Thursday, July 30, 2015

Did my brain make me do it? Neuroscience and Free Will (1)

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 sourcehood 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 sourcehood 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 sourcehood 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’ 
(Harris 2012, 7)

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. 
(List and Menzies 2014)

In other words, the boiling is supervenient upon the underlying microstates, but it is multiply 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.

Right, that’s it for this post. 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? We’ll look into that in part two.

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