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Non-Reductive Physicalism and the Problem of Mental Causation

Internationalisation Fellowship | 06/12/2016

I am philosopher specializing in the metaphysics of mind and science. In this paper, I present one of the central questions in contemporary philosophy of mind: The prospects for a position known as non-reductive physicalism. Roughly, non-reductive physicalism holds that everything is either physical or strongly dependent on the physical (this is the physicalist part of the view), but that there are psychological or mental entities (e.g. beliefs and desires), which are irreducible to physical entities (this is the non-reductive part of the view). A key obstacle to such a position comes in the form of a so-called causal exclusion argument. According to this argument, non-reductive physicalists cannot account for the causal efficacy of mental states and properties, since all the causal work needed to produce behaviour is caused by underlying physical states and properties. My current research project, which is being carried out at the University of Durham and is financially supported by the Carlsberg Foundation, seeks to explore various solutions to this problem.

Physicalism and Reductionism in Philosophy of Mind

In the broadest sense, physicalism is the view that everything not physical depends on the physical in some strong sense. Philosophers like to illustrate this using a thought experiment: Imagine that God is creating our world. If physicalism is true, all God needs to do is create the world’s physical entities, and everything else (chemistry, biology, psychology) automatically comes along for the ride. In the philosophy of mind, the view is typically framed in terms of mental and neural states. The idea is that mental states, such as being in pain or believing that Paris is in France, are strongly dependent on properties of the brain such as having certain neural activity. Put another way: If a person is in pain, he is in pain in virtue of being in a certain neural state.

Non-reductive physicalism about the mind is the view that mental states and properties are dependent on but not reducible to physical states and properties.

Physicalism is a popular view in contemporary philosophy of science. It is motivated by the success of physics, both as far as providing explanations of physical phenomena, but also as far as providing explanations of phenomena in the special sciences. One question that concerns physicalists is whether their view is a reductive one. In the philosophy of mind, the question then is, whether a person’s mental states just are neural states. Philosophers who say yes are reductive physicalists. According to them, to be in pain is nothing more than being in a particular neural state. Reduction provides a straightforward explanation to why physicalism is true (if indeed it is true). Why does God’s work end when he has created all the world’s physical entities? Because everything is physical. Nevertheless, many physicalists are convinced that reduction is untenable. For one thing, it seems that mental states can be shared among individuals who differ greatly with respect to their physical or neural make-up. Dolphins can presumably be in pain, yet they likely share no neural state with a human being in pain. If so, being in pain cannot be identical to any particular neural state. Such considerations have led many philosophers to hold on to their physicalist commitments, but give up on the idea that mental states are reducible to physical states. Such a view is known as non-reductive physicalism.

“If it isn't literally true that my wanting is causally responsible for my reaching, and my itching is causally responsible for my scratching, and my believing is causally responsible for my saying.

…If none of that is literally true, then practically everything I believe about anything is false and it's the end of the world.”
(Jerry Fodor 1990, 156)

The Causal Exclusion Problem and the Threat of Epiphenomenalism

As noted, physicalism about the mind is the view that mental states are dependent on physical (and presumably neural) states. In addition to this commitment, physicalists are keen to endorse the causal completeness of physics. This is the claim that every physical effect has a sufficient physical cause. So, for instance, the physical event of my arm reaching for a glass of water is sufficiently caused by some purely physical prior state, presumably some neural state of my brain. This appears to create a problem for non-reductive physicalists. Recall, according to this view, that mental states are irreducible to any physical state. How, then, can mental states cause physical effects? If every physical effect has a sufficient physical cause, it appears that they can do so only by causally overdetermining their physical effects. That is, physical behaviour is caused by both some physical state and some mental state.

Accounting for the causal efficacy of mental states within a non-reductive physicalist framework requires that we play close attention to the metaphysics of the relation between mental and physical states.

However, this is a very odd view, as it leaves mental states causally redundant. This raises the question of why we should think mental states are causes at all. After all, they seem to make no causal difference in the world. Instead, they appear to be excluded from causal relevance by physical states. This argument is known as the causal exclusion argument. The conclusion is that non-reductive physicalism ultimately leads to epiphenomenalism – the view that mental states are causally impotent. Most philosophers find this an unpalatable view. For such philosophers, it seems, non-reductive physicalism is therefore not an option.

“[The problem of mental causation] is the problem of showing how mental causation is possible, not whether it is possible […]”

(Jaegwon Kim 1998, 61)

My Project and the Support from the Carlsberg Foundation

My current research project (which is supported financially by the Carlsberg Foundation) seeks to explore various solutions to the exclusion problem. According to one solution (which I defend in Christensen 2014), the causal powers of any mental state are included in the causal powers of some physical state. This view allows for the causal efficacy of mental states (they really do have causal powers), but avoids dubious “double causings” of physical behaviour (physical behaviour is the manifestation of just a single causal power, had by both a mental and a physical property). During my project, I have been working on responses to some recent objections to this solution.

Another approach focuses on the nature of causation. It holds, roughly, that construing causation as a matter of counterfactual dependency allows for autonomous causal levels – i.e. mental states cause mental effects while physical states cause physical effects. While I have defended such an approach in an earlier paper (Christensen and Kallestrup 2012), I am currently working on a paper, which highlights some limitations to this approach. 

My Carlsberg Foundation supported project is part of a larger research project in which I investigate the metaphysics of so-called inter-level relations, i.e. relations between various scientific domains, especially physics and special science. The support from the Carlsberg Foundation has enabled me to be part of a major international research centre on the metaphysics of science at the University of Durham (the Durham Emergence Project). This has presented a unique possibility to engage with world leading philosophers and scientists, and the support is directly responsible for one research output under review, and another, which is in its final stages of completion. While my research is largely theoretical, questions about reduction and the possibility of mental and other higher-level or specific scientific causal relations has real-life implications. If psychology and other special sciences are causally redundant, distinctive practical benefits of such sciences are unclear. This, in turn, would be directly relevant with respect to funding allocations and research strategic initiatives. My project holds that such a conclusion is premature and that there are ways to provide a theoretical foundation for the autonomy of special science.


Christensen, J. & Kallestrup, J. [2012]. ‘Counterfactuals and Downward Causation – a Reply to Zhong’. Analysis 72:3, 513-517

Christensen, J. [2014]. ‘Determinable Properties and Overdetermination of Causal Powers’. Philosophia 42:3, 695-711

Fodor, J. [1990]. A Theory of Content and Other Essays, Cambridge, Mass, Bradford Book/MIT Press.

Kim, J. [1998]. Mind in a Physical World. MIT Press