Tag Archives: neural networks

Human brains have to be deterministic (though indeterminism would not give us free will anyhow)

Are human brains deterministic? That is, are the decisions that our brain makes the product of the prior state of the system (where that includes the brain itself and the sensory input into the brain), or does quantum indeterminacy lead to some level of uncaused randomness in our behaviour? I’ll argue here that our brains must be largely deterministic, prompted by being told that this view is clearly wrong.

First, I’ll presume that quantum mechanics is indeed indeterministic (thus ignoring hidden-variable and Everettian versions). But the fact that the underlying physics is indeterministic does not mean that devices built out of quantum-mechanical stuff must also be indeterministic. One can obtain a deterministic device simply by averaging over a sufficient number of low-level particle events. Indeed, that’s exactly what we do when we design computer chips. We build them to be deterministic because we want them to do what we program them to do. In principle, quantum fluctuations in a computer chip could affect its output behaviour, but in practice a minimum of ~50 electrons are involved in each chip-junction “event”, which is sufficient to average over probabilistic behaviour such that the likelihood of a quantum fluctuation changing the output is too small to be an issue, and thus the chip is effectively deterministic. Again, we build them like that because we want to control their behaviour. The same holds for all human-built technology.

There may be some instances where genuine non-deterministic randomness might be useful. An example is running a Monte-Carlo simulation (a technique, widely used in science and engineering, of computing a simulation that allows for all possibilities). But, even here, in practice, one usually uses deterministic pseudo-random-number generators, simply because our computers — despite being built on quantum mechanics — don’t actually do genuine randomness.

Our brains are also built to do a job. They are the product of a genetic recipe, a recipe that is the product of evolution. In evolutionary terms the job of a brain is to make real-time decisions based on that genetic recipe and on the local circumstances, as informed through the senses. And brains are hugely expensive in evolutionary terms, consuming 20 percent of the body’s energy and (for example) forcing big compromises in female anatomy and making childbirth dangerous.

It follows that brains could not have evolved unless they were strongly selected for (they cannot just be byproduct “spandrels”), which means they must serve the interests of the genes that specify the recipe, and that means that brain behaviour (the choices they make) must be strongly influenced by the genes. And, since those choices can be being made decades after the childhood brain develops out of the genetic recipe, it follows that there must be a deterministic chain of causation that holds over generational timescales.

To head off misunderstandings, the above is not saying that behaviour is entirely specified by genes. (Whenever anyone argues for genetics being a major factor in anything, it is often attacked as being the claim that genetics is the only factor; the reality is that everything is always a product of both genes and environment.) Obviously, the brain’s job is to make decisions reflecting the local circumstances, but how to react to particular circumstances must be strongly influenced by genes, otherwise the brain could not have evolved. Nor is this saying that the environment and stochastic factors have no effect on how the genetic recipe plays out and as the brain develops; of course they do. And nor is this saying that the brain’s neural network is static and unchanging. Of course it isn’t (memories, for example, and changes in the network). But this argument does show that there must be a deterministic chain between genes and brain behaviour that holds over multi-decade timescales. It must be the case that, in general, “that behavioural decision could have been different if, three decades ago, that gene had been different”. That includes behavioural decisions that are regarded as “free will” decisions — which presents no problem if one adopts a compatibilist interpretation of free will.

The above argument doesn’t fully rule out a minor role for genuine randomness based on quantum indeterminacy. I would guess that, were a quantum dice-throwing module within the brain of evolutionary benefit to an animal, then such a module could have evolved. But it’s hard to see why it would be evolutionarily beneficial. Just as we make technology to do what we want it to, genes will make brains that behave in ways they program for. That will hold especially for the large component of brain function that is simply about regulating body functions, not producing “behavioural choices”, and for the primitive brains producing fairly simple behaviour, such as in worms. This means that the neural-network junctions (synapses) will have evolved to be deterministic. This is achieved by a neural signal, an “action potential”, being an on–off event (so that it is insensitive to small changes), with a sufficient number of ions needed to trigger one (such that quantum indeterminacy is averaged over). This is pretty much the same way that humans make computer chips to be deterministic. Since larger and more complex brains work in a similar way, just with vastly more neurons and neural connections, it follows that they also will be deterministic.

