...Those two domains can interact causally with each other, so phenomena in the physical world can have supernatural causes and vice versa.
...If causality flows freely between the two domains, then what distinguishes the border from any other arbitrary line drawn through reality?
I didn't say that causality flowed freely.
If "can interact causally" isn't the same thing as "causality flows freely", what restriction on the flow of causality do you have in mind?
Most people in the world start with the assumption of dualism. Belief in a spiritual plane of existence is widespread and taken as a given.
People believe a lot of goofy things that make no logical sense. "Everything has a cause; therefore there must have been an uncaused First Cause.", for instance.
We may be in violent agreement that it is an implausible, untenable assumption, but it is a logical possibility.
But that's not something I agree with. To qualify as an implausible, untenable assumption, a sentence first has to mean something.
People who believe that there are two different planes of reality--spiritual (mental) and physical--don't purport to know how they interact, so this isn't much of a challenge for them. Our earliest experiences were of a distinction between our mental reality of bodily sensation and control vs. external events that we could not always sense or control. Our brains were hardwired to build hypotheses about how external reality worked and to test those hypotheses through observation and experimentation. So dualism of some sort is grounded in early experiences.
That's a self-vs-other division, not a spiritual-vs-physical division. You can control your own arm but not your neighbor's arm; you can perceive your own thoughts but not your neighbor's thoughts.
So a spiritual being such as a god can perform miracles in the physical world, and events in the physical world can cause effects on spiritual beings.
Likewise, a tigerish being such as Mantecore can perform miracles in the untigerish world, and events in the untigerish world can cause effects on tigerish beings. So what's the difference between your calling an effect of a god on a human a "miracle", merely on account of your having chosen to draw a line between gods and humans, and my calling Mantecore crippling Roy Horn a "miracle", merely on account of my having chosen to draw a line between tigers and humans?
Reality contains all manner of things we can't see: things we infer the existence of from observing the behavior of stuff we can see. Gravity waves, for instance. We routinely call such things "physical" and "natural", purely on the basis that we have figured out that causality flows between them and the rest of the stuff we call "physical" and "natural".
OK, sure. You reject that kind of dualism as implausible.
No, I don't. You're interpreting me as meaning the opposite of what I said. I reject spiritual-vs-physical dualism as meaningless, as content-free, as "not-even-wrong". Implausible would be a giant leap forward for it.
So do I. I'm not defending its plausibility, just its clarity as a model of how reality works. You are picking an argument with someone who also rejects Cartesian dualism. What I don't agree with is the claim that the natural/supernatural dichotomy is any vaguer than competing models of reality.
Well then, if next year we discover another entity that affects our observations, what criterion will it have to satisfy to qualify as nonphysical?
If you want to say that there are other possible, even more absurd, models of how things work, I'm not going to argue with you.
Huh? Why would I want to say that? I'm arguing for my views, not for yours.
Build all the straw men you want, and I'll help you knock them over.
What are you on about? I didn't build any strawman -- I didn't tell you what you think. It's you who told me what I think, and botched it.
The problem with calling something a miracle because it involves causality crossing between the tiger and the untiger domains isn't that it's a model that's possible but absurd. The problem is that it's a non-model. It's empty, arbitrary, uninformative, content-free. If somebody told you such interactions are "miracles", that would give you no information about men or tigers; you'd only learn that he conceptually divides reality into a tiger part and an untiger part, and the border in his mind between those domains is important to him.
An implausible but possible model of how things work would be something like Kepler's theory that the planets' orbits correspond to the Platonic solids. It implied a falsifiable prediction: that Uranus didn't exist. In contrast, Cartesian dualism doesn't have what it takes to qualify as a model of how things work, any more than a division of the world into tiger and nontiger domains does. Those are just
labelings of the world. They have no implications for what happens. What prediction can you make that follows from Cartesian dualism?