I didn't come up with this term (that credit goes to @hellgnoll) but it's been useful for describing/understanding A Certain Approach to TTRPGs in other conversations, so I wanna have this online somewhere that people can link & reference. I'm tempted to pose player solipsism in opposition to Something Else, but I think it's most effective & accurate to simply treat it as a set of principles/assumptions that have no clear opposite. With that in mind, to the explanation.
What Is Solipsism?
To simplify a rather extensive philosophical history, solipsism can kinda be thought of as "everything that extends from Descartes' cogito ergo sum (I think, therefore I am)." It takes as its base assumption that you exist (for some ultra-barebones definition of existing) and then tries to develop philosophy without assuming anything else exists. There's a lot of interesting history here—Al-Ghazali, for example, was doing a lot of the same thinking in Islamic scholarship centuries before Descartes—and a lot of interesting ideas borne of solipsism, but most people probably know about it from assholes. Lotta edgy teens & young adults going "if I'm the only one that really exists, then I can kinda do whatever to the people around me because they aren't really people like I am." For the sake of brevity I'll stop here, and move on to connecting this to TTRPG design.
What Is Player Solipsism?
At its most formal, we can describe player solipsism as "a framework that assumes players are the only agentic forces within a game world that shapes itself around those players," but that's a lotta words that probably don't make sense if you don't already get the concept so let's start with examples instead. The most obvious example is the Powered by the Apocalypse approach to storytelling: players roll moves, and then the world changes as a consequence of those moves. Like, conventionally in PbtA when you want to interrogate a character for information, whether or not that character has information (and what kinds of information they have) is defined by the outcome of your roll. This is also why most PbtA-lineage games have the principle "play to find out:" you are literally playing the game to find out what the story actually looks like, as shaped by the players' rolls & the GM's responses.
I want to emphasize that while PbtA makes player solipsism into a core design principle, it's present in plenty of other contexts – and actually was a major part of GM advice in D&D circles before PbtA really became a standard. It's the basis of "just move the encounter in front of your players," "if you succeed in a roll Something Should Happen," and (to a degree) "if the players aren't affecting the story it's railroading." In other words, player solipsism is something you can pretty much always do in a TTRPG, it's just a matter of how much a game assumes/supports you doing it.
To discuss the topic more abstractly then, player solipsism approaches TTRPGs as a story the players are experiencing first and foremost. From this perspective, what's happening off-screen doesn't really matter unless it somehow affects the players. They may not have power within the game world, but they are the ones that drive the plot & decide where it goes. Frequently this design also comes with turning rolls into story beats rather than tests, because it ensures no player's action goes to waste.
This works great for pulp stories & genre fiction, because it puts players in the thick of whatever's happening – but it also makes it incredibly easy for the story to feel rote & artificial. It also makes it a lot harder to incorporate player characters incidentally dying or otherwise leaving the stage, because the game has put that character, specifically at the heart of the plot. Putting someone new in the same position thus requires way more effort than just "roll up a new character." I'll also point out that executing player solipsism requires a lot of comfort with improv & on-the-spot good-enough decision-making. Players need to be willing to drive the story, GMs need to leave holes for player actions, and both parties need to be ready & able to adapt the story they're telling based on dice rolls.
Again, I want to emphasize that this is not "one end of a spectrum of design," nor is it "a thing that a game either is or isn't." It's not even an all-or-nothing decision within the context of individual rules/systems; Blades in the Dark, for example, decides the consequences of a roll only after it's been made, but its risk/effect system allows GMs to show how the world exists outside the context of the players (e.g. "sure you can try to pick the door, but with the guards watching it's a desperate action with limited effect").
To wrap up, I'm not gonna try to formally declare "these are the principles & assumptions of player solipsism that are expressed through game design," this isn't supposed to be that level of effortpost. I will restate my description above though & rephrase it a few ways to hopefully help it stick. So, what do I mean when I say player solipsism is "a framework that assumes players are the only agentic forces within a game world that shapes itself around those players?" I mean that as far as the game is concerned, the players are the center of the universe. I mean that players aren't simply interacting with the game world, they're the primary shapers of it. I mean that NPCs only exist to play a role in the players' stories, not as part of a complex system out of the players' sight. Whether or not this is a good approach to TTRPGs is up to you.
