I used the phrase "metaphysical naturalism"
I suppose that's true if we interpret 'metaphysics' in the sense of 'higher planes' or whatever it is that one finds in 'metaphysical bookstores'. (Do metaphysical bookstores exist any longer? Do
bookstores exist any longer?)
I was using the word 'metaphysics' in a more technical usage, referring to those categories (Aristotle first tried to describe them in his
Categories) that we use to describe and conceptualize reality. Things like 'substance', 'property', 'relation', 'change', 'quantity', 'cause', 'possibility', 'time', 'space' and many more. Together they provide a whole conceptual framework for understanding things.
There's also a concern to identify those things that have being and are real in their own right, things that just
are on account of the kind of thing they are in other words, and distinguish them from things that can ultimately be reduced to combinations or behaviors of these self-existent things. (Atomism is the historical poster child for that, and today's physicists' Theories of Everything are extensions of it.)
Another technical term for what I was just describing is 'ontology', so if 'metaphysics' creates confusion, let's use 'ontology' instead.
https://en.wikipedia.org/wiki/Ontology
The Universe is composed of stuff which exist ie detectectible and measurable
And there you're doing it yourself. Kind of just pronouncing your own ontology. Religions do it when they pronounce the existence of supernatural beings. Atheists do it as well, when they assert their own ontological naturalism. You're doing it. Don't feel bad, everyone does it. We can't help it, we need these kind of fundamental concepts in order to think about things. The challenge is justifying all these categories. Most of them aren't scientific in the sense that they derive from science. If they are scientific, they are scientific in a different way, since science
depends on ideas like these in order to do whatever it is that science does.
One of the ways that the advent of relativity and quantum mechanics were so
revolutionary a century ago was because they kind of forced a reconceptualization of a few of these very fundamental ways that people had been thinking about reality since ancient times. That reconceptualization isn't near complete in quantum mechanics, and physicists just sort of accept QM as a calculating device that they use to predict experimental outcomes. They can't really say how reality is such that their experiments turn out that way.
I'm going to focus in on this thing you said: "stuff which exist ie detectectible and measurable." That's about as clear a statement of ontological naturalism as one could want.
The challenge is: How do you
know that existence is equivalent to detectability and measurability? It's exactly the same kind of question that atheists incessantly throw at theists: How do you know that your God exists? In both cases it demands that one justify his or her fundamental ontology. Something that's very difficult to do.
Think of your dog. There are many things that seem to exist in reality or are necessary to fully understanding reality that your dog doesn't know and can't even conceive of. Black holes, galaxies, bacteria, ribosomal RNA, Maxwell's equations... the list is endless. The same can be said of every other animal. Only humans can know these things.
So... why must we assume that we humans are the apex of all possible intelligence? Why can't we imagine (entirely hypothetically, don't freak out) super space aliens whose intellectual powers are as far beyond ours as ours are beyond the dog's? And why can't we imagine that there are aspects of reality that such a space alien can know that humans can never even suspect? That analogy comes from Albert Einstein btw. I got it from a professor that used to work with Einstein in Princeton. In his later years, Einstein thought of reality as a mystery that human beings can probably never fully understand. Nobody knows what the unknown contains or how far it extends. He saw his life's work as just shining a weak little flashlight a short distance farther into the mist.
Now think of the problems that this analogy presents for your ontological naturalism. If the analogy has any force, how can we so blithely exclude the possibility of very real things that humans don't currently know and perhaps can never know? How can we so blithely restrict the boundaries of reality to what this one very limited and contingent species on this one planet can detect and measure?
Well if you wish to divide reality into TYPES of reality be my guest
There's mathematics and all sorts of abstract things like universals. There's material substances and relations between them. There's parts and wholes. There's space and time. There's actualities, possibilities and impossibilities. It seems to me that one can never really get a conceptual grip on things unless we make these kinds of distinctions.
Personally I am not inclined to go looking for other types of reality. One flavour reality good enough for me
Fine (for you... maybe). Your choice of your favored ontology is your choice. But doesn't that 'It's my belief and I'm sticking to it' put you at the same place as the religious believer? Both of you would seem to be asserting views of fundamental reality that you can't really justify or know as fact.