Another point: our brains are simply too big and too complex to be about producing quantum “dice throwing” decisions. A device producing an indeterminate output would have to be small (again, averaging over anything more than a couple of dozen ions gives you a deterministic output). Yet our brains have 100 billion neurons made of 1026 molecules. What is all that for, and how did it evolve, if our decisions are effectively a dice throw? The only answer is that it evolved to process the information from our senses, and make decisions based on that, and making decisions based on input information is (by definition) a deterministic process.

Lastly, twin studies tell us that behavioural traits are highly heritable, with typical heritabilities of 50%. Again, this requires a causal chain between gene expression at the stage of brain development and behavioural choices made decades later. (There’s also a large role for the environment, of course, but being affected by environment is just as much a deterministic process as being affected by genes.)

Anyhow, I was told that I was wrong, and that quantum indeterminacy plays a role in our brains, especially when it comes to “free will”, and was pointed to a review paper arguing this by neurobiologist Björn Brembs of the Universität Regensburg.

Obviously a professor of neurobiology knows more about brains than I do, but, for now at least, I’m sticking to the above arguments. So what counters does Professor Brembs give? The paper first points out that the underlying physics is indeterministic. I agree, though, as above, that does not necessitate that brains are. The main argument presented, however, is the need for an animal to be unpredictable. Clearly, a gazelle running from a cheetah will benefit if the cheetah cannot predict which way it will dart. This will hold for any animal vulnerable to predation.

I agree on the need for unpredictability, but this does not require quantum indeterminacy. The world is simply way too complex for any of us to be able to predict it, even if that were “in principle” possible given enough information and intelligence. All that matters is that, in practice, predators stand no chance of making such predictions. The nematode worm (Caenorhabditis elegans) has only 302 cells in its nervous system. But even there, different individuals have those 302 cells wired up differently, owing to inevitable differences as the embryo developed. And if I gave you a complete map of the neural network of a mere 302 cells, could you look at it and predict how it would respond to various stimuli? I certainly couldn’t. The cheetah hasn’t a hope in hell of predicting the exact behaviour of the gazelle’s brain, even if that brain is entirely deterministic, and even if it had a complete and accurate neural-level map of that gazelle’s brain (which of course it doesn’t), and even if it had complete knowledge of the gazelle’s sensory experiences (which it also doesn’t).

So you don’t need indeterminacy to have in-practice unpredictability; the world is just too complex. And, while a prey animal wants unpredictability, it does not want to make dice-throwing decisions. There are some directions to dart — straight into the paws of the cheetah — that would be a rather bad choice. The gazelle still wants to make best use of all the information from its senses, and that requires a deterministic neural network.

And that’s about it: the above argument, that “predictability is one thing that will make sure that a competitor will be out of business soon”, and thus that “deterministic behaviour can never be evolutionarily stable” is pretty much the only argument that Professor Brembs presents for brains being indeterministic. He does argue at length for brains needing to produce “behavioural variability”, and that they need to flexible and adaptable and responsive to their environments. I agree entirely. But this is a separate issue from them being non-deterministic. Indeed, being responsive to their environments is itself a deterministic concept. The whole point of quantum indeterminacy is that it is not the result of anything, and so is independent of local conditions.

As an example, Brembs argues that:

“… the temporal structure of the variability in spontaneous turning manoeuvres both in tethered and in free-flying fruitflies could not be explained by random system noise. Instead, a nonlinear signature was found, suggesting that fly brains operate at criticality, meaning that they are mathematically unstable, which, in turn, implies an evolved mechanism rendering brains highly susceptible to the smallest differences in initial conditions and amplifying them exponentially. Put differently, fly brains have evolved to generate unpredictable turning manoeuvres.”

But sensitivity to small differences and a non-linear response is not the same thing as being non-deterministic. Deterministic systems often behave like that. A roulette wheel is non-linear and unpredictable in practice, even if it is deterministic. As another example, modern fighter jets are designed to be aerodynamically unstable, in order to be highly manoeuvrable — just like the fruit fly — and it would be straightforward to write a black-box computer code to produce a flight plan that another human (having no sight of the code) could not then predict.

