is his “l’art pour l’art” attitude and his preference of illusion (in the arts) over any objective truth.
Nietzsche was on the brink of becoming a professional musician, by the way.
Rabenaas wrote :
Heh, neither, I couldn’t grasp what you meant by ‘decadent artistry’ in the given context
The Artistry/art (on both Nietzsche and the Sensates.), by itself mind you though, aspect is clear now. Further I get decadence by itself, but the two together, i.e. decadent artistry, no. (haven’t read The Chase of Wagner btw).
Nietzsche does not see himself as a decadent artist after having adopted this creed; ‘ “l’art pour l’art” attitude and his preference of illusion (in the arts) over any objective truth.’ Right?
The cant in the early PS gamebooks reminds me of Shadowrun material. The "belief alters reality" idea reminds me of Mage. The breakdown of characters into factions reminds me of White Wolf in general (Mage traditions, the Camarilla).
I take it you refer to Nietzsche’s notion of eternal recurrence (ewige Wiederkunft). Though there is controversy surrounding the meaning of this concept, I for one, find a literal understanding of it quite far out – indeed, if one considers Nietzsche’s train of thought in its context. I’d be keener to the interpretation done by the contemporary American philosophers’ Robert C. Solomon and Kathleen Higgins, for instance, that it is a test; to see if you truly love life the way you live it at the moment; do you really live in the most optimal way according to yourself. Exemplified by the hypothesis, by Nietzsche, of a demon showing up and asking the question of whether one would accept to live one’s life, the exact same life, again and again and again for all eternity, what would be one’s answer in such a circumstance? … To me, and Solomon and Higgins this is not to be taken literally. But meh.
Well, and maybe he just needed an excuse for his negative world view. But the point is not how its being interpreted, but rather could it possibly have inspired Planescape (especially the Takers). Rabenaas made a good point there, though.
I'm pretty sure that the bleakers are based on the absurdist movement of the 20th century. Their philosophy pretty much reflects exactly what that was all about. They never preached or forced people to accept it. What was the point? Nothing matters. They simply portrayed life in the way that portrays people looking for answers in places where there are none. The internal purpose that bleakers strive to develope is also prevelent in the philosophy. However the insanity that comes with it less so.
Wahoo! I'm no longer clueless, I'm a cager!
Honestly, the Bleakers always struck me as fairly existentialist.
Existentialism and Absurdism is more or less the same thing. Absurdism branched off from it. The whole idea that life is so unbelievably absurd, sheer and utter nonesense, is kinda the crazy part of the bleakers.
But your right I usually use the term existentialist when I'm actually talking about Absurdism, as I said more or less the same thing.
Arguably, each faction has an obvious real-world parallel philosophy or religion behind it... unless it's an amalgam of a few of them.
My major guesses:
Harmonium -- Communism, Confucianism, or Fascism (the Hardheads' actions have elements of all of them..)
Revolutionary League -- Anarchism (with the nickname they have, it should be darned obvious, but geared towards the style of action favored of the violent 19th century movements)
Athar -- Atheism (again, this should be DEAD obvious) + Agnosticism (not the "I'm not sure if a god exists" type... more the "I'm not sure how I should worship the deity I believe in because I can't know what the deity wants" type)
Bleak Cabal -- Existentialism (nothing matters, life sucks... yadda yadda yadda)
Xaositects -- Dadaism or Discordianism
Dustmen -- Platonism + nihilism
Fated -- Social Darwinism + Utilitarianism
Fraternity of Order -- Positivism.. kinda...
Mind's Eye -- Trancendentalism
Sodkillers -- Legalism + Hobbesian Soical Contract
Sons of Mercy -- Aristotelianism
Trancendent Order -- New Age philosophy in general or maybe Taoism
Sensates -- Empiricism
Free League -- less violent anarchism + moral relativism (or as close as you can get in Planescape...)
Ring-Givers -- Sikhism w/o the religious leanings
Note, these are basically guesses. But if you look into it, the models' relations to the various factions generally should fall into place REALLY easy if you have studied philosophy at all deeply. Some of the more major philosophical trends are oddly unrepresented; however, in a game where morality is objective, the Kantian categorical imperative is likely to be the standard, so it's not entirely unsurprising, but even then, the Harmonium probably operate on such a standard. However, the breadth and depth of theories is impressive; for the most part, you can play a character w/ almost any philosophical leanings and one of the factions will probably be appropriate, should you care to look...
Possible definition: (somewhat sick) exaggerated aesthetic lifestyle and/or state of mind, to produce and consume art as a way of / the meaning of life
Examples: Dali, Wilde, Baudelaire, Beardsley, Toulouse-Lautrec …
I also see elements of Buddhism in the Dusties - the whole nonattachment thing.
