How can the mind causally influence a world that is, ultimately, made up of physical stuff? This is one way of asking the mental causation question, where mental causation is the type of causation in which either the cause or effect is a mental event or property. The question can also be put this way: How can mental events or properties (such as beliefs, desires, sensations, and so on) cause other events? Discussion of the mental causation question dates back to at least Princes Elizabeth of Bohemia’s challenge to Descartes, who took the mind to be a non-physical substance. Elizabeth’s question to Descartes was how one can make sense of the idea that the mind could move the body, or the body could influence the mind, if they are two distinct substances as such.
We take mental causation to be real. The reality of mental causation is so central to our philosophical thinking that the view that there is no such thing as mental causation, namely epiphenomenalism, has a crucial dialectical role in philosophical argumentation in metaphysics of mind. As with Elizabeth’s criticism of Descartes, sometimes views in the metaphysics of mind are evaluated on this basis. In terms of their roles in philosophical argumentation, I find epiphenomenalism and radical scepticism to be very similar. In epistemology, radical scepticism is the view that there is no such thing as knowledge of the external world. Although pretty much everyone takes radical scepticism to be false, some epistemologists still devote time to showing why this is the case, as a view’s implication of radical scepticism is taken to be reason enough to dispense with it. Likewise in metaphysics of mind, nearly everyone thinks that epiphenomenalism is false, but there is a very sizable literature trying to show how this is so. For this reason, we often find charges of epiphenomenalism in reductio arguments.
Although there may have been ways of tackling Princess Elizabeth’s challenge to Descartes, the difficulty of doing so moved many contemporary philosophers towards an ontologically physicalist view according to which, at least in the actual world, there are only physical substances. Now, once we get rid of all non-physical substances from our ontology (substance physicalism) and yet still hold on to the existence of minds (realism about the mind), the next set of questions is: What should we do with the properties of such minds? What are mental properties? Can mental properties be reduced to physical properties?
For the sake of brevity, I shall not recite the reasons why such a reduction cannot be maintained, so let’s just assume that mental properties are not physical properties. (For seminal work on this point, see Putnam 1967.) In a world with purely physical substances, some of which have irreducibly mental properties, it might look as if the mental causation question can be answered easily. Mental events can cause physical events (or vice versa); such a causal relation doesn’t require the interaction of physical and non-physical substances, so the problem of causal interaction evaporates.
Emergentism is a view, or rather a group of views, according to which substance physicalism is true and mental properties are irreducibly mental. There are (at least) two varieties of emergentism. The weak variety, which sometimes goes by the name “non-reductive physicalism”, takes mental properties to be realized by physical properties. (For my work on what it is for a property to be realized by another property, see Baysan 2015). The strong variety, which goes by the name (surprise surprise!) “strong emergentism”, holds that (at least some) mental properties are as fundamental as physical properties to the extent that they need not be realised by physical properties. (See Barnes 2012 for an account of strong emergence along these lines. For joint discussions of weak and strong emergence, see Chalmers 2006 and Wilson 2015.)
Some contemporary metaphysicians of mind, most notably Jaegwon Kim (2005), think that epiphenomenalism is still a threat to emergentism. It is thought be a problem for the weak, non-reductive physicalist, variety because of the following line of thought. The physical world is supposed to be causally closed in the sense that if a physical event has a cause at any time, then at that time, it has a sufficient physical cause. Thus, if a physical event is caused by a mental event (or a property), it must be fully caused by a physical event (or a property) too. If all this is true, then every physical event that has a mental cause must be causally overdetermined. (Here, the idea is that causation implies determination, and having more than one fully sufficient cause implies overdetermination.) The acceptance of such systematic causal overdetermination is taken to be absurd; the world can’t have that much redundant causation. Therefore, the combination of non-reductive physicalism and the reality mental causation is not tenable. That is the charge anyway.
Now, what about strong emergentism? In a nutshell, defenders of this view can reject the idea that the physical domain is causally closed in the way that non-reductive physicalists typically assume. Given its anti-physicalist assumption that some properties other than the physical ones can be fundamental too, rejecting the causal closure principle is definitely a live option for strong emergentism. However, according to some, that is precisely the problem with this view. From a scientific or naturalistic point of view, how can we defend such a view if its best way of accommodating mental causation is through rejecting the causal closure of the physical domain?
