Tuesday, April 30, 2019

Two interaction problems

Yesterday I realized something that should have been obvious: there are two separate interaction problems for dualism.

  1. Metaphysics: How does the soul manage to cause effects in the body?

  2. Physics: Wouldn’t such causation violate the laws of physics?

I used to think of the interaction problem as just (1), and hence I thought it was spurious once one learned from Hume that all cases of causation are equally mysterious.

But problems (1) and (2) are pretty independent: one can have a solution to each without a solution to the other. For instance, an indeterministic physics provides a solution to (2), but says nothing about (1), while occasionalism and hylomorphism provide solutions to (1), but say little about (2).

While I think the questions are interesting, I don’t really think either poses a serious problem for interactionist dualism.


Brian Cutter said...

Yeah, when I teach phil mind I usually only call the first the "interaction problem" and the second the problem of the causal completeness of the physical.

Since you think neither is a real problem, I'm curious what you think the best answer to the second is. Just say that physics is indeterministic? But of course, high-level processes can be *effectively* deterministic even if microphysics is indeterministic. Maybe there are behaviorally-relevant neural processes that are sensitive to quantum indeterminacy, but it seems plausible that there *could* (nomologically) be a creature with a mind/consciousness but whose "brain" had the high-level structure of an effectively deterministic physical system (e.g., a creature whose brain is more like a digital computer). For creatures like that, it seems that any mental causation would have to either violate the laws of physics (or, at least the frequencies predicted by probabilistic laws), or be redundant in an overdetermination-like way.

Alexander R Pruss said...


I don't think (2) is the problem of the causal completeness of the physical. One has problem (2) even if one takes the (incorrect) view that physics says nothing about causation. And the problem of causal completeness is not solved by positing macro-indeterministic brains, while (2) is. So I now think causal completeness is a *third* problem. But one path to causal completeness is through determinism together with a causal interpretation of the physics.

Regarding your question about effective determinism, we just don't know right now whether the brain has or does not have "behaviorally-relevant neural processes that are sensitive to quantum indeterminacy". If we were to know that it does not have them, the second problem would be a real one.

I am not, however, at all worried about the mere nomological possibility of a creature whose brain was physically deterministic. There are at least two available options: either such creatures would have miraculous powers of transcending physical law or they wouldn't be free. These two options are viable for these *hypothetical* creatures even if they are not viable for us, in much the same way that there is no problem with admitting the nomological possibility of brains in vats, while there would be problems (e.g., for ethics) in thinking *we* are brains in vats.

Brian Cutter said...

I agree about our own case (we don't know whether any behaviorally relevant neural processes are sensitive to micro-level indeterminism). Re: the second case, you say "either such creatures would have miraculous powers of transcending physical law or they wouldn't be free." But I wasn't thinking about *freedom*, but mental causation generally (e.g., an animal's feeling of pain causing avoidance behavior). I guess your view would be that anything whose brain is physically deterministic (at behaviorally relevant levels) is either (i) not conscious, or (ii) has epiphenomenal consciousness, or (iii) has physics-transcending powers. (I'm not saying this is wrong; I think I agree with the disjunction, actually. I just wouldn't describe myself, as you do, as "not at all worried" about this consequence, especially because it seems not-too-unlikely that there really are conscious creatures whose brains are physically deterministic at behaviorally relevant levels.)

Alexander R Pruss said...

I was thinking only about creatures with freedom, since I thought that for other creatures problem (2) is very easy, given deterministic physics. By choosing the right initial conditions and laws (both on the mental and physical side) God can surely arrange it that deterministic mental causation lines up with what the laws of physics predict in such a way that the outputs of mental causation conform to the laws of physics (though the outputs are not causally explained by the laws if we want to avoid overdetermination--but I doubt that overdetermination is a serious problem given theism, since the main problem with overdetermination is that it seems too much of a coincidence, whereas it's not a coincidence if God arranged it).

On reflection, though, I think there is a special problem of type (2) when the laws of physics are indeterministic, a problem that I don't remember ever seeing discussed. Either the mental life is deterministic or not. If it is not deterministic, then we have the problem of how to make two different indeterministic processes consistently line up without overriding either. If it is deterministic, then the problem is how to make the indeterministic physics line up with the deterministic mental life.

Things are easiest if both the mental and physical lives are deterministic.