By Alin Christoph Cucu
The mind-body problem is as old as philosophy itself (already Plato and Aristotle dealt with it). One may wonder, though, how there is a problem in the first place. We have minds, and we have bodies, and the two clearly seem to interact. My wanting a sip of water is the cause of my reaching out for the glass; the itch I feel is the reason why I scratch myself. Those events are among the most mundane we can think of. How in the world is there a problem?
It arises when one starts to think about the nature of the mental and the physical, respectively. The mind-body problem then becomes the problem of explaining how two such fundamentally different things as mind and body can relate to, or even interact with, one another.
Upon reflecting on it, one cannot but concede that the mental has characteristics wholly absent from physical states:
- a) There is a raw, qualitative feeling, a “what it is like” to have a mental state like a pain.
- b) At least many mental states have intentionality directed towards an object—they are about that object.
- c) Mental states are internal, private, and immediate to the subject who has them.
- d) Mental states require a subjective ontology, i.e., they are necessarily a property of the sentient subjects who have them. There are no “free-floating” mental states.
- e) Mental states lack crucial features (e.g., spatial extent, location, a structure composed of parts) that characterize physical states, and they generally cannot be described using physical language.
The mind-body problem (MBP) is thus first an ontological question: are the mental and the physical two distinct realms, or is one type a subclass of the other? But secondly, it is also a causal question: how, if at all, do the mental and the physical interact?
Answers to the causal question are determined by the answers to the ontological question. The latter admits of two basic options: either there is only one type of thing/property (monism), or there are two such types (dualism). We shall look at the most important monist theory, physicalism, as well as at two dualist views.
Physicalism is mostly propounded as the thesis that physical events can only have physical causes (the “causal closure of the physical”). This means that if one allows the mental to be of an ontologically different kind than the physical, it cannot be causally efficacious; a price many physicalists consider too steep (see Fodor’s introductory quote). To ensure mental causation, they therefore take mental events to be physical events which fulfill certain functions (like protecting the body from harm by making the hand retract upon touching a hot stove). Physicalism thus attributes a causal role to the mental, but only at the expense of making the mental physical.
The problem is that this, rather than explaining the mental, explains it away. Take phenomenal consciousness, the “what it is like to be” in a mental state, e.g., feeling a pain. These feelings are so-called qualia. Physicalists claim that qualia are just functions, realized in the matter of our brains; thus the feeling of pain is the function of self-protection (e.g., causing hand retraction). But this does not explain the phenomenal aspect of pain. Property dualist David Chalmers argues that the exact same functions, with the exact same physical processes, could be at work in a physical duplicate of him, albeit without consciousness. If it is logically possible that there are such zombie duplicates of people, then the qualia I have must be something other than physical properties.
A second line of argument asks whether rationality can be wholly explained in terms of physical processes. We are rational whenever we think or act for good reasons. Reasons are, of course, mental events which relate logically or rationally to one another (like “A because of B”). The problem is that physical events have no such relationship at all. There is only causality (“B is the cause of A”), governed by the laws of nature. If our mental processes are nothing but physical brain processes, then what guarantees our rationality? 
Contra property dualism
Property dualism (PD) assumes that the world fundamentally contains mental properties alongside physical properties, but no mental entities. PD can thus explain qualia: they are irreducible mental properties.
Still, PD has some ultimately fatal problems:
- Mental properties become causally irrelevant because property dualists typically hold to the causal closure of the physical, and because mental properties are usually considered to depend for their existence on physical properties that do all the causal work.
- The firm tying of mental properties to physical properties again raises the question of the validity of rational thought. If physics/biology does all the causal work, and the mind follows its movements like a shadow follows a body, where are the distinctively rational relations between thoughts?
- If one reintroduces sui generis mental properties, one must consequently reintroduce a subjective ontology: there must be a subject which has these properties. That this subject is physical seems implausible for several reasons:
- Our consciousness as subjects is an indivisible unity. But physical objects are divisible. It is unclear how a divisible subject can have a unified consciousness.
- We are self-reflexive, i.e., we can ourselves be objects of our thinking. This does not sit well with a physical ontology of the self, because, insofar as we grasp the essence of our self, it does not seem to be physical.
- We are free as subjects—or, ultimately as persons. Matter, however, is subject to the laws of nature which do not admit of freedom in the relevant sense.
These considerations show that the question about the mind is ultimately a question about persons. It is persons who have minds, and hence mental properties. What we therefore need in the context of the MBP is an adequate ontology of the human person.
Pro substance dualism
Just such an ontology is offered by substance dualism (SD). Here the human person is a soul (a mental entity) and has a body. On SD, consciousness is unified, because it is the consciousness of a simple substance, the soul. Likewise, the freedom of the person is assured (because the soul is not governed by the laws of nature), as is the causal efficacy of the mental. Self-reflexivity also becomes more intelligible, as it stands to reason that the object of a reflection in which the object appears as mental actually is mental. Finally, rationality is assured: the rational aspect of rational thought is entirely located in the soul. The brain may be needed to get those thought processes going, but it does not provide rationality.
The weightiest objection against SD is the so-called interaction problem: How can an immaterial soul move the material body? Christians at any rate should have no problem with such causality. Has not God, the ultimate Spirit (John 4:24), often intervened in the physical world? Second, are we certain of having understood the nature of physical causality? What ensures that masses, or opposing charges, attract each other? There is no logical necessity in this. It could be otherwise. By contrast, we know with great certainty (namely, introspectively) that we mentally cause things in the physical world.
Then there is the question of the laws of nature. If souls can interact with bodies, don't they break natural laws? The most popular version of this objection is the argument from conservation laws: if souls could interact with brains, they would violate the conservation of energy and/or momentum, and since that cannot be, there can be no interaction. But the objection is based on a misunderstanding of conservation laws. These are not formulated categorically, but biconditionally: if and only if certain symmetries of a physical system are satisfied (the Noether symmetries),then energy/momentum are conserved; if not, then not.
This is not to say that SD has no difficulties of its own. But at least it explains the “mind” part in “mind-body problem” adequately.
 Fodor, Jerry A. 1983. “Making Mind Matter More.” In Mental Causation, edited by John Heil and Alfred R. Mele. Oxford: Clarendon Press.
 See Chalmers, David. 1996. The Conscious Mind. New York: Oxford University Press.
 C. S. Lewis famously made this point in his Miracles (Revised Edition 1960, New York: Macmillan.)
 See Moreland, J. P. 2018. “Substance Dualism and the Unity of Consciousness.” In The Blackwell Companion to Substance Dualism, edited by Jonathan J. Loose and Angus J. L. Menuge. Hoboken, NJ: Wiley Blackwell.
 Avicenna uses this reasoning in his famous “floating man” argument (see also Adamson, Peter, and Fedor Benevich. 2018. “The Thought Experimental Method: Avicenna’s Flying Man Argument.” Journal of the American Philosophical Association 4 (2): 147–64.
— Alin Christoph Cucu (PhD, University of Lausanne) is a lecturer and postdoctoral researcher in philosophy at the University of Lausanne, Switzerland. He is the author of a PhD thesis as well as several peer-reviewed papers on mind-body-interaction, especially its scientific aspects, and co-editor of an anthology with the title “Religious Hypotheses and the Study of Human Nature” (in Romanian, Institutul European, forthcoming). His co-authored paper “How Dualists Should (Not) Respond to the Objection From Energy Conservation” (with J. Brian Pitts) was awarded the Best Paper Prize of the Mind and Matter Society. In recent years he won three Templeton grants for research projects at the intersection of theology, philosophy and science. You can follow his work at his newsletter, Pensees.