The picture that I have portrayed thus far seems to suggest that unless we go all the way and reduce mental properties to physical properties, there isn’t any room for mental causation. This is what Kim and others have been trying to persuade us over the years. But, is the reasoning that has led us here really solid? Should all of the argumentative steps briefly sketched above be accepted? I have some doubts.
First, there is an emerging (pun intended) consensus that the causal argument against non-reductive physicalism sketched above has some flaws. Some philosophers aren’t convinced that non-reductive physicalism, as Kim portrays it, really implies causal overdetermination (see Yablo 1992 for a seminal account). Very roughly, the idea is that such causal overdetermination appears to obtain when a whole event and its parts are causing an event too. But taking a whole and its parts separately is surely “double counting”. Also, in presentations of the causal argument against non-reductive physicalism, we often come across the idea that if an event has two distinct sufficient causes, it must be genuinely causally overdetermined—this is known as “the exclusion principle”. But a principle with such a crucial dialectical role needs some backing-up, and some authors have noted that there doesn’t seem to be any positive argument for the truth of the exclusion principle. (For a criticism along these lines, see Arnadottir and Crane 2013).
Second, the reason to resist strong emergentism that is sketched above can be questioned too. Do we really have good reasons to think that the physical domain is causally closed? I don’t think that we can play the causal closure card unless we carefully study the reasons that are given in favour of it. Considering its importance in argumentation in the metaphysics of mind, it would be fair to say that there hasn’t been enough attention given to the positive reasons for holding it. I am aware of three arguments for the causal closure principle: (1) Lycan’s (1987) argument that it is absurd to think that laws of conservation hold everywhere in the universe with the exception of the human skull; (2) McLaughlin’s (1992) suggestion that the failure of the causal closure principle was a scientific hypothesis in chemistry which was eventually falsified (in chemistry!); and (3) Papineau’s (2002) argument that the principle is inductively verified by the practice of 20th century physiologists. This is not the place to carefully examine these three arguments in detail, but I think it is fair to say that these arguments don’t even attempt to be conclusive. The closure principle may turn out to be true, but whether that is the case or not will be an empirical matter of fact, and until we somehow established it empirically, we need to devise more solid philosophical arguments for it.
I hope this short discussion has persuaded you that whichever view in the metaphysics of mind turns out to be true, the mental causation question will play some role in determining its plausibility.
Árnadóttir, S. and Crane, T. (2013). ‘There is no Exclusion Problem’, in Mental Causation and Ontology, eds. S. C. Gibb, E. J. Lowe, and R. D. Ingthorsson (Oxford: Oxford University Press).
Barnes, E. (2012). ‘Emergence and Fundamentality’. Mind, 121, pp. 873–901.
Baysan, U. (2015) ‘Realization Relations in Metaphysics’, Minds and Machines 25, pp. 247–60.
Chalmers, D. (2006). ‘Strong and Weak Emergence’, in The Re-Emergence of Emergence , eds. P. Clayton & P. Davies (Oxford: Oxford University Press).
Kim, J. (2005) Physicalism or Something Near Enough. Princeton, NJ: Princeton University Press.
Lycan, W. (1987). Consciousness. Cambridge, MA: MIT Press.
McLaughlin, B. (1992). ‘The Rise and Fall of British Emergentism’, in Emergence or Reduction?: Prospects for Nonreductive Physicalism, eds. A. Beckermann, H. & J. Kim (De Gruyter).
Papineau, D. (2002). Thinking about Consciousness. Oxford: Oxford University Press.
Putnam, H. (1967). ‘Psychological Predicates’, in Art, Mind, and Religion, eds. W.H. Capitan & D.D. Merrill (Pittsburgh: University of Pittsburgh Press).
Wilson, J. (2015). ‘Metaphysical Emergence: Weak and Strong’, in Metaphysics in Contemporary Physics: Poznan Studies in the Philosophy of the Sciences and the Humanities, eds. T. Bigaj and C. Wuthrich (Leiden: Brill).
Yablo, S. (1992) ‘Mental Causation’. Philosophical Review, 101, pp. 245–280.