Neuroscience and Free Will
Consider the following passage from Ian McEwan’s novel Atonement. It concerns one of the novel’s characters (Briony) as she philosophically reflects on the mystery of human action:
She raised one hand and flexed its fingers and wondered, as she had sometimes done before, how this thing, this machine for gripping, this fleshy spider on the end of her arm, came to be hers, entirely at her command. Or did it have some little life of its own? She bent her finger and straightened it. The mystery was in the instant before it moved, the dividing moment between not moving and moving, when her intention took effect. It was like a wave breaking. If she could only find herself at the crest, she thought, she might find the secret of herself, that part of her that was really in charge.
Is Briony’s quest forlorn? Will she ever find herself at the crest of the wave? The contemporary scientific understanding of human action seems to cast this into some doubt. A variety of studies in the neuroscience of action paint an increasingly mechanistic and subconscious picture of human behaviour. According to these studies, our behaviour is not the product of our intentions or desires or anything like that. It is the product of our neural networks and systems, a complex soup of electrochemical interactions, oftentimes operating beneath our conscious awareness. In other words, our brains control our actions; our selves (in the philosophically important sense of the word ‘self’) do not. This discovery — that our brains ‘make us do it’ and that ‘we’ don’t — is thought to have a number of significant social implications, particularly for our practices of blame and punishment.
Or so a popular line of argument goes. Is this line of argument any good? Christian List and Peter Menzies’s article, ‘My brain made me do it: The exclusion argument against free will and what’s wrong with it’, claims that it is not. In this two-part series, I want to closely examine their arguments. Although I sympathise with parts of their critique, I think their attempt to apply this critique to the recent debates about neuroscience and responsibility are somewhat misleading. I’ll explain why I think this in part two. For the remainder of this part, I’ll focus on their primary argument.
1. The Challenge from Physicalism and Neurosicence
What does it take to be free? Two conditions are said to be important. The first is the alternativism condition, according to which we must be capable of doing otherwise in order for actions to be free. The second is the sourcehood condition, according to which we must be the source of our action in order for it to be the product of our free will. Both conditions are threatened by popular philosophical theses. The thesis of determinism threatens the alternativism condition, and the thesis of physicalism threatens the source-hood condition.
We could talk about the impact of determinism on the alternativism condition, but we won’t. Instead, we will focus on the impact of physicalism on the source-hood condition. In particular, we will focus on what List and Menzies call the ‘exclusion argument’ against free will. The main substance of their article is directed towards this argument, so we need to understand it if we are to understand the article. The argument works a little something like this (note: the numbering of the premises does not follow the numbering in List and Menzies article — this might make cross-comparison a little awkward):
- (1) Someone’s action is free only if it is caused by the agent, particularly by the agent’s mental states, as distinct from the physical states of the agent’s brain and body (call this the ‘Causal Source Thesis’)
- (2) Physicalism rules out any agential or mental causation, as distinct from causation by physical states of the agent’s brain and body (call this the ‘Purported Implication of Physicalism’)
- (3) Therefore, there can be no free actions in a physicalist world (call this the ‘Source-Incompatibilist Conclusion’)
The argument is a little underwhelming at first glance. Although we might be inclined to accept premise (1), premise (2) is going to be unconvincing to many physicalists. They will accept that the mental and physical are one and the same thing: that mental states are constituted by particular patterns of brain states, but they will deny the implication that this rules out agential causation. They will just say that, provided the actions are caused by the right kinds of brain states (i.e. the ones that constitute the right kinds of mental states), there is agential causation and hence the source-hood condition is satisfied. It does not matter that there is no ‘distinct’ class of mental causation.
This is where the exclusion argument comes into play. The exclusion argument derives from the work of Jaegwon Kim, a famous proponent of physicalism. Kim argues that physicalism entails mental supervenience (i.e. the mental supervenes upon the physical), and that mental supervenience entails epiphenomenalism (i.e. that the mental has no real causal role in our actions). This means that there is no mental causation on physicalism, which means that premise (2) is sound.
