Whatever else is the case, I think it is obvious that the word "existence" means different things when applied to physical things than it does when applied to at least some spiritual things. This can be shown if one asks "How do I know x exists?" because that question not only gives us a direct answer (at least in theory) but it also indirectly answers what we mean by "exists".
In other words, the operational meaning of the word "existence" varies depending on whether one is talking about material or immaterial, physical or spiritual, things.
And I think it goes beyond that. What do we mean by the term 'physical things'? I'm sure everyone agrees that chairs and trees and computers are physical. It becomes a little less clear, to most people, when asked if light is physical. Is gravity? Are electrons? How about neutrinos?
Now, in all of those cases, I am unhesitant to say they are physical. Light, for example, is a form of electromagnetic radiation that is studied by physicists with good results.
Similarly, even though neutrinos are incredibly hard to detect and trillions of them go through the Earth every second with no effect, I would say that they are physical.
So the question becomes *why* are those elusive things all physical? Why are they given the label of 'physical'?
And the answer, as far as I can see, is that they interact in predictable ways to things we have already identified as physical (ultimately, things like trees and chairs). Ultimately, they are physical because they interact in some way with something that is physical (a recursive definition, if you will).
OK, now let's pivot. What does it mean to say that something exists? More specifically, does it make sense to say that something exists that does not interact with anything else?
In my view, it doesn't. If something doesn't interact, then there is nothing that distinguishes it as existing.
And then, we get to the original question. We see that it makes no sense to say that something exists that doesn't interact, but also that anything that interacts with the physical is, itself physical.
So is it even meaningful to say that something is non-physical? As far as i can see, under this formulation, it just isn't meaningful. And that has the immediate consequence that anything based on the idea of a non-physical is also incoherent.