I always thout of the Dustmen as really depressing Budhists, and I dislike both philosophies for the same reasons.
Ring-Givers -- Sikhism w/o the religious leanings
The Fated and the Ring-Givers are a paralax of a concept found in pre-christian germanic cultures (note that they both originate in Ysgard). This being the idea that gifts given are to be returned in kind. The repeated cycle of gift exchange strengthens the social bonds between the two parties, and breaking the cycle would not only weaken the bonds, but probably cause a loss in standing in the community as well. Note that this is a gross over-simplification.
The Fated deal with this by guaranteeing reciprocity at the time of the giving, probably due in part to a lack of trust of the other party's attitude about the gift exchange. The Ring-Givers deal with this by trusting their own power of belief in their philosophy to sort things out for them.
Libertarianism
Bleak Cabal: basically, this group is founded on skepticism and, from all other factions, has the strongest connection to Nietzschean philosophy (there is no valid interpretation of the external reality, all interpretation - and therefore law, morality, meaning - comes from within, as a free creation of self). I always disliked this derpressive manner the Cabalists were described (becouse such a depressive attitude simply cannot be derived from their beliefs - and that blasé pose, I guess, is the only affinity to existentialism here).
Nietzschean concept of Ubermensch can't be reduced to Fated beliefs by any means. It's so much more than any 'give me your candies' attitude.
The Fated's beliefs shouldn't be reduced to a "give me your candies" attitude either.
The Fated's beliefs shouldn't be reduced to a "give me your candies" attitude either.
I didn't want to imply either.
What of the Doomguard? That one I can't pinpoint?
^^ possibly Nihilist... and, well, there's an "entropy" crowd amongst physicists (or at least the physics buffs that I know) but they're not really a philosophical posse (though they real down with entropy)
Well after much deliberation, contemplation and rehabilitation I believe that the doomguard philosophy. Does to some extent come from the laws of thermodynamics. One of them but I couldn't say which one exactly. I don't know the law very well.
I would suggest the 2nd Law of Thermodynamics:
"In simple terms, the second law of thermodynamics states that for a system, the intensive thermodynamic quantities such as temperature, pressure, and chemical potential tend to become more uniform as time goes by, unless there is an outside influence which works to maintain the differences." - Wiki
My major guesses:
...
Athar -- Atheism (again, this should be DEAD obvious)
...
I always looked at it from the following perspective:
"Aether (also Æthere, Greek: Αἰθήρ), in Greek mythology, is one of the Protogenoi, the first-born elemental gods. He is the personification of the upper sky, space, and heaven, and is the elemental god of the "Bright, Glowing, Upper Air." He is the pure upper air that the gods breathe, as opposed to the normal air (Ἀήρ, aer) mortals breathe." - Wiki
In this way the outer and inner planes "could" have a unifying concept: the Aether / Ether. I have been speculating that a magic users' understanding of these concepts is what allows them to manipulate energy into action through spells.
Were the releases close enough together that they had to both be in development simultaneously, or is it fairly certain that one influenced the other?
The former.
The Baator is clearry inspired by the first book of the Divina Commedia (Inferno), while the seven skies of mount Celestia are from the second book (Purgatorio). Factions are inspired by different real worlds philosophies (i am less sure about those and i hope i won't become too obvious or rude):
The Planescape cosmology (minus Sigil, the Infinite Staircase, and several locations) were all designed by Gygax and friends back in the 1E days. The planes are modelled as following:
Abyss- The Fiery/Bottomless Pit from Revelations, last book of the Bible
Baator- Dante's Inferno
Beastlands- Based off of the Amerindian apellation of heaven, known as "The Happy Huntinggrounds". I don't know if this was a pre-contact or post-contact construct, however.
Carceri- based off of Tartarus, the dark pit and prison from Greco-Roman mythology (it is also mentioned in the New Testament of the Bible as a place where some of Satan's demons are chained up)
Gehenna- This is based off the Hebrew hell by the same name.
Gray Waste- Based off of Hades, Annawn, and Hel, primarily, but also other 'generic dreary underworlds' such as Sheol (Hebrew grave and equivalent of Hades)
Mechanus- supposed to be Nirvana, but Nirvana isn't a place in Buddhism. There are many heavens in the Dharmic religions however, which are characterized by clouds and lots of floating lotuses.
Sign of One: I think they are loosely based on post-Kant ideas of solipsism.
Yeah, that's a no-brainer.
I think its important to emphasis that the Factions/sects are inspired by real life things, but are quite different if by nothing else then by the virtue of the fact that the Planscape universe and our own are different.
