Is causal interaction consistent with substance dualism?

One of the main objections people always raise against substance dualism is that there’s a correlation between brain states and mental states. This correlation can be proved by giving a person drugs. The drugs cause a chemical reaction in the brain that immediately causes a change in a person’s mental state. This is supposed to disprove the notion that the mind and brain are distinct substances.

In response I would say it is perfectly consistent with substance dualism that there would be causal interaction between the mind and the brain. In fact, I would argue that the nature of the causal interaction is not only consistent with substance dualism, but that it is actually inconsistent with a denial of substance dualism.

J.P. Moreland explains the differences between substances and properties in chapter 3 of Scaling the Secular City. Basically, a property is a universal, and a substance is a particular. A property, like redness, can be in several different places at the same time, but a substance, like a dog, can only be in one place at a time. Properties are had by substances (e.g. a ball can be red). A substance can change by gaining or losing properties. A round ball of wax can be shaped into a square, but it remains the same piece of wax. Roundness, however, does not change into squareness. Roundness is roundness, and squareness is squareness. Likewise, a substance can change from red to green, but the property of redness does not turn into greenness. Most importantly for our purposes, a substance has causal power, but a property doesn’t. The property of roundness, by itself, doesn’t cause anything, but substances that have the property of roundness do.

If the mind is an emergent property of the brain, then whatever the brain state is, the mental state will conform to it. All the causal interactions happen strictly in the brain, and the various mental states emerge as a result. To illustrate this, let’s let “B” represent brain states, and “M” represent mental states. The arrow sign, -> represents the direction of causation. The emergent property view would look like this:

B -> M

The state of the brain is constantly changing, and the mental states all change as a result. But all the causal interaction is between brain states.

B1 -> B2 -> B3 etc.

Each of these brain states results in a corresponding mental state.

B1 -> M1
B2 -> M2
B3 -> M3
Etc.

But the mental states themselves don’t cause anything since they are only emergent properties. One mental state cannot cause another mental state. Neither can the direction of causation go backwards. A mental state cannot cause a brain state.

If the problem with the emergent property view isn’t already obvious, I’ll just spell it out. Desires, inclinations, and the will are all mental states. But if the mind is an emergent property, then the will has no causal power. A person cannot act on a desire. He cannot will to lift his hand. The will is itself a brain state. Actions (such as lifting the arm) work because the brain sends a signal to the muscles in the arm causing them to contract. So a brain state can cause the arm to rise. Let’s let “A” stand for any action, such as lifting the arm. The emergent property view would look like this:

A <- -="" b=""> M

The mental state may be something like a desire, or the will to lift the arm. The action may be lifting the arm. The brain causes them both. But there is no causal chain between the will to act and the action, because the mental state is an emergent property, not a substance.

Only if the mind is a substance can it have causal influence on the brain. Only then can we will to do anything. On the substance dualism view, it is possible for a person to mentally decide to act, and then initiate an action by causing a brain state that then causes an action.

M -> B -> A

Think how strange it would be if the emergent property view is true. Mental states would really be superfluous. Since all actions are caused by brain states, and brain states can only be the effect of previous brain states or chemical reactions, then all the activity of humans (i.e. having conversations, reacting to the environment, eating, etc.) would go on exactly the same if there were no mental states at all. Though we continue to have the perception of having a will that initiates actions, our minds are really passive. We’re just riding along seeing it all. Our perception of a will capable of initiating an act is just an illusion caused by brain states.

Imagine now that we are aliens observing humans. We see them interacting with each other. We see them “preparing” for the next thing to come along. But there would be no reason to think anything mental was going on at all. Robots might very well do the same things. What we’d be witnessing, for all we knew, might be highly sophisticated artificial intelligence—computers just mimicking real intelligence.

But we know this isn’t so. We know we have a will that initiates actions. Our minds are not passively observing all that we do. We know our actions are intentional. But that can only be so if the mind is a distinct substance from the brain. Only then can the mind have any causal power.

So the substance dualism view is consistent with drugs affecting the mind, because causal interaction can go both ways when there are two substances involved. Brain chemistry has causal power on the mind, which is why drugs work, and the mind has causal power on brain chemistry, which is why we are able to act intentionally.

Next: The rational argument for substance dualism