I find Goff's questions perplexing/confused. You quote him asking:
"where do distinct subjects come from? what ensures they share a common world of experience? i find these qs easier to answer on panpsychism than idealism." — spirit-salamander
I am an immaterialist on the basis of the evidence. That is, I believe there are epistemic reasons to believe immaterialism about the mind - and immaterialism about everything - is true. Whether there are prudential reasons or moral reasons or aesthetic reasons to think some other view is true, is neither here nor there. For those kinds of reason are not the kind that evidence is made of. As I am sure even Goff would accept, if there are prudential, moral and aesthetic reasons to think God exists, that would not constitute evidence that God exists. Likewise, if there are good prudential or moral reasons to think materialism is true, that's not evidence it is true.
I am not a dualist. I am an immaterialist about the mind. But, following Berkeley, I think everything that exists is made of minds and their contents. Immaterialism is a monistic theory, like materialism. It should not, then, be conflated with dualism - a theory that does no more than add the problems of materialism to an otherwise problem-free immaterialism.
Anyway, he asks 'where do distinct subjects come from?' Nowhere. Everyone has to say that about something. And note, as a panpsychist he would have to say 'nowhere' to the question 'where do the consicous states of the most basic units of existence come from?' So when it comes to the basic units of existence - that from which all else is made - the question 'where do they come from?' is misapplied. They don't come from anywhere, whatever they are. THey just brutally exist.
That's what I say about my mind. It doesn't 'come from' anywhere. It just exists. And that's true of all minds. They just exist. The external world 'comes from' them, or one of them. But they themselves come from nowhere, for they are not in a location and they are not made.
Consider this argument for immaterialism:
1. If an object is material, it is infinitely divisible
2. No object is infinitely divisible
3. Therefore, no object is material
4. If no objects are material and some objects exist, then immaterialism is true
5. Some objects exist (my mind, for instance)
6. Therefore, immaterialism is true
My mind is an indivisible thing - half a mind makes no sense - and thus has no parts (for if it had parts it could be divided into them). If a thing has no parts - is indivisible - it is simple. That is, it is not made of anything more basic than itself. It has no ingredients. As such, if an object is indivisible it has not been created, for there is nothing more basic from which one could create it.
Thus our reason tells us, if we care to listen to it, that we are not created - we are uncreated simple things. Immaterial things.
Goff, then, by asking 'where to subjects come from' shows only that he does not really understand how one might arrives at immaterialism about the mind.
What about the second silly question - 'what ensures they share a common world of experience?'?
Again, he seems not fully to understand the position he's addressing. There 'is' a common world that we are experiencing - he would agree to that, of course. The question is 'what is it made of?'. If we answer that question, that will help us answer his. Well, it is made of mental states. For I am aware of the world via my sensible experiences. But what do I experience? I experience sights, sounds, smells, tastes, textures. These are sensations. Thus the world of common experience - the world we each seem independently to be aware of via our own sensible experiences - is made of sensations. Not mine or yours, of course. My sensible experiences give me an awareness of a world, but do not constitute it. He would accept this too, of course, for otherwise what's he asking about? We all seem to be aware of a single, unified world - that is, our individual sensible experiences give us each a partial awareness of a single world of sensations. Thus that common world is not existent in our own minds, but is external to them.
Sensations can only exist in a mind. As the world of common experience is made of sensations - sensations external to my mind and yours - the world of common experience is made of the sensations of a single external mind.
So what ensures our minds share a common experience of that sensible world: well, what else but the mind whose sensations the world is made of?