Yes, but with a few exceptions, most of the factions and sects are based on rather generic, abstract ideas. For instance, the Doomguard always struck me as just being a different twist on the apocalyptic nutbags, that twist being that they actually worship/revere the armageddon (whereas apocalyptic nutbags want armageddon to occur because they believe a better world will be borne from its ashes). The idea of a mortal ascending to godhood is another really generic one that can be found in quite a few circles, including the Church of Scientology (ooh, I just made them block our site, LOL!), as the Operating Thetan folks prettymuch are gods comparatively (despite how much the CoS might want to deny this, just like the whole body thetan thing is just a tired old rehash of demonic and evil spirit possession-- the whole Xenu backstory doesn't add any originality whatosever since it's still the ol' generic tortured spirits story.)
Though not exactly the same thing as folks seeking divine ascension, most despots of the 20th century and heck, throughout history have viewed themselves as gods *and their country as a divine promised land*. Hitler made everyone salute anything in his likeness, Kim Jong Il has his people pray to him, etc. and then of course we have the whole "ruling family descended from the king/queen of the gods" cliche in imperial Japan, imperial China, pre-Christian Celts, pharoic Egypt, and to a degree with the whole "Divine Mandate" crap that the western monarchs tossed around (which was as close to "I'm a descendant of God" as the monarchs could get away with)
The Harmonium are pretty generic, as well, and I always took them to be based off of "neighborhood watch" groups that always start overreaching and end up bullying/oppressing the people they swore to protect. They're more generic than even that, however-- the harmonium signify humanity's struggle to balance freedom with security, and treading the fine line between security and oppression (or moral panic) The Harmonium have about half of one foot over the line into oppression.
PPS.: Nietzsche is difficult to pinpoint and echos of his teachings can be found in the Athar (“God is dead.”) as well as in the Sensates with their decadent artistry.
I always thought that the Athar were mostly just the Planescape equivalent of Atheists.
as well as in the Sensates with their decadent artistry.
I don't think the Sensates are based on anything particular, they're just the Planescape equivalent of hedonism, and if you've ever encountered a hedonist IRL or online, you'll know that most of them are the "try anything once" types.
The "belief alters reality" idea reminds me of Mage.
"Belief alters reality" is actually from some New-Agey hippietastic stuff that unfortunately found its way into various self-help cults and scams (such as "willing your cancer away" or "willing money to come to you" and crap)
Bleak Cabal -- Existentialism (nothing matters, life sucks... yadda yadda yadda)
The difference between Existentialism and Absurdism is that Existentialists (at least the ones I've encountered) usually follow a "just because there is no purpose to existence doesn't mean that our lives are without meaning/purpose". Most usually take the philosophy that either the purpose is whatever meaning you give your existence or that the purpose is to leave one's mark on society via good deeds/humanitarian efforts (of course, this tends more towards humanism)
I don't know if anyone mentioned it already but there is Gord the Rogue short story in the Night Arrant collection titled 'The Weird Occurrence in Odd Alley' that features eerily Sigil-like planar city.
Also there was illustration in the old Manual of the Planes that featured a group of adventures in front of the Sigil like city
here's the link to the ad in question
http://www.gmoracle.com/wp-content/uploads/2010/07/motp.jpg
Sort of. Some of the later material seems to paint the Athar as closer to being Deists, with their belief in the Great Unknown. They reject the anthropomorphized powers with their flaws in favor of a distant, impersonal, unknowable creator entity.
The difference between Existentialism and Absurdism is that Existentialists (at least the ones I've encountered) usually follow a "just because there is no purpose to existence doesn't mean that our lives are without meaning/purpose". Most usually take the philosophy that either the purpose is whatever meaning you give your existence or that the purpose is to leave one's mark on society via good deeds/humanitarian efforts (of course, this tends more towards humanism)
Yeah, that's something I really like about the Bleak Cabal. You can play them either as nihilists who've given up on most everything, or as oddly optimistic humanists who work for the good despite a seemingly dark worldview. (Or ideally some members of each.)
One of the best metaphors for absurdism that I've heard was the following:
A man in a tuxedo comes up to a person on the streets of Las Vegas and starts displaying various amazing displays of dexterity with a deck of cards (making the cards flip from one hand to the other, etc.) He then fans out the cards and asks the on-looker to "Pick a card, any card."
The passer-by picks a card, looks at it and then (as instructed) puts it back at a random spot in the deck.
The tuxedoed man then shuffles the deck repeatedly and then tosses the entire deck into the air. As the cards flutter down, he quickly grabs one card out of the air and shows it to the on-looker.
He asks, "Is this your card?"
"Um...no."
"To an absurdist, this means nothing."
Since most people don't get this, I'll explain it a little more. (My apologies to those to whom I'm being condescending.)
Absurdism posits that people attempt to impose meaning and intentions on things just to give ourselves a sense that we can control or expect certain outcomes.
