Three possibilities come to mind:
Is there an evolutionary purpose?
Does it arise as a consequence of our mental activities, a sort of side effect of our thinking?
Is it given a priori (something we have to think in order to think at all)?
EDIT: Thanks for all the responses! Just one thing I saw come up a few times I’d like to address: a lot of people are asking ‘Why assume this?’ The answer is: it’s purely rhetorical! That said, I’m happy with a well thought-out ‘I dispute the premiss’ answer.
Experience is definitely real, but there is no such thing as “subjective experience.” It is not logically possible to say there is “subjective experience” without inherently entailing that there is some sort of “objective experience,” in the same way that saying something is “inside of” something makes no sense unless there is an “outside of” it. Without implicitly entailing some sort of “objective experience” then the qualifier “subjective” would become meaningless.
If you associate “experience” with “minds,” then you’d be implying there is some sort of objective mind, i.e. a cosmic consciousness of some sorts. Which, you can believe that, but at that point you’ve just embraced objective idealism. The very usage of the term “subjective experience” that is supposedly inherently irreducible to non-minds inherently entails objective idealism, there is no way out of it once you’ve accepted that premise.
The conflation between experience with “subjectivity” is largely done because we all experience the world in a way unique to us, so we conclude experience is “subjective.” But a lot of things can be experienced differently between different observers. Two observers, for example, can measure the same object to be different velocities, not because velocity is subjective, but because they occupy different frames of reference. In other words, the notion that something being unique to us proves it is “subjective” is a non sequitur, there can be other reasons for it to be unique to us, which is just that nature is context-dependent.
Reality itself depends upon where you are standing in it, how you are looking at it, everything in your surroundings, etc, how everything relates to everything else from a particular reference frame. So, naturally, two observers occupying different contexts will perceive the world differently, not because their perception is “subjective,” but in spite of it. We experience the world as it exists independent of our observation of it, but not independent of the context of our observation. Experience itself is not subjective, although what we take experience to be might be subjective.
We can misinterpret things for example, we can come to falsely believe we experienced some particular thing and later it turns out we actually perceived something else, and thus were mistaken in our initial interpretation which we later replaced with a new interpretation. However, at no point did it become false that there was experience. Reality can never be true or false, it always just is what it is. The notion that there is some sort of “explanatory gap” between what humans experience and some sort of cosmic experience is just an invented problem. There is no gap because what we experience is indeed reality independent of conscious observers being there to interpret it, but absolutely dependent upon the context under which it is observed.
Again, I’d recommend reading Jocelyn Benoist’s Toward a Contextual Realism. All this is explained in detail and any possible rebuttal you’re thinking of has already been addressed. People are often afraid of treating experience as real because they operate on this Kantian “phenomenal-noumenal” paradigm (inherently implied by the usage of “subjective experience”) and then think if they admit that this unobservable “noumenon” is a meaningless construct then they have to default to only accepting the “phenomenon,” i.e. that there’s only “subjective experience” and we’re all “trapped in our minds” so to speak. But the whole point of contextual realism is to point out this fear is unfounded because both the phenomenal and noumenal categories are problematic and both have to be discarded: experience is not “phenomenal” as a “phenomenal” means “appearance of reality” but it is not the appearance of reality but is reality.
You only enter into subjectivity, again, when you take reality to be something, when you begin assigning labels to it, when you begin to invent abstract categories and names to try and categorize what you are experiencing. (Although the overwhelming majority of abstract categories you use were not created by you but are social constructs, part of what Wittgenstein called the “language game.”)
We don’t need more metaphysical elements, we need less. We need to stop presuming things that have no reason to be presumed, then presuming other things to fix contradictions created by those false presumptions. We need to discard those bad assumptions that led to the contradiction in the first place (discard then phenomenal-noumenal distinction entirely, not just one or the other).
This is basically the Many Worlds Interpretation. I don’t really buy it because we can’t observe wave functions, so if the entire universe is made of wave functions… how does that explain what we observe? You end up with an explanatory gap between what we observe and the mathematical description.
The whole point of science is to explain the reality which we observe, which is synonymous with experience, which again experience just is reality. That’s what science is supposed to do: explain experiential reality, so we have to tie it back to experience, what Bell called “local beables,” things we can actually point to identify in our observations.
The biggest issue with MWI is that there is simply no way to tie it back to what we actually observe because it contains nothing observable. There is an explanatory gap between the world of waves in Hilbert space and what we actually observe in reality.
What you’ve basically done is just wrapped up the difficult question of how the invisible world of waves in Hilbert space converts itself to the visible world of particles in spacetime by just saying “oh it has something to do with our consciousness.” I mean, sure, if you find that to be satisfactory, I personally don’t.