As I mentioned above, List and Menzies direct most of their critique against this exclusion argument. They identify two variations upon the argument, and argue that both rely on a mistaken understanding of agential causation. Once the correct account of agential causation is substituted-in, the argument becomes less plausible. There is, consequently, no reason to suspect that physicalism rules out mental causation of the appropriate kind. List and Menzies also try to argue that something very much akin to the exclusion argument underlies much of the current ‘my brain made me do it’ rhetoric in the neuroscience community. Consider Sam Harris’s statement, from his 2012 book Free Will:
‘Did I consciously choose coffee over tea? No. The choice was made for me by events in my brain that I, as the conscious witness of my thoughts and actions, could not inspect or influence’
There is something exclusion-argument-esque about this, for sure. But, although I’m inclined to agree with List and Menzies in their critique of the physicalist challenge to sourcehood, I’m less inclined to agree with them about the neuroscientific challenge. I’ll get to that in the next post.
2. Two Versions of the Exclusion Argument
Before we do anything else, we need to gain a deeper understanding of the exclusion argument. List and Menzies maintain that this argument comes in two major forms. The first, simpler form, relies on a straightforward physicalist causal closure principle (i.e. on a principle claiming that the physical world is causally closed: physical causes are sufficient for all physical effects). This will be familiar to anyone who has debated the merits of Cartesian dualism vis-a-vis physicalism. The second, more complex form, relies on slightly more general claim about the nature of causation and causal sufficiency.
The first version of the argument works like this:
- (4) An agent’s action is free only if it is caused (in a relevant sense of causation simpliciter) by the agent’s mental states.
- (5) Any effect that has a cause has a sufficient physical cause (i.e. a causally sufficient physical condition) occurring at the same time.
- (6) An agent’s mental states are not identical to any physical states, but rather supervene on underlying physical states.
- (7) If an effect has a sufficient cause C, it does not have any cause C* (simpliciter) distinct from C, occurring at the same time (except in cases of overdetermination).
- (8) Therefore, there are no free actions.
The second version of the argument simply changes premise (5) to the following:
- (5*) Causation implies causal sufficiency.
The conclusion then follows in the same manner, provided you also accept this lemma:
Lemma: If C* is causally sufficient for some effect E, and C* supervenes on C, then C is causally sufficient for E.
This lemma is easily proved because the supervenience relationship is a necessary one. In other words, if C* supervenes on C, then whenever C is present, so too is C*. It follows then that if C* is sufficient for E, then C is also sufficient for E. If you are confused, see my previous post on the nature of the supervenience relationship.
List and Menzies are at pains to point out that most of the premises of both versions of the argument are plausible. I won’t explore the matter in quite the same detail as they do, but I will give a quick run-down of the salient points.
I’ll start with premise (4). This premise looks to be a pretty uncontroversial statement of the sourcehood condition: in order to freely will an action you (your mental agency) must be the source of that action. This premise should be acceptable to most people, irrespective of their philosophical worldview.
Premises (5) and (5*) are slightly more controversial, but still highly plausible. Premise (5) simply states a standard physicalist account of causal closure. It is also quite weak in its claims. It states only that if an event has a cause, then physical causes are sufficient to produce that event. This is consistent with the existence of some non-physical events with no causes. It should, consequently, be acceptable to virtually all physicalists. Premise (5*) is even more relaxed in its claims. It doesn’t appeal to physicalism at all. It states that if an event C causes an event E, then C is causally sufficient for E. This is potentially compatible with all versions of causal determinism. The premise could also be refined so as to incorporate a probabilistic version of causation. Still, despite its more relaxed nature, there is something worth disputing. Everything depends on how you understand the concepts of causation and causal sufficiency. List and Menzies think that an incorrect understanding of both concepts permeates the exclusion argument. We will return to this problem below.
