Planar Conundrums (Potentially Illogical, Unsustainable things that crop up in Cosmology design.)

Not necessarily. The physical space of a given plane may be infinite, but it doesn't necessarily follow that a given resource is equally infinite. For all we know, there may be an infinite variety of finite resources. Maybe once all the, say, gold in Earth and Mineral are mined out, Prime sources of it dry up as well. And there's nothing to say that this resource depletion hasn't happened before. (Whatever *did* happen to all that oricalcum?) This could provide a good excuse/reason for legendary materials that simply aren't available any longer.

(And sorry about the jumping around. My mind is milling over the ideas and I'm just dumping whatever strikes me)

Petitioners are probably infinite, but they're more like set dressing, more interested in living in tune with the plane/their alignment/their power, than with any inter-planar plots or mechanations. Plus, their numbers are in constant flux, with new ones arriving all the time and probably a similar number that are merging with their plane/power. This would keep even a finite plane from being overrun.

Also, I don't recall ever reading that the number of exemplars are infinite. Countless maybe, but never infinite. And for groups like the Gehreleths, they're explicitly said to have finite numbers.

I suspect that the planes are functionally infinite, without ever needing to define the scope of that infinity (like in my Excel file example). So when dealing with the problem of infinities in a setting like this, I go with this idea of "There's always something more".

I don't know if it would draw on the psyche of the creator. The Outer Planes at least are aligned, so how would it draw on the psyche of somebody whose alignment didn't mesh with the plane?

(I need to get going, so I'm just going to put this up and see what y'all do with it)

This is great. I've been thinking about something that may connect with this. Both the infinite and infinitely-expanding models seem to imply a frontier, a point at which settlement stops and then there is simply vastness. But the outer planes are so based on belief that the mood of a town can relocate it to another plane entirely, in which case all of this empty space doesn't really make sense.

Maybe I'm just restating Wicke's Excel metaphor, but what if the planes (at least the outer planes) are simply as big as they need to be? And it might take some serious quantity of belief to actually create new real estate. A party of adventurers could walk for years into virgin territory and it would never take more than several days to get back, simply because they aren't creating new space despite what they're experiencing.

Maybe this whole thing calls for a Cosmology Renovation Project. Obviously this isn't the Far Realms - the planes have direction and landscape and generally make experiential sense. That said, perhaps they're not at all "normal" 3D spaces on the macro scale, which is where a lot of these paradoxes seem to come from.

I am bothered by the fact that Gehennan real estate (the mounts themselves) is not infinite. Given the exploitative nature of yugoloths, every possible resource in the Infernal Mounts (iron, valuable stones, and so on) would have been totally mined out ages ago, which would certainly not be any good for the loths, since all of their competitors are sitting on infinite planes.

If I were designing Gehenna, I might have the surface of the Infernal Mounts be finite, but the interior of each of the mounts would be infinite.

Here's my take on the second point:

What is the relationship between the Outer Planes and the Prime Material? What about the Prime to the Inner Planes? Or the Astral or the Ethereal? Hyena of Ice wrote this in the Positive Energy Plane Renovation thread: "The planes were... created (came into being) in this order: Ethereal, Elemental, Astral, Prime, Outer." That means we have pure creation (spiritual or metaphysical), followed by the building blocks of the 'verse (physical matter), followed by the genesis of thought (the mind), followed by the Prime (our home), followed by the Outer Planes (thought given form).

This suggests that the Outer Planes are entirely dependent on the continued existence of every other plane.

My next question is this: What are the gods? What is their relationship with mortals? Or the Prime? Why do they have followers? My answer is to define the relationship between the worshipper and the object of worship. Gods gain their power from their worshippers. When a god is no longer worshipped, it "dies." But why is that? What is so special about the act of worshipping that it powers and sustains a being as powerful as a god? I see worship as a form of energy. It's a very special type of energy, and it's something that only gods can tap into.

So gods derive power from worship. The more followers they have, the more powerful they become.

