What’s really at stake in the mind-body debate? Jonathan Birch looks at some of the explanatory differences in approaches to the metaphysics of consciousness.

Officially, the mind-body problem is all about the metaphysical relation between conscious experiences and brain activity. But it’s also a clash between three very different explanatory projects. Competing theories about the mind-body relation are often hard to tell apart, and sometimes seem to blur (exasperatingly) into each other. Yet the differences between their associated explanatory projects are striking. I tend to think that what’s really at stake in the debate is not so much who is right (because it’s too soon to tell), but whose explanatory project is most worthy of pursuit.

The materialist project

Let’s start with materialism. There are various different relations between conscious experience and the brain that are acceptable to a materialist, leading to multiple, slightly different versions of materialism. For example, perhaps conscious experiences are identical to physical events in the brain, or perhaps they are constituted by or realized by or grounded in physical events in the brain.

When we set things up like this, we soon start to worry about what exactly materialism means. What are physical events? A first stab: they are events that can be fully characterised in terms of properties, entities, processes and laws that feature in the natural sciences, including chemistry and biology as well as physics. They are events involving things like neurons, neurotransmitters, action potentials, but no mysterious “mind stuff” or “mind force”.

Of course, the natural sciences are a moving target – they change over time. The science of today is not the science of ancient Greece or early modern Europe, when there were still recognisable materialists. The physics of a hundred years from now may look very different from the physics of today. It’s tempting, given this, to define materialism in terms of a complete, future natural science and to say that “physical events” are events that would be fully characterised by this future science.

But now we seem to be losing our grip on materialism. What if a complete future natural science includes mind stuff or mind forces? You can’t leave that option open and still call yourself a materialist. We need to rule out some possible future trajectories for science as incompatible with materialism.

A popular option here is to say that “physical events” are those that would be fully characterised by a future, complete natural science broadly continuous with current physics. In other words, a future science that still endorses something that resembles the “standard model” of contemporary physics, in so far as it continues to posit a fairly small set of fundamental particles and fundamental forces, with no place anywhere for mind stuff or mind forces. This gives us some grip on what materialism means, but the grip is rather looser than we might ideally like.

What’s much easier to get a grip on, I think, is the materialist explanatory project. This is the project of explaining everything that really needs explaining about conscious experience using only the resources that the sciences provide.

For the materialist, what really needs explaining? We need to identify and explain the mechanisms involved in conscious experience (e.g. attention, working memory) and explain how they differ from those involved in unconscious processing. We also need to explain the way we think and talk about our experiences, and our susceptibility to being misled by thought experiments (like the “zombie” thought experiment or the “Mary” thought experiment) that make us think conscious experience is something non-physical when it isn’t.

The materialist’s project is to explain all this without ever positing anything mysterious (like mind stuff, mind forces, or non-physical properties) that goes beyond what is already posited by science. A materialist needs to draw on resources from psychology and cognitive science, and there is work too for evolutionary biology: the work of explaining how we evolved to be creatures who think about their own brain activity in the peculiar way we do.

Daniel Dennett’s Consciousness Explained (1991) is a great entry-point to the materialist project. Dennett pursues both the psychological and the evolutionary project with panache. The subtext on every page is “and that’s all that really needs explaining!”. There is no further residue of non-physical qualia or mind forces or mind stuff that requires a special, extra explanation of a type that psychology, cognitive science and evolutionary biology can’t provide.

This is why readers with dualist sympathies usually think the book is missing the point. They say it is explaining away consciousness, not explaining it. But for a materialist, it is explaining what actually needs explaining.

The interactionist project

A dualist says conscious experiences are not identical to, constituted by, realized by or grounded in physical events in the brain. But since the dependence of conscious experience on events in the brain is clear (as shown by psychedelic drugs and brain injuries, for example), they will typically add that conscious experiences are related to brain activity by psychophysical laws. This commitment to special extra laws that add something to the basic stock of fundamental laws posited by physics is a key feature of dualism.

There are different ways to be a dualist, just as there are different ways to be a materialist. You can be an epiphenomenalist who thinks that (to use T. H. Huxley’s metaphor) conscious experience is like the steam trailing behind the steam train, generated by brain activity but doing absolutely nothing to influence it. Or you can be an interactionist who thinks conscious experiences, while distinct from physical events, nonetheless pull the strings in the brain, somehow nudging the neural activity one way or another. Although epiphenomenalism is an important view, I’ll focus on interactionism here.

For the interactionist, some causal power must be added to the list of fundamental forces posited by physics. Physics says: there’s gravity, electromagnetism, and the weak and strong nuclear forces, and that’s it. An interactionist says: there must, in addition, be some entry point at the fundamental level of reality for mental causation.