Premise (6) requires some commitment to non-reductive physicalism. That is, to the view that mental states depend on (supervene on) physical states but are not identical or reducible to them. This, of course, means that reductive physicalists and non-physicalists have a route out of the argument. That’s to be expected. But it is worth noting that non-reductive physicalism has tended to be the dominant position in the philosophy of mind for the past century or so. It is also the view that seems most at home with a scientifically oriented worldview, which is the sort of worldview shared by List and Menzies, and the neurosceptics.
That leaves us with premise (7). This is the most problematic one, according to List and Menzies, because it assumes an incorrect theory of causation.
3. A Difference-Making Account of Causation
Let’s try to unpack their critique in more detail. There are two main types of causation:
Production-Causation: This is a metaphysical account of causation according to which causes produce effects via some metaphysical source. As List and Menzies describe it ‘[c]ausation here involves a causal ‘oomph’, i.e. the production of an outcome through some causal force or power’ (List and Menzies 2014).
Difference-Making Causation: This is a probabilistic or counterfactual theory of causation. It says that to be the cause of an effect is to make some sort of difference to the occurence of that effect across possible worlds. More precisely, it holds that C causes E if, and only if, two conditionals are satisfied:
The Positive Conditional: If C were to occur, then E would occur.
The Negative Conditional: If C were not to occur, then E would not occur.
List and Menzies argue that the difference-making account is much more consistent with the scientific worldview. The kinds of experimental evidence of causation that scientists discover usually involve playing around with the conditionals in the manner envisaged by the difference-making account (e.g. the randomised placebo-controlled trial in medicine). Furthermore, the production account seems to require a metaphysical ‘leap of faith’.
In addition to this, they argue that the difference-making account is the most natural way to understand agential causation. In other words, to say that an agent mentally causes an event is to say that the agent (and the relevant mental states) made a difference to that event. When the relevant mental state is present, so too is the effect, and when it is not, neither is the effect.
The crucial thing about the difference-making account of causation is that it casts premise (7) into doubt. This is because the difference-making account allows for cases in which certain microphysical states might be the production-causes of an event; but higher-level, supervenient events, might be the difference-making causes of the event. Here’s an example. Suppose you have a flask of boiling water that breaks because of the pressure inside. The movements of the particles (or some subset of particles) within the flask might be causally sufficient for the break. These microstates would then be the production causes of the event. But it is the boiling of the water (which supervenes on various microstates) that is the difference-maker. It satisfies the positive and negative conditionals. As List and Menzies point out:
If the boiling had occurred, but had been realized by a slightly different microstate, the flask would still have broken, and if the boiling had not occurred, the flask would have remained intact…Although it is true that if the microstate in the flask had been exactly as it was, the flask would be broken, it is not true that if the microstate had been slightly different, the flask would have remained intact. The boiling could have been realized in many different ways, through different configurations of molecular motion, and would still have led the flask to break.
In other words, the boiling is supervenient upon the underlying microstates, but it ismultiply realisable by those microstates. This means that it (not the microstates) is the true difference-maker. The same thing could then hold true for mental causation. Mental states could be multiply realisable. Different physical states of the brain could give rise to the same mental event. Where those different physical states give rise to the same event, we can say that the supervenient mental state is the true difference-maker. The result is that the exclusion argument fails: if we adopt a difference-making account of causation, there is no reason to think that physicalism rules out the appropriate style of mental causation.
I’m broadly in agreement with this line of argument, though I would note that much depends here on how fine-grained or coarse-grained we are in our understanding of what constitutes a common or distinct event or mental state. Daniel Dennett’s paper ‘Real Patterns’ is quite good on this topic, for those of you who are interested.