From this, I derive two reasons for the Outer Plane inhabitants to be so concerned with the Prime Material. 1) It's the source of mortal worship. The more territory a god can influence (through his petitioners, planars and proxies), the more potential exists for worship. 2) It's the source of the Outer Planes themselves. The more territory anyone (like a faction or a demon lord) can control, the more influence they have on the shape of the Outer Planes.

The second point needs more details: We know that a place in the Outer Planes can shift location based on the belief. But we also know that it's very difficult. It happens, but not very often. What if it's easier to seize control of real estate in the Outer Planes by first taking control of Prime Material worlds?

So the incentive to control the Prime is based on the needs of gods (the desire to sustain and expand their source of worship) and the needs of everyone else (the desire to control and/or shape territory in the Outer Planes).

This is the motivation behind influencing the Prime worlds. And pretty much everyone in the Outer Planes wants to influence the Prime. With so many different factions and religions vying for control, it stands to reason that someone, at some time in the past, created limits to that control. So demons and angels can only travel to the Prime under specific conditions. Gods can send avatars, but never their true being. Planars have more freedom of movement to the Prime, but they're limited by a lack of innate planeshifting abilities (well, most of them are).

Of course, this is all based on my reasoning and it's how I run my Planescape games. There might be details that conflict with canon, or you may disagree for your own reasons. Take what you will.

They're not fighting over real estate; they're fighting over belief.

The real estate is just set dressing. There's as much as there needs to be (even on Gehenna - the fact that it has edges doesn't mean there isn't plenty of room within that bounded space). It's effectively infinite, but in the Outer Planes there is no real space (it's all just the idea of space beyond the Astral Plane, just the idea of infinity). It isn't an actual, physical realm (though it's solid enough to hurt you). It's a realm of reified concepts.

Belief is not infinite. It's a finite resource; that's the whole point.

There are certainly not an infinite amount of planar beings, either. There's only as many as the current levels of souls/belief available to them support.

The Inner Planes are different. They're a realm of physical matter rather than concepts, but space is a tricky thing there, too. You can cross from one infinite plane to another (or anywhere in the plane) in a few days if you know the way, but if you don't it literally takes forever. Even there, though, the natives fight over philosophical differences rather than raw resources. They want to wipe out their philosophical rivals, and the way space works in the Inner Planes it's impossible to get so enough away that your rivals can't find you.

"Why would outsiders bother fighting anywhere but on the mortal world where creation rests?"

There are so many bad assumptions in that question. Remember the Center of All rule: there is no center of the multiverse. The Material Plane is no more important than any other part of the cosmos. When the Outer Planes and Inner Planes shift and change due to conflicts among planar beings, the Material Plane shifts accordingly. When Gehenna captures (conceptual) land from Baator, the Material Plane shifts away from Law and further toward Evil. When Bytopia captures (conceptual) land from the Outlands, the Material Plane shifts toward Law and Good. When fire elementals destroy a smoke paraelemental city, fires burn brighter and cleaner. It doesn't matter that the land in question is effectively infinite, because the meaning of the planes and concepts and elements change when symbolically important victories are won.

In the same way, affecting mortal believers can change the planes. But it works both ways, and Creation doesn't "rest" on any one plane. Invading the Material Plane isn't any more effective a strategy than invading an Outer Plane.

As for dragons, they care about themselves, mostly. There was a guild of draconic portal guardians described in 3rd edition, but I don't imagine it's a big thing for them. Gods, nature spirits, and pacts among outsiders all insure that apocalyptic invasions are rare.

I'd like the prime to really mean something in a setting.

It's funny to see these positioned as mutually exclusive - it seems to me that the prime has the potential to be as or more exotic as anywhere else on the planes. There are plenty of spots in the Log X thread, for instance, that don't seem to clearly fit anywhere on the planes (ethereal aside). The prime presents the opportunity to unfold literally any idea you might have into a cohesive world without being encumbered by the themes and philosophies that define the planes (that is: supervillain laying low in a pastoral wonderland while quietly raising a secret army? not on Elysium he's not…).

