Inscrutable Mental Causation
It is a matter of common observation that the sun causes plants to grow. Our ancestors will have noticed this long ago and wondered why it is. The answer is by no means obvious. Maybe it’s because the sun is a god and he commands plants to grow; maybe the sun god sends out waves of love that are absorbed by plants; maybe it has something to do with the shape of the sun or its motion or its size or its color; or maybe the sun is a distant fiery object that emits radiation and this interacts chemically with tiny receptors on the surface of plants. We now know that it is the last of these: it is in virtue of the process of photosynthesis that plants grow by absorbing sunlight. This process is well understood down to the fine details: light energy gets converted to chemical energy which is stored in carbohydrate molecules synthesized from carbon dioxide and water and involving electron stripping and oxygen release. But it is conceivable that our ancestors could have got stuck at the macro level of the sun causing plant growth, never achieving the kind of micro understanding now taught in biochemistry classes. If so, they would never have understood the nature of the causation involved; they would know there is a causal connection but not what it depends upon. For that they would need to provide an analysis of light and an analysis of plants that revealed their intrinsic nature. This is a highly non-trivial enhancement of the cognitive abilities present in simply observing a causal connection. A creature might be blocked from achieving that kind of causal understanding by innate intellectual limitations.
Now think about mental causation. This is a vexing and obscure subject. We observe certain broad macro causal relations: desires cause decisions, perceptions cause knowledge, reflection causes changes of heart. We also observe instances of such general causal relations: my desire to have a drink caused my decision to go into a bar, my seeing my cat at noon caused me to believe that my cat was home at noon, my reflection about a friend’s actions caused me to drop that friend. Not everyone agrees that there are real causal relations within the mind—partly because they cannot see what kind of causation might be involved—but it is certainly a natural way to talk about the mind and its states. We can then ask how this causation works: in virtue of what do mental states cause other mental states (or actions)? Immediately we are brought up short: we don’t have a story comparable to the story about sunlight and plant growth. We don’t even have a rough outline of such a story. We can’t say what it is about desire that allows it to cause decision, or what it is about seeing that brings about knowledge, or how reflection causes changes of mind. We don’t have a story about the properties of the cause that lead to its effects, comparable to the photon theory of light and the concept of energy transformation. We don’t have a causal theory to go with our causal observations—we have no analogue of electron stripping and oxygen release. We have no understanding of the mechanisms of mental causation.
It might be suggested that there is such machinery—processes in the brain: so we do have a story about mental causation. It may not be as well worked out as the theory of photosynthesis, but it plays the same logical role: it tells us in virtue of what the macro level causal relations obtain. Brain science tells us the nature of mental causation as biochemistry tells us the nature of botanical causation. But this line of thought faces formidable difficulties, well known and extensively debated. The problem, in short, is that it leads either to epiphenomenalism or to materialism. It leads to epiphenomenalism because it threatens to make mental properties causally redundant, and it leads to materialism because materialism is the only way to avoid epiphenomenalism. I won’t rehearse this familiar story: intuitively, the problem is that causation in the brain is at the wrong level to account for mental causation—it is too extrinsic, too foreign. This is why materialism starts to seem unavoidable: it is the only way to ensure that mental causation is possible. A materialist theory of mental causation leads to materialism about the nature of the mental. If neurons play the role of energy-converting cells in photosynthesis, then neurons must be what the mind is made of. But that kind of reductionism has its own difficulties, also well known. Materialism about light and plants is not a problem, but materialism about the mind is a high price to pay for the causal efficacy of the mental.
Why not just locate the causal machinery in the mental states themselves? It is in virtue of being a desire that desires cause decisions; it is in virtue of being a desire for a drink that this desire causes me to decide to go to a suitable bar. The causally operative properties are just mental properties themselves, conceived as existing separately from underlying brain states. This is the analogue of asserting that it is in virtue of being light that plants grow in sunlight—with nothing further to be said by way of causal elucidation. That is, mental causation is primitive. But this position is hard to accept because mental states are not basic properties of the universe; they are less primitive than biochemical properties of plants. Is there really nothing more to causation in the mind than our ordinary catalogue of mental properties? Also, how exactly does this irreducibly mental causation work? How does simply being a desire bring it about that a decision is made? We need some sort of explanation, some account of the causal connection. Simply being light doesn’t explain how light produces plant growth, so why should simply being a desire explain how desires have the effects they do?