In the tale above, we are led to believe that the correct card will be retreived. But if you look closely, the card shark never says "I'm a magician" or "Let me show you a card trick". If the same story was told about a 6-year old playing with a deck of cards, we wouldn't be surprised by the 51 in 52 chance of it being the wrong card.
Absurdists would argue that expectation of things to behave a certain way (e.g. bad people will be punished somewhere down the line) are just as illogical as expecting the 6-year old to pick the right card. Sure certain people might dress up things to make it look like the improbably expectation will come true; but ultimately its just a form of self-deception.
In a way, I guess it is the opposite of the "Belief alters reality" theme in Planescape. An Absurdist would probably say, "Belief puts a false veneer on reality"
One thing I think has not been mentioned yet:
The Fraternity of Order has a fairly scientific outlook on reality:
1) There are underlying laws governing all processes.
2) We can observe the outcomes of these processes and, with careful examination, determine the laws governing them.
3) If we know those laws exactly, we can find ways to work with, or around them (this is also a bit of an engineering attitude).
Back to novels close to Planescape: His Dark Materials trylogy by Philip Pullman (Northern Lights, the Subtle Knife and the Amber Spyglass). Multiverse travels and phisolophical battles, not "good" vs "evil"... Actually I'd like to put the Subtle Knife in my campaign, I love it and my party didn't read the books....
Shouldn't be too hard to work their whole cosmology in to Planescape as long as the players avoid Lyra and Will's original worlds, which are rather too high-tech for Planescape to work well (although Lyra's is closer). Apparently this collection of Primes mostly features people whose afterlife is on a particular Prime, and eventually they merge with the Prime itself. That's not completely unknown in Planescape; Fharlangn and Vecna are said to have their godly domains on the Prime. Their celestials are advanced souls, and their fiends are the Spectres. Of course it basically overlooks Pullman's entire thesis to have immortal souls and actual gods running around elsewhere -- though instead of atheist, you might call the protagonists' point of view Athar.
The Subtle Knife is an artifact-level item, able to create and close gates at will as well as cut anything, though fortunately the required loss of two fingers and the death of the previous wielder do make it unattractive for PCs to use. So is the Golden Compass, of course (perfect divination? Sure). The Amber Spyglass isn't so much, fortunately -- true seeing, basically.
There is very little communication with the Multiverse at large here. If you put it into the regular Prime, the lack of active deities and Outer-Planar exemplars in the region may be a problem to explain. A very isolated region, perhaps? Surrounded by a barrier, like the Grey around Athas? Or it could be another cosmology with its own rules, reached by traveling through the Far Realm. Or, taking a tangent from the high-tech world that gave rise to Annavaree in Guide to the Ethereal, it might be accessible through strange byways there, or one of the other paths through Shadow (which D&D has claimed before is the route to our world).
Double post.
No, not Ringworld. See "Eon" by Greg Bear, which featured a ring-shaped city full of portals to alternate timelines, whose inhabitants floated above the streets and spoke to each other in pictures that floated above their heads. It's a science fiction novel, not fantasy. I don't know for sure that David "Zeb" Cook read it, but the parallels are eerie.
"The Dragon in the Sword" by Michael Moorcock featured a ring-shaped set of planes where the inhabitants kept complex charts to keep track of the shifting portals that connected them. A race of bearlike scholars - the Ursine Sages - is one of the best sources for these charts; they remind me somewhat of the ursinals. Moorcock's Elric books are a definite inspiration, since that's where the elemental planes and elemental rulers came from, and the word "multiverse," as well as the concept of Law and Chaos as forces independent from Good and Evil. Michael Moorcock got the idea of Law and Chaos as these huge antagonistic forces that drive entire races and worlds in vast cycles from Poul Anderson's "Three Hearts and Three Lions," but he transformed it and stripped away the moral bias, and created a more complex multiverse as his writing progressed. Those are influences to D&D in general, though, not just Planescape.
Specific inspirations that David Cook has admitted to are listed on the Planescape Inspirations site: lots of Borges and Calvino, the movies "City of Lost Children," Un perro andaluz, and Orphee, and Giovanni Piranesi. Colin McComb's influences include Nine Inch Nails, Joel Peter Witkin, the movies Hellraiser I and II (source of the kytons) and Swimming With Sharks. Tony DiTerlizzi has his own influences, including Brian Froud and a bunch of other people who used to be listed on his website, but I can't find any list of his influences there anymore.
Monte Cook has cited the Chronicles of Thomas Covenant the Unbeliever by Stephen R. Donaldson as a major influence on his work in general, not necessarily Planescape specifically.
Obvious RPG predecessors to Planescape include the 1st edition Manual of the Planes, "For Faerie, Queen, and Country" by David Cook and Carl Sargent (the origin of the cant and the tiefling traits table, especially), and "Vampire: the Masquerade" for its idea of player factions (also used in every other World of Darkness setting).