I suspect a popular reaction to learning about (but not necessarily visiting) the planes goes like this: "Why should I care? I'm the center of the multiverse."

Along these lines, I've always wondered about this stark prime/planar division - sure, some gnome who's never left Bytopia may know about the planes, but I'm not sure how that knowledge translates into any actual planar savvy. Most Americans seem perfectly aware of China and India and how large they are and perhaps even how fast they're moving, but how many people seriously consider that the world may ever look meaningfully different than the one they know? Or that any of it even matters at all?

I'm not sure how well that analogy holds up, but my point is that folks are pretty good at taking in information as true without absorbing the accompanying existential body blows. Or maybe my point is just that most planars are probably center-of-the-multiverse yokels, too.

Sure, but would they feel any differently about another plane getting paved over, either?

Cool thread. Here's one that's been in the back of my mind for a while:

PS adventures seem to assume that the clueless on the other side of one of the Lady's portals really are clueless about it. Which makes sense if it's a temporary portal; but not so much if it's a permanent or even shifting portal. I mean how many weirdos have to show up seemingly out of nowhere before the locals start buying these weird travelers drinks, asking questions, and finding out about the portal and its key?

And what then? Even a shifting portal would develop its own road (or at least a beaten trail) pretty soon after discovery, to make trade and tourism easier. Otherwise, it'd be like modern locals of a rural town not having a road to, or maybe not even hearing of the nearest big city.

Opinions?

The rules of the setting allow for such a thing. If you wanted to, you could put a stable, permanent, non-shifting, two-way portal the size of the Arc de Triomphe in a major city like Paris and the key could be something easy like "being alive." Merchants could go in and out of the portal every day, bringing the wealth and terror of the planes to the mortal world. Just as you suggested, everyone would know about Sigil at least in rumor. The factions could be major players in the politics of the world (or, at least, the city around the portal). Planar Common might be the lingua franca of the planet and migrating to the Upper Planes would be as simple as Europeans migrating to the New World.

In practice, it usually doesn't work that way.

Because while the rules technically allow for portals that are stable and well-known, they also allow for portals that are one-shot, or which only activate once a century, or which shift in patterns so complex that no one understands them, with secret or seemingly impossible keys. A world's sole portal to Sigil might be a sewer grate in a backwater hamlet that activates once every five hundred years, whose key is the seed from which mountains grow, whose destination in Sigil varies between a ring hovering upside down 500 feet above street level directly over a shark pit, an ogre mage's hat, and a hole chewed into a dungeon wall by giant rats depending on the time of day. Or a given world in the nigh-infinity of the Material Plane may have no portals to Sigil at all. Portals to Sigil appear at the Lady of Pain's whim and the DM's fiat, and they certainly don't have to be convenient or known to anyone at all. If a portal becomes too well-known, the Lady of Pain can move it. Or the Grixxit, a rogue petitioner who's made it her mission to destroy portals, might destroy it. Or something else might happen on either side to break or lock the portal, either through a spell or just by damaging the frame. The Infinite Staircase is much more problematic in my mind, though I suppose its portals might shift periodically too as the points of greatest creativity in a city change, or as portals are locked or destroyed.

Obviously a permanent portal to Sigil is potentially incredibly convenient, changing a campaign setting that would otherwise largely be confined to a single world to one intimately tied to the multiverse as a whole, and possibly even making easy for a party of low-level adventurers to get anywhere in the planes. Not every world should have one.

Here are some official and unofficial examples.

Blood Hostages by J. Robert King
The first novel in the "Blood Wars Trilogy," this is one of only five novels to bear the Planescape logo, and the only one to deal with a group of clueless primes discovering the planes for the first time. In this novel, the portal to Sigil is a cabinet in a rural cabin. The protagonists, the twins Nina and Aereas, have never heard of Sigil before, and their uncle's cabinet may well be the planet's only route to the City of Doors. At the time the book begins, their uncle is the only one in the world that knows about it, and on the Sigilian end it's a secret kept by a gnomish tobacco merchant until some yugoloths discover it.

