He wasn’t asking whether there is interaction between spiritual phenomena and material phenomena. Obviously there is. He asked how such an interaction works, given that these phenomena are the result of two different substances, one material substance and one spiritual substance.

I’m asking how this is philosophically possible. If the difficulty is so serious, maybe it is more reasonable to accept a materialist or idealist explanation (or a monist one).
Let’s say spirit affects matter—when exactly does that happen? How is that even possible, and is it acceptable?

I’m asking this בעקבות a study of Richard Taylor’s book, “Metaphysics,” where he raises this argument and says that a dualist view is not necessarily more plausible than a materialist one.

You do understand how interaction between spirit and spirit works? We don’t know how to describe mechanisms of spiritual processes. That has nothing to do with whether these are different substances. Light and matter are also different substances, and there is no problem understanding interaction between them. These questions are just an empty slogan. People are looking for a scientific explanation, but science deals with matter and therefore cannot explain spirit. That has nothing to do with the question of whether this is an interaction between two substances or not.

It seems to me the paradox is this: if spirit acts physically, then it is physics again and not spirit. If I decide in my soul to move my hand, then one of two things must be true: either my decision is a physical cause, since after all it moved electrons, or my decision is spiritual and did not affect matter. There is no third option. If there was an effect on physics by a plane whose essence is non-physical, then where exactly was the physical cause of the effect? Or at a more tangible resolution: who kicked the ball?

No paradox at all. It’s just begging the question. You assume that if it acts on a physical object, then it itself is physics. But that is nothing more than the same assumption in different words—in other words, you assume that action between spirit and matter is impossible. I see absolutely no reason in the world to assume that. Don’t assume it, and you won’t have paradoxes.

My assumption is simply the laws of physics. An object does not move unless a physical force is exerted on it. Of course there is no paradox if we simply accept an exception to the laws of physics, but then we would have to note that for interactionist dualism we are breaking physics as it is known to us.

Besides that, I don’t think the comparison to light and matter is correct. The difference here is more essential, because spirit is essentially a different plane from matter. Just as you argue against emergence from matter to spirit, so too one would argue against the emergence of physics from spirit.

To clarify: it seems that the claim against emergence also applies to the ability of matter to bring about a change in an alien plane (spirit), even assuming that it exists—not only against the ability to bring that plane itself into existence.

The laws of physics deal with interactions between physical entities, not with the effect of spirit on matter. Beyond that, you spoke about a paradox; that is a logical concept, not a scientific one. There is no paradox here, and no difficulty either. It is simply begging the question. Indeed, when one assumes a spirit that acts on matter, that goes beyond the laws of physics, because this is not physics.
What is the problem with that?
And regarding the comparison to light and matter, you’ve decided that this is not the same thing, even though they act on one another. Again, the same begging of the question.
Emergence is not relevant here, precisely because of the distinction you wrote. The identification of spirit itself with matter is nonsense. The claim that spirit acts on matter, or vice versa, has no problem whatsoever. It is simply a fact of life.
I think we’ve exhausted this, because everything is just repeating itself in different words.