To briefly recap, the exclusion argument claims that physicalism rules out free will because, on physicalism, we are not the sources of our actions. But, as we have just seen, this argument assumes an implausible theory of mental causation. If we adopt a difference-making account, then there is no reason why supervenient mental states cannot count as the causes of our actions. How does this affect the debate about neuroscience and free will?
Discoveries in neuroscience, and the science of behaviour more generally, pose a challenge to the existence of free will. But this all depends on what is meant by ‘free will’. The term means different things to different people. Philosophers focus on two conditions that seem to be necessary for free will: (i) the alternativism condition, according to which having free will requires the ability to do otherwise; and (ii) the sourcehood condition, according to which having free will requires that you (your ‘self’) be the source of your actions. A scientific and deterministic worldview is often said to threaten the first condition. Does it also threaten the second?
That is what Christian List and Peter Menzies article “My brain made me do it: The exclusion argument against free will and what’s wrong with it” tries to figure out. As you might guess from the title, the authors think that the scientific worldview, in particular the advances in neuroscience, do not necessarily threaten the source-hood condition. I discussed their main argument in the previous post. To briefly recap, they critiqued an argument from physicalism against free will. According to this argument, the mental states which constitute the self do not cause our behaviour because they are epiphenomenal: they supervene on the physical brain states that do all the causal work. List and Menzies disputed this by appealing to a difference-making account of causation. This allowed for the possibility of mental states causing behaviour (being the ‘difference makers’) even if they were supervenient upon underlying physical states.
If that seems at all confusing, that’s because the remainder of this post switches the focus from physicalism (a philosophical doctrine) to findings from contemporary neuroscience. List and Menzies argue that many modern day neuroscientists are sceptical about free will. But their neuroscepticism exhibits the same flaw that they found in the exclusion argument. I’m not so sure about this. I’m going to try to explain why.
1. List and Menzies’ Interpretation of the Neurosceptical Argument
To start things off, I need to explain how List and Menzies’ understand the neurosceptical position. A paradigmatic statement of neuroscepticism can be found in Sam Harris’s bookFree Will. I quoted this in the previous post, but it is worth repeating here:
’Did I consciously choose coffee over tea? No. The choice was made for me by events in my brain that I, as the conscious witness of my thoughts and actions, could not inspect or influence’
List and Menzies use Harris’s work as their main scratching post in their paper, highlighting another section of the book where he refers to free will as an ‘illusion’ because ‘thoughts and intentions emerge from background causes of which we are unaware and over which we exert no conscious control’ (Harris 2012, 12).
Using quotes of this sort as their source, the authors argue that Harris and other neurosceptics rely on the following (oftentimes implicit) argument (numbering continues on from part one):
- (9) If an agent’s choices and actions are wholly caused by neural states and processes that are inaccessible to his or her consciousness, then these choices and actions are not free.
- (10) Human choices and actions are wholly caused by neural states and processes that are inaccessible to the agent’s consciousness.
- (11) Therefore, human choices and actions are not free.
Let’s consider how one might defend the two main premises of this argument.
The first premise states a condition for free will. It holds that causation by consciously accessible mental states is essential to free will. This is very similar to the sourcehood condition that I mentioned above. It is also very similar to the principle used to motivate the exclusion argument that was discussed in part one. If you recall, the opening premise of that argument stated that an agent’s mental states had to cause their behaviour in order for it to be free. The only real difference is that this new argument demands that the mental states be consciously accessible. I’m guessing this means that the agent’s conscious mental states must be causally responsible for their behaviour. I think this is a plausible sufficient condition for free will (or, at a minimum, for responsible behaviour), but I’m not sure if it is necessary. Neil Levy’s recent book argues that it is, but I have not read it yet. List and Menzies think it is plausible, citing some survey evidence from Eddy Nahmias suggesting that most ordinary people think that premise (9) is correct.