To Baator and Back
This is an adventure in the Well of Worlds anthology designed to bring clueless primes to the planes. There's no portal directly from the Material Plane to Sigil in this adventure; the party stumbles into a portal to Hell in an abandoned wizard's tower, and only discover a portal to the City of Doors after trekking across the plains of Avernus. Again, if the DM desires, this roundabout method may be (at least currently) the only way to reach Sigil from that particular world.

For the Price of a Rose
This is an introductory adventure from the Planescape Campaign Setting boxed set. The portal to Sigil in this adventure is shifting and temporary, appearing on a different world periodically while the Sigilian end stays stable. The key from the Prime end is a freshly cut flower. This portal is basically designed to strand the PCs on the planes, as by the time they want to get back it's probably moved on to another world, and it may well cycle through hundreds or thousands of different planets before it returns to theirs. Plus, it's controlled by a gang of thieves who are certainly not inclined to tell the locals about the secret door they're using to commit crimes.

Dragonlance
The Dragonlance Legends trilogy implies that the sole permanent planar portal in Krynn at the time was in the Tower of High Sorcery in Palanthus, and that was a portal to Takhisis's realm in the Lower Planes. Tarholt's family of dwarves made it to Sigil somehow a few centuries ago, but other than that, Krynn seems a very isolated world. Tarsheva Longreach (in the Planewalker's Handbook) somehow knows about the Inn of the Last Home, so it's possible that there's some kind of portal near Solace, but as Solace is obviously not a cosmopolitan planar trade city, the portal must be fleeting, difficult to unriddle, or well-guarded. Maybe it's hidden away in pre-Cataclysmic ruins. After the Summer of Chaos, all the existing planar routes are broken, and there probably isn't any way to Sigil from Krynn until Dalamar finds a way to meet with Elminster and Mordenkainen again much later.

Forgotten Realms
Toril is full of portals, but a lot of them are hidden away in dungeon ruins, such as the ruins of the Netherese and Imaskari civilizations, which usually mean hunting through a desert to even find where the ruins are or fighting through the monsters and traps of Undermountain. There's a canonical portal to Sigil beneath the Abbey of the Sword in Cormanthor, but no one knows what the key is.

Greyhawk
Oerth has its share of portals, from the portal to the Demonweb Pits in Erelhei-Cinlu to the many portals beneath Castle Greyhawk and the Lost Caverns of Tsojcanth to the mysterious stone circle of Tovag-Baragu. Most of these are hazardous, though, and I'm not aware of any canonical portals directly to Sigil (in Dead Gods, it's something of an issue that the PCs have to hunt down rumors of portals to the Inner Planes in the Underdark in order to eventually get back to the City of Doors). The closest thing I'm aware of is the portal described in the short story "The Weird Occurrence in Odd Alley" by Gary Gygax, published in his anthology Night Arrant through his New Infinities company way back in 1987, well before Sigil as such was invented. In the story, the seasoned adventurers Gord and Chert are directed by a gypsy fortune-teller to an alley in Greyhawk City so obscure that most of the city's natives have never heard of it. Neither hero had any idea there was a portal there, and they're only able to find the portal after many hours of searching and accidentally finding a passage only visible at dusk. They would never have found the key (a distinctive coin forged from an otherworldly metal) if they hadn't ambushed and mugged a nonhuman planewalker who happened to be using it at the time. The fact that somebody had a key handy indicates there's more than zero traffic between Greyhawk and the extraplanar city beyond, but not enough that seasoned adventurers Gord and Chert had ever heard of the place. The city on the other side of the gate is functionally identical to Sigil - it's a place of interplanar trade, with hundreds of portals to countless worlds and planes. However, the portal they entered through was one-way, they have no idea how to get back, and the guide they go to is horrified at the idea that they'd want to go some place as unsafe as Oerth. That said, the city apparently has portals leading to a variety of places on Oerth, and they eventually follow a man to a portal that opens, unexpectedly, in a ceiling twenty feet up in a noble's house in the city of Rel Mord (disrupting a dinner party when they plummet on to a table). The implication is that there is regular traffic between Oerth and the city beyond the gates, but that it's kept secret from the majority of the public.

