> What does this even mean? It means that you need to concentrate the actual code that manipulates the state and perform all state changes through this code to

> What does this even mean? It means that you need to concentrate the actual code that manipulates the state and perform all state changes through this code to ensure that the state is always correct. OO solves this through the concept of encapsulation, but that is just one way of doing it. If you have code all over the place that can manipulate the state, then it becomes extremely hard to ensure that the state remains valid. I'm not talking about the ownership of the particular instance, I'm talking about what code is allowed to, and responsible for, making alterations to state instances in general. If I ask you what code can make changes to state, you should be able to point to a small-ish part of your codebase and say: only these functions can make alterations. Each of those functions should guarantee that the state they return is a valid state, that is: they are responsible owners. All functions that operate on the state have the main responsibility to keep the state valid (e.g. no player character inside an object). Each specific function has additional responsibilities (e.g. move the character if possible). In the example, the move function can take an existing valid state, and turn it into an invalid state, the player can be moved inside an object. So you when you think about what that function's responsibility should be: it is to attempt to move the main character to a new, valid position. If you think of it like this, you quickly realise that the move and collision detection functions should be combined. > The only ownership that is happening that needs to be worried about is the parent function passing a copy of state to a child function for the duration of that call, right? If you're passing a copy of state from one function to the next, and every function can just modify it in whatever way it wants to, they you basically have globals but with more copying.