Premise (10) is where the discoveries in neuroscience come into play. Neurosceptics typically appeal to a widely-known set of evidence suggesting that our behaviour is caused by neural events that are largely beneath or outside our conscious awareness. I’ll mention three such sources of evidence here. This is for illustrative purposes only; it is not intended to be exhaustive. First, there are Benjamin Libet’s famous studies on intention and behaviour. By getting people to perform simple actions and recording associated brainwaves, Libet’s studies found that the conscious intention to act post-dated the neural causation of the action by nearly half-a-second. These studies have been scrutinised and challenged over the years (I always enjoyed Dennett’s discussion of them). Second, there are the more recent studies by the likes of Haggard and Haynes which seem to confirm and extend Libet’s results. These studies suggest that neural causes precede conscious awareness by even longer periods of time, perhaps by up to 10 seconds. Third, and finally, there is the work of Daniel Wegner, particularly the work found in his book The Illusion Conscious Will, which brings together a diverse set of studies, all pointing to the same conclusion: that the conscious will does not direct or control our behaviour. Rather, our consciousness confabulates a mental cause of our behaviour after the fact. This evidence all confirms Harris’s view that we are mere ‘conscious witness[es]’ to the true, underlying, neural causes of our actions.
If this is all correct, then the neurosceptical position is confirmed.
2. List and Menzies’ Critique of Neuroscepticism
But, obviously, List and Menzies do not see it that way. They argue that the neurosceptics make the same mistake as the physicalists. This is unsurprising since most neurosceptics are resolute physicalists, but it is worth going through the mistake to see exactly how it applies to the neurosceptical position. The main flaw comes with the motivating principle stated in premise (9). This premise appears to claim that the existence of a sufficient neural cause rules out the existence of a mental cause. But this is wrong. Just as the physicalists mistakenly assumed that sufficient lower-level physical causes ruled out higher-level mental causes, so too do the neurosceptics mistakenly assume that sufficient lower-level neurological causes rule out higher-level mental ones.
To be more precise, the mistake lies in the assumption that a sufficient neurological cause rules out a higher difference-making cause. It could well be that for every single action there is a sufficient neurological cause, but also a difference-making mental cause. Remember the example from the previous post. Suppose you have a flask of boiling water and the flask cracks. What is the cause of the cracking? You could attribute it to a particular arrangement of the (expanding) molecules of water, or to the act of boiling. The particular arrangement of molecules is a sufficient cause of the cracking; but the act of boiling is the difference-maker. This is because boiling could have led to a different arrangement of water molecules that was also sufficient for cracking. It is the true-difference maker because its presence or absence makes a difference to the outcome across an appropriate set of possible worlds. The particular arrangement of water molecules does not.
The key point is that the same could be true of the relationship between sufficient neural causes and supervenient mental states. Neuroscientists might discover that a particular pattern of neuronal firing is sufficient for the act of raising one’s hand. But it is possible that the same act could be caused by a slightly different pattern of neuronal firing. The only thing shared by the two distinct patterns of neuronal firing might be a supervenient mental state (e.g. the intention to raise one’s hand). This mental state would then be the true difference-maker.
The upshot of this is that premise (9) would need to be reformulated if the neurosceptical position were to be persuasive. List and Menzies suggest the following reformulation, one that respects the difference-making account of causation (note I have amended this from their original discussion):
- (9*) If an agent’s choices and actions have a difference-making cause at the neuronal level, and they do not have any other difference-making cause at the mental level occurring at the same time, then the agent’s actions and choices are not free.
With this reformulated premise in place, the debate switches to premise (10), or rather to a suitably reformulated version of that premise. This one would claim that neuroscientific evidence points to difference-making causes at the neuronal level, not co-occurrent with difference-making mental causes. But List and Menzies reject this premise. In doing so they make two points, one conceptual and one empirical.