Mystara
There are no known stable portals between Mystara and Sigil. The shadow elf Farrow ended up in Sigil after being banished to the planes by a Glantrian archmage.

4th edition
In the 4th edition Dungeon Master's Guide 2, Tradegate is a town on the mortal plane, and it has a prominent gate to Sigil in it. The key needed to operate the portal is known only to Master Trader Duncam, however, and those who pay him 1,000 gp for the knowledge. I don't know what would stop those people from telling others for free, though, unless the key changes regularly and only Duncam has the resources needed to figure out what it's changed to.

Robert Asprin's MythAdventures series
The equivalent of Sigil in these books is the Bazaar on Deva, an otherworldly marketplace that caters to travelers from hundreds of alternate worlds. The Bazaar on Deva is well-known to dimensional travelers, but in the first novel the protagonist Skeeve has never heard of it, even after working as an apprentice to a seasoned planewalking mage for several months. Initially, he thinks all otherworldly travelers are devils from his world's legends. The books later reveal that not everyone on Skeeve's dimension of Klah is as ignorant of Deva as Skeeve was, however.

Something From the Nightside by Simon R. Green
The equivalent of Sigil in Simon R. Green's Nightside books is the Nightside, a secret metropolis accessible from London "where it's always three A.M." The Nightside was founded thousands of years ago as a neutral zone unclaimable by either Heaven or Hell, and it has portals to countless worlds and times, including Faerie, alternate Earths, Heaven and Hell, places where legends go when they're forgotten, Camelot, and the far future. The Nightside is infamous in occult circles, but most ordinary people have never heard of it. The protagonist gets there in the beginning of the novel through a supply closet in the London Underground; the key is speaking the word "Nightside" into a phone on the wall and then a journey through horrifying, blood-red tunnels and a secret subway station, though it's possible to also stumble into the Nightside accidentally.

I don't think I would do this with Sigil but I think it would be interesting to have two cities on different planes but with a permanently open major gate connecting them.
In the real world, there are a number of officially separate cities that grew interconnected (for example the cities of Buda and Pest in Hungary - separated by a river but connected by bridges until it effectively became the unified city of Budapest) In such a set-up, the residents would be much less concerned with crossing between the two planes than they would be travelling a few miles in the "wilderness" to reach the next town.

Perhaps this has already been done but I don't know of one.

The town that Glim mentions doesn't seem to have a name, unless someone in the Living Greyhawk campaign came up with one (I wouldn't know). Greyhawk Adventures says only (page 97): "A large town has grown up near the center of the conflagration, where there is a gate to the plane of Fire." And yes, the City of Brass is on the other side, though it may (or may not) be a bit of a walk.

Of course, individual DMs can absolutely build permanent, easily usable gates to Sigil in their worlds, or in their versions of official worlds, if their campaigns are very planes-heavy and such a thing seems desirable. This wouldn't change Sigil at all, but might change the Prime world considerably. It just seems that officially, portals to Sigil are not (and Sigil itself is not) inevitably known to the public at large.

In general, there are many examples of gates with towns on both sides. Sylvania and Thrassos, Ribcage and Darkspine, Fortitude and the previous Fortitude, Bedlam and the Madhouse, Excelsior and Heart's Faith, Glorium and Himinborg (there's a sea in between), Plague-Mort and Broken Reach, Pashrita and Savitri's realm, Gloomport and Gloomwrought (another sea in between), Moonstair and Celduilon, Absalom and Shadow Absalom, maybe Curst and the Bastion of Lost Hope. I created the city of Void's Edge to be on the other side of the portal from Torch.