I am an idealist, which I define as the claim that there is nothing outside of consciousness. My main reason for being an idealist is that

- all materialisms (physicalisms) have the intractable problem of how mentality can be caused by non-mentality (the so-called "hard problem of consciousness"). (Property dualism does not have a reason why the underlying substance with physical and mental properties should be considered physical.)

- substance dualism has the intractable problem of how the two substances interact.

- while idealism has problems, they are tractable.

By "intractable problem" I mean that no one, in the centuries in which these ontologies have been around, has the slightest idea of how the problem can be addressed, much less solved. All conjecture, speculation, or what have you is stopped at the outset. Idealism's main problem is why there appear to be things outside of consciousness, like rocks. This problem can be addressed, for example, by noting that a rock in a dream is clearly within consciousness, and conjecturing that the physical world is simply a shared dream with much more restrictive rules than our private dreams. I am not saying this is the solution, just that it could be, and at least shows that the problem is addressable.

I would say that hylemorphic dualism is two steps away from being idealism. The first step is to recognize that everything is in God's consciousness, and is kept in being 24/7 by God. Because of the doctrine of Divine Simplicity, God's sustaining power is identical to God as Pure Act which is identical to God's Thinking, and so every thing can be understood to be an idea of God. The second step is to replace the concept of prime matter with this power of God's thinking. That is, replace 'hyle' with God as Pure Act. This, to be sure, upends the actualizing role in the act/potency distinction for physical entities. Under this scheme, God, as the formless power of Thinking, actualizes form (thoughts) by thinking them into existence.

Jeremy Taylor wrote:

The problem for Descartes was he wanted to restrict causation amongst material things to physical causation. Otherwise, I'm not sure why two substances shouldn't interact. Indeed, I agree with William Hasker that the interaction problem is one of the most overrated objections in philosophy.

How do they interact, then? 

Jeremy Taylor wrote:

How does what interact? Descartes had a problem because he implied what is immaterial and non-extended could exert physical force (essentially push-force) on what is extended and material. So, I agree with you in that case, if that is what you are referring to. There is an interaction problem there.

But if you mean all claims to immaterial-material interaction, I don't see the problem. If we don't assume the immaterial has to act as a physical cause, then the how here is just that the immaterial is a cause of a certain kind with certain material effects. I am not sure what more needs to be said.

What more needs to be said, as I see it, is how this gets one passed the "somehow" stage. As the reductive materialist is limited to saying that somehow the brain produces mentality, so the dualist is limited to saying that somehow mind and matter interact. Just positing an immaterial cause with material effects gets one no further.

With idealism, on the other hand, we know of immaterial causes having immaterial effects (e.g., our thinking). If we note that, as we drill down into apparently material things, that what gets moved is just as well described as information (that is, a form, an idea) rather than as as piece of mindless matter, we have an imaginable system of forces and that which forces act upon, all immaterial.
 

My solution would be the hylemorphist one.

There have been interesting proposals at solving the problem of interactionism even for those who aren't exactly classical hylemorphists. Father Robert Spitzer defends a mixture of what he calls "trialist interactionism" with hylemorphism especially as he gathers it from Lonergan and Polanyi. On the trialist interactionist side, his model closely resembles John Eccles's solution.

Jeremy Taylor wrote:

But what makes the case of immaterial-material causation any different to immaterial-immaterial causation (or, indeed, material-material or causation)? You need to be more specific about just what getting passed means. What is wrong with saying that an immaterial cause can have a material effect? Perhaps the issue is you are looking for something like a physical account of causation, like we aim to get (ignoring Humean worries) in natural science? Otherwise, I don't see the different between immaterial-material and immaterial-immaterial causation.

The difference is that immaterial-immaterial causation is known, while immaterial-material and material-material causation are not. (I am taking 'material' to mean that which is assumed to exist outside of any experience.)

What I said in my first post in this thread can be rephrased as pointing out that idealism starts from what cannot be consistently doubted (that there is mental activity), while substance dualism and materialism start out with that which can easily be doubted (claiming the existence of something outside of experience), and has no means of refuting that doubt.