There seems to be a fair amount of Planescape chatter around the interwebs recently. I've been considering running a game of Planescape myself some time ago but that got supplanted by the Kalesh game.
Anyway, what I wanted to talk about is this: the only real contact I've had with the Planescape setting was through the Planescape: Torment computer RPG. So when someone asks something like "what does Planescape mean to you?" I can only reply in terms of what the game meant to me. The videogame took certain liberties with the system and the source material that made it its own thing.
And, looking back, the game was deeply existentialist. I'll explain that in more detail, but let me just say that's the reason why I always found D&D a not-so-good fit. Sure, (classic) D&D is, at its core pretty existentialist. Like Zak has pointed out on his blog recently, it's not a game about Luke (the chosen one, destined for a fate) but about Han Solo (making his own fortune).
But Planescape took that nail and hammered it pretty hard, much more than the original ruleset ever conceived for.
In D&D, when you pick a class, you're stuck with it. Some editions allow for dual-classing or multi-classing, but that's it. Planescape on the other hand, allows for you to switch your class entirely. No edition of D&D has ever accounted for that. In Planescape you can, if you want, become a mage or a thief in the middle of the game. And then switch back or whatever.
In D&D it's a pretty traditional aspect that races are monocultural, essentialist. If you're a dwarf, you're a dwarf. This isn't as clear for PC races but definitive for monster races. In Planescape it's not entirely clear what race the main protagonist is. Probably human. In either case it doesn't mater. He is what his actions make him. The other members of your party are even more interesting in this aspect - Fall-from-Grace is a succubus that has deserted her race and is pursuing intellectual pursuits, Dakkon is a gith that is essentially a heretic, Nordom is a rogue modron, we don't know what Vhailor is, but it's not his race that defines him, but his belief and dedication and so on. All over, the NPCs aren't cliches of their race, but rather outcasts, making their own, personal journey. Not strengthening the stereotype, but subverting it. They all reject their origin, their "essence" and choose existence.
The Nameless One that you play is just one of the countless incarnations of the same man. And you see traces and clues and memories and reports of your previous identities throughout the game. Thing is, except his immortality (and about that in a second), there is nothing defining about him. Sometimes he's a bad guy, sometimes he's paranoid, sometimes he's good. He isn't one thing. He is what the circumstances make him, and what he makes of himself. There's nothing that pins him down as his essence.
4. "What can change the nature of a man?"
This is a question you get asked in the game. It's an important question. Often in computer RPGs when a question like this is asked you must give the correct answer. In PS:T, there is no right answer. Whichever reply you click, that's the one. There isn't a definitive answer. The hag that asks you the question was really just interested in your answer, not a right answer. What can change the nature of a man? Whatever you say. There isn't a real "nature", because there is no essence.
The protagonist, The Nameless One is immortal. That's the whole hook of the game. He will die and be reborn again over and over and it's implied this has been going on for a very long time. And even though you resolve this situation in the end (in several possible outcomes), there's still this theme of eternal suffering or should I say Torment, where this man is forced to repeat the same thing over and over. Perhaps in different circumstances, but it's still a repeat. This not only echoes Nietzsche's eternal return, but perhaps alludes to this tale. The Nameless One keeps pushing. The fact that there is a resolution perhaps undermines this theme, but hey, then again PS:T is one of the few computer games where you can, to a degree, address your own themes.