The conceptual point focuses on what it takes for something to be a difference-making cause. It requires the satisfaction of two counterfactual conditionals. First, there is the positive conditional ‘if C occurs, then E occurs’; then there is the negative conditional ‘if C did not occur, E would not occur’. List and Menzies’ argument is that mental causes will tend to satisfy these two conditional tests, whereas neuronal causes will not. As they put it themselves:
[W]hen we understand causation as difference-making, we are likely to conclude that the cause of an agent’s action is not the agent’s brain state, but his or her mental state. Only the supervenient mental state, but not the subvenient brain state, may satisfy the two conditionals for difference-making.
They then illustrate this conceptual point in more detail by drawing out a map of possible worlds and the different possible causes that one can identify across these possible worlds. This is supposed to show how, under a difference-making account of causation, higher level mental causes can potentially exclude lower-level neural causes. They call this their ‘downward exclusion’ result. I find this slightly redundant, however, as the map is their own construction and merely serves to repeat their main conceptual point, which is that mental states are more likely to be the difference-makers.
This brings us to their empirical point. They accept that whether neural or mental causes are the difference-makers is an empirical question. And they also accept that a suitably constructed psychological study could tell us which is the case. But they say nothing more, suggesting that they think no such study currently exists (or, at least, the current set of studies are not decisive one way or the other).
3. Criticisms and Concluding Thoughts
What are we to make of all this? As mentioned the last day, I think List and Menzies are broadly correct in their critique of the exclusion argument from physicalism. But I think they are much less persuasive in their dismissal of neuroscepticism. It is not that I disagree with them entirely, or that I am a resolute neurosceptic, it is just that their interpretation of the neurosceptical position seems remarkably uncharitable and their engagement with the empirical evidence insufficient.
Their lack of charity stems from their original formulation of premise (9). They interpret the neurosceptic as holding that the existence of neural causes excludes the existence of mental causes. Perhaps there are some neurosceptics who rely on this principle. But as best I can tell, the neurosceptical position advanced on foot of the studies by Libet, Haynes and Haggard (who List and Menzies explicitly reference) and on foot of Wegner’s work (which they do not reference) has nothing to do with the alleged sufficiency of neural causation. It has everything to do with the timing of conscious mental states and thetiming of unconscious neural events. The claim in these studies is always that the mental states come after the fact: people confabulate and reinterpret their behaviour as having a mental cause but it really doesn’t. In other words, I think the neurosceptics already embrace something akin to premise (9*). They think that neural causes (or external causes) are the difference-makers not mental states, because the neural causes precede and initiate actions, whereas the mental states do not.
This is also why I think their engagement with the empirical evidence is insufficient. At the end of the article they appeal to the possibility of psychological experimentation helping us to work out whether neural causes or mental causes are the difference makers. But it seems to me that the experimental evidence from the likes of Libet, Haynes and Wegner already helps us in this regard (maybe not directly, but certainly indirectly). For example, my interpretation of the evidence is that Libet-style experiments do not undermine the existence of difference-making mental causes because conscious mental states always seem to be required for the performance of actions in those experiments (the experimental set-up is such that the subject is primed well in advance to consciously will an action). My interpretation of the Wegner-like evidence is slightly different. You would have to read Wegner’s work to get a fuller picture (and I confess it has been several years since I read it myself). Nevertheless, it does seem to me like Wegner identifies many cases in which conscious mental states are not the different makers. For instance, patients with hemiplagias often perform actions with one side of their bodies that they subsequently deny or confabulate a reason for performing. In these cases, the hemiplagia seems to be the difference-maker, not the conscious mental state. Admittedly, these kinds of cases are exceptional as they involve some sort of pathology. The question is how far do similar causal sequences creep into our everyday lives. I am not sure.
John Danaher is an academic with interests in the philosophy of technology, religion, ethics and law. John holds a PhD specialising in the philosophy of criminal law (specifically, criminal responsibility and game theory). He formerly was a lecturer in law at Keele University, interested in technology, ethics, philosophy and law. He is currently a lecturer at the National University of Ireland, Galway (starting July 2014).
He blogs at http://