Alicornutopia

Brainspace is a setting based around the magic system of the same name.

In the beginning there was God and nothing else. God invented the laws of physics and invested them with a magical substance called detail to make them real. Then God waited, billions of years later sentient life arose on a planet in this world. God chose to share it's magic with these new sentients, gifting each of them the same amount of detail from it's infinite supply.

Humans and other sentients in this world each have their own brainspace in which they can use the detail that was given to them. People with brainspace can go from where they are to their brainspace at any time. If someone would die outside their brainspace they instead find themselves in their brainspace. Brainspace starts out as an emptiness but each person can use detail to create places or things inside their brainspace. When a user creates a thing in their world they are imbuing an idea from their imagination with detail. This detail can later be reclaimed at the user's discretion. Users can create almost anything inside their brainspace including new magic systems and interactive entities.

If you specifically invest a lot of detail into an interactive entity you begin to get a sense of fullness. If you continue adding detail past that point that entity may become a person. If they do, you get back all the detail you've invested and magic fills in enough additional details to give the person you've just created a consistent backstory and fills in their memories accordingly. This new person gets the standard amount of starting detail and their own brainspace which they can choose to leave your space into.

Users of the magic system can also imbue detail into invitations for others to visit your brainspace or requests to visit the brainspace of others. You could invite your friend specifically or all the people with a green shirt on or all the people who's favorite Avenger is Hawkeye. Optionally you can attach your feelings at the time you make the invitation to that invitation. People receiving the invitation can recognize it's from you if they've previously met you, and they'll also know what the query you were using in inviting them is and the emotions you attached if you choose to do so. You can also invite a person to a brainspace you're currently in that isn't yours if you have permission to do that from your host. God gives everyone permission to invite people into it's space. Invitations can be ignored and removed from your conscious attention with no feedback to the person extending the invitation. When you accept an invitation the person who invited you gets feedback on what invitation of theirs you are accepting.

Separate from the process of issuing invitations you can create ground rules for your space which restrict others from bringing certain things into your space including certain types of avatars. God’s rules for it’s space only allow for relatively low-key magic and required secondary powers for non-human avatars.

Given permission you can also place an anchor in another's brainspace that allows you to return at your discretion. God also grants this permission to everyone for it’s own, you can only have one anchor per brainspace.

When entering another’s space the host may offer you assistance in conforming to the aesthetics of their space. You may refuse this assistance but if you do it’s possible that the host’s ground rules will prevent you from entering. God’s assistance adds enough details to objects including avatars that they are compatible with the laws of physics if that’s possible. Certain overly abstract or magical objects and avatars are beyond the reach of this assistance.

Sounds interesting. I have a couple of questions:

- Is the entire world God's brainspace, or is there an external world and *also* an open-source brainspace belonging to its creator?
- What is an avatar?
- Is it possible to create a person accidentally?

Alright, so the main physical dimension is God's brainspace. If someone got portalsnaked into this world they would end up in God's brainspace and get the local magic in the bargin. Other people's brainspaces are their own, god has no special powers inside other people's brainspaces or in general, God just has an infinite well of detail so it can make a much more detailed world than anyone else can manage.

Your avatar is your body, if you start your life in God's brainspace your body will start out physically detailed but you probably can't retain all of those details when moving to your own brainspace. Your mind will be preserved but by default you won't experience largely physical things like hunger and puberty in your own brainspace or that of other humans. You'll preferentially retain those features that are important and evident to you by default you can consciously retain different sets of traits when switching. When moving from your brainspace to God's you can also guide how the details are filled in to a degree based on how you've edited your simplified avatar. In brainspace you can trivially change your height or haircolor and with a bit more difficulty your gender and God's assistance would physically realize those changes.

You can never accidentally make a person without knowing something special is about to happen. The feeling of fullness isn't explicitly screaming this will make a person though so someone who hasn't been informed of what it means could decide to keep going anyway. Someone who does know what they're doing could be trying to edge as close to the line between NPC and person as possible while not getting a person and misjudge.

Not related to your questions I forgot to include how you get more detail.
There are two mechanics outside extra dimensional magic. First if you are using almost all of your detail whenever you reclaim some or all of the detail from something you have made then you'll get back a tiny bit extra, less than a percent of what you invested but this can add up. Second if someone gifts you an item or other creation made with their detail and you store it in your brainspace then you slowly get enough extra detail to maintain that object yourself. This is slow though think like a year to get enough detail this way to have as much detail as is in a chair.

Speaking of gifts, you can take on the detail cost of an object you receive from someone else if they're willing to let you. Otherwise if the detail making up the object themselves remains theirs they can reclaim it at any time.

You are clearly using "detail" in some technical sense but I don't know what you mean by it.

I read it as a limited resource which you can add to an idea to make it more real in some sense, at least in your own brainspace.

yes, I'm using detail for lack of a better term to describe the resource you need to do magic in this world which can be spent to add more detail/complexity/information to things. I tried to find another term that worked aesthetically but I was unable to.

I'm vaguely considering renaming detail substance to be less confusing. Would people find that more clear? Also I edited the first couple paragraphs to try to avoid misunderstandings.