They are useful to us, but every now and then it's helpful and humbling to remember it's a fiction we assign, rather than fundamental.
Criminal justice or the concept of culpability is one of these areas. I know I've seen material by Robert Sapolsky, a neuroscientist who does not believe in free will, talking about how off the mark criminal justice and punishment for crimes can be.
However I didn't just assume a lack of free will. I also assumed a lack of identity. Do you realize that who you are is socially defined? When you breathe in, the air in the room around you becomes part of you. When you breathe out, you lose certain gases. When you eat your food, similar story. There's a good case to be made that "you" are in the entire room or the entire food chain. That does make causality and culpability hard to assess objectively. When we do so, we do so subjectively.
In that case I dub you a mushroom.
We would do well to remember that every now and then. People who get too into pretending their perspective is objective reality tend to do stupid things.
> like a virus doesn't become part of me -- the immune system acts a biological discriminator between what is part of me, and what is not
But what about symbiotic organisms? What about your microbiome? Or the mitochondria, which began its existence as a separate organism? Or, they say our DNA includes many viruses that our ancestors contracted. A number of these things do stick with us and we sometimes even become totally dependent on them to function.
To pick one possible simple and broad definition from WP, "Identity is the set of qualities, beliefs, personality traits, appearance, or expressions that characterize a person or a group." -- you made it sound like that set is empty, which doesn't make sense to me. My identity is part of "I". Every being has an identity; it's not something you can get rid of?
In any case, I find your comment very interesting. I'm studying quantum computing at the moment, and I've had to read the different interpretations of quantum mechanics, including Everett's many-worlds interpretation. As a non-physicist, I've found the different interpretations fascinating.
The many-worlds one, as far as I understood it, says that all the possible outcomes of a quantum measurement actually "happen" in different worlds. I have the impression that you would be able to give a much better explanation.
In any case, in the joke the gun is shot in the macro world, not in a quantum state. It's possible that it is a quantum gun, but probably not.
Let's say "overruled" then.
Collapse theories, in contrast, state that at specific points in time (the "measurements"), the wave function stops following the Schrödinger equation, and instead collapses to a single slice/branch/world, thus upending the natural proliferation of branches implied by the normal time-evolution of the wave function according to the Schrödinger equation.
Even in many-worlds, however, the wave function doesn't necessarily contain all conceivable worlds. It only contains the worlds, following from some initial quantum state, that follow from the Schrödinger equation. While it's true that all possible outcomes of a quantum measurement become real (because they are all contained in the wave function in superposition), "possible" here means specifically what the equations allow, not any imaginable world.