I’m not sure if we’re talking past each other or if there is genuine disagreement, but I’ll expound a bit.
When asleep or in a coma, the mind doesn’t interact with the environment at all.
The sleeping/comatose mind does interact constantly with the environment in two ways. For starters, it’s well established that external sensory input (specifically sounds and touch) regularly makes its way into the conscious experience of dreaming and comatose state. But that’s just a side issue here. At a more fundamental level, every living thing interacts 24⁄7 with its environment through its metabolism.
That confuses causality with necessity (metabolism causally preceding understanding doesn’t mean that metabolism or continuous input are necessary for it).
Maybe this is the crux of a misunderstanding. I don’t claim that “continuous input” in the sense you (seem to) mean is necessary/causally antecedent to semantics. E.g., I’m not saying that I have to constantly look at a tree out in the woods in order to think about what a tree is. I’m only saying that any thought I have, and whatever language and semantics attached to it, are the result (causal/necessity if you like) of my metabolic processing. (Using metabolism in the broadest sense to mean any chemical pathways that use energy and produces entropy in the body, which includes neural activity). If that’s not the case, then something non-biological makes human language possible, which I assume you don’t intend. Either way, that would be a hypothesis for a different type of discussion forum.
The point I’m trying to express (and clearly failing at) isn’t conceptualism or solipsism, at least not in the way my own semantic modeling interprets them. As I interpret them, the idealism of, say, Berkeley, Buddhism et al amounts to a re-branding of reality from being “out there” to “in my mind” (or “God’s mind”). I mean it differently, but because I refer constantly to our mental models, I can see why my argument looks a lot like that. Ironically, my failure may be a sort of illustration of the point itself. Namely, the limits of using language to discuss the limitations of language.
In fact, the point I’m trying to get to is not so much about “the nature of reality” but about the profound limitations of language. And that our semantic models tend to fool us into assigning a power to language that it doesn’t have. Specifically, we can’t use the language game to transcend the language game. Our theories of ontology and epistomology can’t coherently claim to refer to things beyond human language when these theories are wholly expressed in human language. Whatever model of reality we have, it’s still a model.
The objection of realism is that our models are not created in isolation, but by “actual reality” interacting with our modeling apparatus. My response is: that is a very useful way to model our modeling, but like all models, it has limitations. That is, I can make a mental model called “realism” in which there are mental models on the one hand and “real reality” on the other. I can further imagine the two interact in such a way that my models “carve reality at the joints”, or “identify clusters in thingspace”. But all of that is itself manifestly a mental model. So if I then want to coherently claim a particular model is more than just a model, I have to create a larger model in which the first model is imagined to be so. That can be fine as far as it goes. But realism – the claim of a “reality” independent of ANY model—commits one to an infinite nesting of mental models, each trying to escape their nature as mental models.
This situation is a close analog to the notion of “truth” in mathematics. Here the language game is explicitly limited to theorem-proving within formal systems. But we know there are unprovable statements within any formal system. So if I want a particular unprovable statement to count as “true”, I need a larger meta-system that makes it so. That’s fine as far as that goes. But to use the language game of formal systems to claim an unprovable statement is true independent of ANY proof, I would need an infinite nesting of meta-systems. That’s clearly incoherent, so when mathematicians want to claim “truth” in this way they have to exit the language game of formal systems – i.e. appeal to informal language and the philosophy of Platonism.
Personally I’m not a fan of Platonism, but it works as a philosophy of mathematics in so far as it passes the buck from formal to informal language. But that’s also where the buck stops. The sum of formal and informal language has no other system to appeal to, at least not one that can be expressed in language. To sum it all up with another metaphor: the semantic modeling behind the philosophy of realism overloads the word “reality” with more weight than the human language game can carry.