Does Quantum Mechanics allow for Free Will?

Do the laws of physics rule out free will? The neuroscientist Kevin Mitchell has argued for the following theses:

• If the laws of physics are deterministic, then free will is ruled out

• If the laws of physics are indeterministic, then this makes room for the reality of free will.

It’s a really interesting paper but I have some disagreements, which I would like to outline here.

A quick preliminary remark: there is a huge debate in philosophy as to whether or not human freedom is compatible with determinism (we can define determinism as the thesis that the initial state of the universe, in conjunction with the laws of nature, determine everything that will happen subsequently). Compatibilists believe is that free will and determinism can happily co-exist. Mitchell is well aware of this debate, and briefly motivates his commitment to incompatibilism: that view that if the universe is determined, we’re not really free. For the purposes of this blog post I will assume incompatibilism.

Given the assumption of incompatibilism, Mitchell argues that the way to avoid the clash between freedom and indeterminism is to embrace the indeterminism entailed by many interpretations of quantum mechanics. Now, many have pointed out that mere randomness isn’t what we need to make sense of freedom, and Mitchell acknowledges this. But he thinks that the absence of total determinism in fundamental physics provides some “causal slack” that mental processes at the neurophysiological level can then take advantage of:

“The inherent indeterminacy of physical systems means that any given arrangement of atoms in your brain right at this second, will not lead, inevitably, to only one possible specific subsequent state of the brain. Instead, multiple future states are possible, meaning multiple future actions are possible. The outcome is not determined merely by the positions of all the atoms, their lower-order properties of energy, charge, mass, and momentum, and the fundamental forces of physics. What then does determine the next state? What settles the matter?”

Mitchell’s answer to this posed question is the agency of the organism: physics leaves certain options open, the organism then decides from among those options.

It sounds intuitive, but I don’t think this strategy ultimately works. Even among indeterministic interpretations of quantum mechanics, although the physics doesn’t conclusively settle what will happen, it does determine the objective probability of what will result from any given physical circumstance. Although we can’t predict with certainty, say, where a given particle will be located when we make a measurement, the Born rule tells us, for any given location in the universe, precisely how likely we are to find the particle in that location. It’s not determinism, but it’s not a “free for all.”

Mitchell worries that if the physics determines what I’m going to do, then I’m not really free. But physics determining the objective probability of what I will do is no less constraining. If whether I water Susan (my Madagascan dragon tree) is really up to me – in the strong incompatibilist sense – then surely the physics can’t fix how likely it is that I will water Susan. If it’s just totally up to me, then it could go either way depending on my radically free choice.

Here’s a little thought experiment to make the point clear. Take the moment when I’m about to decide whether or not to water Susan. Let’s say the Born rule determines that there’s a 90% chance my particles will be located in the way they would be if I watered Susan and a 10% chance there’ll be located in the way that corresponds to not watering Susan (obviously this is a ludicrously over-simplistic example, but it serves to make the point). Now imagine someone duplicated me a million times and waited to see what those million physical duplicates would decide to do. The physics tells us that approximately 900,000 of the duplicates will water Susan and approximately 100,000 of them will not. If we ran the experiment many times, each time creating a million more duplicates and waiting for them to decide, the physics tells us we would get roughly the same frequencies each time. But if what happens is totally up to each duplicate – in the radical incompatibilist sense – then there ought to be no such predictable frequency. The number that do and don’t water the plant should change each time, as the radically free choices of each individual varies.

In other word, indeterministic quantum mechanics doesn’t provide the kind of causal slack Mitchell is seeking. The problem is that objective probabilities constrain our choices as much as deterministic laws. After all, a deterministic law is just a law that yields an objective probability of 1 that a given outcome will occur.

All is not lost, however. I think Mitchell is conflating two claims:

• The laws of physics are deterministic

• The universe as a whole is deterministic

How could these come apart? They come apart if the laws of physics are ceteris paribus laws, i.e. laws that tell us what will happen in the absence of other causal influences. On this interpretation of physical law (defended by my colleague the esteemed philosopher of science Nancy Cartwright) the probabilities yielded by the Born rule are the objective probability of what will occur in the absence of some other causal influence. Such other causal influences might include the kind of irreducible causal powers Mitchell believes reside at the level of neurobiology. Mitchell seems to be concerned to avoid a violation of the laws of physics. But if the laws of physics are ceteris paribus laws, then higher-level causal powers should be thought of as complementing the laws of physics rather than contradicting them. If physics basically tells us “X will happen unless there are some higher-level causal forces,” and X doesn’t happen precisely because there are higher-level causal forces, then nothing occurs that is inconsistent with physics.

It does not follow that there are no empirical challenges for a proponent of incompatibilist free will. Some philosophers have argued that we have strong empirical grounds for thinking there are no irreducible causal influences of the kind Mitchell believes in, that everything that happens at higher levels can be reduced to the causation we find in physics. But this is an empirical question about neurobiology. My point here is just that the question of whether the laws of physics are deterministic has no bearing on the question of whether the universe as a whole is deterministic, and hence no bearing on the question of whether or not we have free will.

Photo by Bianca Ackermann on Unsplash

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