We now have three possible theories of mental causation: first, it doesn’t exist at all; second, it reduces to physical causation in the brain; third, it is primitive and inexplicable. For completeness I will add the “theory” that mental causation is magical and requires divine intervention. None of these theories is attractive; in fact, they are all pretty horrible. They seem more like symptoms of panic than sober theorizing; none of them could claim the mantle of obvious truth. They are what you come up with when you can’t think of anything better. Are we then clean out of options? Must we accept defeat? Consider again our causally curious ancestors puzzling over the sun and plant growth: I remarked that they might never have been able to hit on the theory of photosynthesis; they might, indeed, never have progressed beyond the sun god theory. The problem was not elementary and required sophisticated physical concepts—atomic physics no less. Might we be in a similar position with respect to mental causation? Is it that we can’t construct or discover the properties and principles that actually lie behind mental causation? There is something about desires that enables them to cause decisions, but we don’t know what it is, not even sketchily. It isn’t brain correlates and it isn’t just the mental properties themselves (as we conceive them); it’s something else that eludes our understanding. It might not always do so—we might with diligence produce a good causal story: but we also might not. At any rate, nothing we now know about the mind provides us with anything comparable to the theory of photosynthesis. We don’t even grasp the psychological analogue of physical analysis, i.e. investigating things in their fine structure to tease out how they work. We don’t have an analogue for the mind of the macro-micro distinction: we don’t have a notion of micro causation to go with our notion of macro causation. That is, we can’t analyze the mind into simpler parts that operate together to produce observable results—a kind of atomic psychology. Of course, the mind is composed of many elements (faculties, states, modules, concepts), but we have nothing analogous to the atomic understanding of matter, or indeed basic chemistry. This is why we are at a loss to provide an account of mental causation: we don’t know all there is to know about the mind; therefore we have a poor understanding of mental causation. We don’t know how one mental state leads to another. We are like people that observe the sun’s effects on plants but don’t have a clue how to explain it. There is machinery in there somewhere but it is inscrutable to us.
This position implies that there is more to the mind than what we know about, including the neural basis of mind. There is a whole level of description of familiar mental phenomena that eludes us, at present or permanently. This is because we lack any plausible account of mental causation—we don’t know in virtue of what properties and principles this kind of causation works. Mental causation is an enigma to us. The two suggestions we have considered—brain causation and primitive mental causation—are both unsatisfactory, but nothing else suggests itself. The nature of mental causation is not visible from the perspective of the brain or from the perspective of introspection. We must be missing something, something vital. No doubt what we are missing is intimately connected to properties of the brain and to mental properties themselves, because these are not irrelevant to mental causation; but neither identifies the level at which mental causation is to be explained. That is, neither corresponds to the level occupied by the theory of photosynthesis. The level of description provided by a computational theory of mental processes is the kind of level we need, but I don’t believe that computation can play the same kind of explanatory role as photons, energy transfer, and carbon synthesis. It is not in virtue of computations that one mental state causes another, though computations may well be involved in many mental processes. Computations are too abstract to be causes; they are not energy mechanisms.  We would need to connect them to neural processes, but then we are back with the brain theory of mental causation. Rather, there are unknown causal factors at work in mental causation.
These are no doubt difficult issues, in which obscurities combine and deepen, and ignorance seems the most likely hypothesis. The mind-body problem and the problem of causation: put them both together and the going is bound to get tough. I suspect we know almost nothing about the real nature of mental causation, despite knowing many causal truths about the mind. We notice it in ourselves, but we have no deeper understanding of what is involved.
 How does one computation lead to another? Is it by neural realization or is it a primitive property of computations? This is the same dilemma as before—not surprisingly since computations are modeled on actual mental processes such as conscious calculation.