I do wonder whether a rejection of determinism might be motivated by the false presumption that a “deterministic” response must inevitably be a simple, linear and inflexible response that is not responsive to the local circumstances? But this is not so. Responding to local circumstances is (by definition) a deterministic process, and behavioural variation and flexibility is exactly what you’d get from a complex but deterministic neural network.

But Professor Brembs wants a role for indeterminacy as an ingredient for his conception of free will.

While some argue that unpredictable (or random) choice does not qualify for their definition of free will, it is precisely the freedom from the chains of causality that most scholars see as a crucial prerequisite for free will.

I consider this to be misguided. As a compatibilist, I assert that the only “free will” that we have is entirely compatible with determinism. We have a “will” and often we are “free” to act on it. And yes, that will is indeed a product of the prior state of the system. The sort of “will” that arises independently of the prior state of the system does not exist (unless one wants to argue for dualistic souls that tell matter how to move).

But many people dislike that conclusion; they reject dualism but want to rescue a “will” that is “free from the chains of causality”. They hope that some mixture of causation and indeterminism might do that. Thus Brembs argues (as have others) for a “two-stage model of free will”:

One stage generates behavioural options and the other one decides which of those actions will be initiated. Put simply, the first stage is ‘free’ and the second stage is ‘willed’. […] freedom arises from the creative and indeterministic generation of alternative possibilities, which present themselves to the will for evaluation and selection.

This may be a failure of my imagination but I don’t see how this helps. Yes it gives freedom (from the causal chain) and it gives will, but the part that is free is not willed and the part that is willed is not free from causation. So it doesn’t give a “free will” if that is a will that is free from the causal chain.

The “free” part is simply generating a list of possibilities. The “will” part, the bit that is doing the choosing from the list, is still a computational process. Non-deterministic choice making could not have evolved.

Invoking the first stage serves only to salve an intuitive dislike of the idea that we are bound to the same processes of cause-and-effect that govern the rest of nature. People consider this to be beneath human dignity. But human intuition is unreliable, and a mechanism invoked purely to salve human dignity is unlikely to be how things actually are. If you think that a dice-throwing component is needed to generate options that could not otherwise be available, then you’re likely underestimating the degree to which the world is so complex that a deterministic neural network would already produce ample behavioural flexibility.

In short, I suggest that appealing to quantum indeterminacy in any discussion of free will is a red herring that has intuitive appeal but that cannot be made into a coherent account of a will that is uncaused. We should, instead, accept that there is nothing wrong with our wills being caused.

But it seems that Brembs is motivated by a dislike of the idea that we are at the end of a causal chain:

I hope to at least start a thought process that abandoning the metaphysical concept of free will does not automatically entail that we are slaves of our genes and our environment, forced to always choose the same option when faced with the same situation.

So what would be the problem if that were indeed the case?

In fact, I am confident I have argued successfully that we would not exist if our brains were not able to make a different choice even in the face of identical circumstances and history.

I don’t agree that the argument has been successfully made. And, anyhow, in practice, “identical circumstances and history” never recur. Walk around outside and look around you. The local environment that you are looking at is made up of something like 1032 individual atoms. That is so many that, if you were to point to them individually at a rate of one every second, it would take you 100,000,000,000,000 times the age of universe before you’d finished. The idea of “identical circumstances”, with every one of those atoms being in the same place and behaving the same way, is a valid philosophical thought experiment but is not of practical significance. The world already contains enough complexity and variation that there is no need to invoke quantum indeterminacy in order for a cheetah to be unable to predict the darting of a gazelle (if the cheetah counted neurons in the gazelle’s brain at a rate of one a second it would take it a mere 3000 years).

It makes no practical difference at all whether a different decision arises because of a quantum dice throw, or because the circumstances were slightly different. I don’t see why the former is preferable, or has more “moral” salience, or is more in accord with our human dignity (not that that actually matters; since the universe is not obliged to accord with our preferences).

Our Earth is not at the centre of the universe. The universe as a whole does not have a purpose. It was not created for us, we are just one product of an evolutionary process. And, being a product of the universe, we are material beings produced by and governed by the same laws of cause and effect that describe everything else.