Victor Reppert is an American philosopher who has been influential in his development of the argument from reason. Reppert develops a number of different versions of the argument and one of these is the argument from mental causation. This is an argument against naturalistic atheism, rather than an argument for theism.
If naturalism is true then no event can cause another event in virtue of its propositional content, or no mental event can cause other mental events.
But some events do cause other events in virtue of their propositional content and some mental events do cause other mental events. (This is implied by the existence of rational inference.)
Therefore, naturalism is false.[i]
Premise 1 can be defended as follows. If naturalism is true, we must expect that every mental event has a sufficient cause in some brain event. This would be true even if some form of non-reductive materialism is true. (Non-reductive materialism is the idea that mental events cannot be wholly reduced to brain events, but that the mind still exists in virtue of the brain.) It is most reasonable to think that if naturalism is true, then materialism about the mind also has to be true. This means that the mind must be explainable in terms of some material substance (the most obvious candidate is the brain). Otherwise, naturalists have the task of explaining what natural substance apart from the brain could account for the mind. If the mind and therefore all mental events must be caused by brain events, then mental causation should not be possible. All mental events should have a sufficient explanation in a brain event (even if we don’t suppose that the mental event is identical with the brain event). But if mental causation is true, then there are mental events that are caused by other mental events and not by brain events. Or mental events will have to cause brain events to cause other mental events. However, the fact that it is the propositional content of a thought that is involved in the causation of another mental event makes doubtful that a brain event is involved even in this way, because then propositional content of thoughts would somehow need to be translated into a neurochemical process, which seems implausible. But, either way, mental causation is incompatible with naturalism about the mind, reductive or non-reductive.
Reppert defends premise 2 in the following way. Let’s look at a simple deductive argument: 1) All men are mortal. 2) Socrates is a man. 3) Therefore, Socrates is mortal. “If rational inference is to be possible, it must be the case that someone can come to believe that (3) is true in virtue of one’s being in the state of entertaining and accepting (1), entertaining and accepting (2), and that those two states cause the thinker to reach a state of accepting (3)…But there seems to be more involved even than the mere existence of mental causation. One mental event must cause another mental event in virtue of the propositional content of those events.”[ii] It is only by understanding the propositional content of premise one and two that we are caused to think that the proposition in (3) is the case. “If all causation is physical causation it might be asked how the content of a mental state could possibly be relevant to what causes what in the world.”[iii] Brain states don’t have propositional content; only mental states do. This means that naturalism cannot account for our reasoning process. Our reasoning process requires propositional content in our mental states to cause other mental states with a specific sort of propositional content.
The Mind Can Act Independently of the Brain
Also, mental causation implies that the mind can act independently of the brain. The mental event that caused an effect on the brain in mental causation cannot itself be caused by the brain, because then that wouldn’t really be mental causation. The mental event that represents mental causation cannot itself be represented by a brain event, because then the brain has simply caused an effect on itself. And then we are still fully determined by brain events that we don’t control. So the mental event must act independently of the brain in order for mental causation to happen. But if that’s the case, then the mind cannot be fully explained in terms of the brain.
As an Argument for God’s Existence
So this implies that naturalism is false, but we can also use this as an argument for God’s existence.
Mental causation is required by the human reasoning process.
If mental causation is required by the human reasoning process, then it actually obtains.
Therefore, mental causation obtains.
Mental causation implies that the mind can act independently of the brain.
If the mind can act independently of the brain, then it is probably not physically explicable or reducible to another substance.
If mind represents an independent reality, then it is best explained by a greater mind that could have created human minds.
Therefore, a greater mind exists.
We’ve established that human minds are not explainable in terms of the physical. However, human minds must have an explanation, because human minds could have failed to exist (they are contingent). Also, human minds clearly began to exist, so they require a cause as well. If minds are non-reducible, then mind represents a fundamental feature of reality. If mind is not reducible to non-mental properties, then the most simple way to explain it is to explain it in terms of another mind. This mind must be capable of creating all human minds (so it must be very powerful). It must be metaphysically necessary, because otherwise we will have to posit an infinite regress of contingent or impermanent minds (which is absurd). (For more information on why an infinite regress would be absurd, look at the arguments against an actual infinite under the Kalam Cosmological argument). This mind must also be personal, because minds are always personal. It must be immaterial, because physical things cannot be metaphysically necessary. Also, we’ve established that the mind must be able to act independently of the brain, so it is not necessary to think that the being has a body. So, we’re looking at a metaphysically necessary, immaterial, personal mind that created all human minds.
[i] Victor Reppert, C.S. Lewis’s Dangerous Idea: In Defense of the Argument from Reason (Downer’s Grove: Intervarsity Press, 2003) 77-80. Kindle Edition
[ii] Ibid., 78
[iii] Ibid., 78