Interactionists have their own explanatory project, and it’s very different from the materialist project. The interactionist’s task is to find that elusive entry point, the point at which mental causation slips inside the workings of the brain. This is a very unfashionable project indeed. So unfashionable, in fact, that it’s hard to imagine a project of that general shape receiving funding from anyone. But we should ask: Can we completely rule out, on current evidence, that interactionism is right?

In the late 20th Century, the Nobel-prizewinner John Eccles, an expert on the synapse (the place where neurons meet and signal to each other), became convinced that there must be some role for mental causation in how a synapse works. He embarked on an idiosyncratic quest to find the entry point. In a remarkably creative paper, Eccles and Friedrich Beck set out a hypothesis on which activity at the synapse depended on quantum tunnelling, and mental causation was able to nudge the quantum probabilities one way or another by tiny increments. That particular hypothesis has been shown to be false. But it gives a flavour of the general type of hypothesis that someone serious about interactionism needs to put forward and test.

Some may be inclined to say: How on Earth did such a wild paper even get published? My inclination is to say: Why isn’t there more work like this going on? We can’t say that interactionism has been decisively ruled out if we can only point to one example of a serious interactionist hypothesis being developed and tested. What’s the harm in developing, and testing, more hypotheses like this? These hypotheses are always going to sound eccentric, but isn’t it sometimes worth testing eccentric hypotheses?

Interactionism has become rather like cold fusion: an idea dismissed as nonsense on the grounds that no one can currently see how it could work. “It would be lovely it if it were true, but why waste resources trying to find something that’s obviously not there?” Yet it’s hard to be confident something is not there until you’ve invested serious resources in trying to find it, a point Huw Price has made in relation to cold fusion.

The panpsychist project

There is a third option. One of the most visible developments in philosophy of mind over the past twenty years has been the revival of panpsychism as an alternative to materialism and dualism.

As with materialism and dualism, it isn’t easy to state panpsychism clearly. It’s not the view that all the elements of human conscious experience can be found inside fundamental particles. No one is suggesting that quarks consciously reflect or deliberate (“I’m tired of being uranium, fancy trying lead?”). The idea is rather that the most puzzling elements of conscious experience – its qualitative and subjective character – can be found in a simple form in at least some fundamental particles. But what counts as a “simple form” is up for debate.

The picture is further complicated by panprotopsychism, the view that, while the basic elements of conscious experience are not there in fundamental particles, some important precursors to those properties are there. One might wonder (and many authors have wondered) whether some materialist views might also count as panprotopsychist views. It’s all a bit blurry.

It’s easier, I think, to understand how the panpsychist’s explanatory project differs from those of the materialist and the interactionist. The project is not to explain away our susceptibility to anti-materialist intuitions (about zombies, Mary, etc.), because these intuitions are regarded as correct. Nor is it to find some entry point for mental causation in a physical world, because the physical world is taken to be pervaded with conscious experience at the fundamental level.

The panpsychist’s project is something else: what needs explaining is how vast multitudes of “micro-experiences” (or perhaps “micro-proto-experiences”) in fundamental particles combine to form the single, unified “macro-experience” of a conscious creature. In other words, panpsychists need to solve the so-called combination problem.

Panpsychism may sound like an idea from the darkest recesses of speculative philosophy, but some scientists are interested in it too. I see the currently popular “Integrated Information Theory” (IIT) of consciousness as a contribution to the panpsychist project. The theory proposes that the wonder of combination happens in systems that are highly integrated in a specific sense, a sense captured by the notorious “phi“. Even very simple systems like photodiodes and thermostats are said to have some degree of phi, and thus some simple conscious experiences. The view is panprotopsychist, rather than panpsychist, because systems with zero phi are said to have no conscious experiences at all. But all matter has the potential to achieve conscious experience if arranged in a highly integrated way.

This is why IIT feels like an “odd one out” among current scientific theories of consciousness. Other theories are best seen, I think, as contributions to the materialist project. IIT is not.

The panpsychist project strikes many people, at least initially, as ludicrous and ill-conceived. It seems to be leaving behind not just the materialist’s concern with cognitive science, psychology and evolution, but also the interactionist’s concern with biophysics, in favour of unconstrained speculation. The question of how to constrain the speculation – how to bring it into serious contact with hard evidence – is a crucial one for any panpsychist. But the panpsychist can argue that we may have to increase our tolerance for projects that initially seem ludicrous and ill-conceived, if we one day hope to understand consciousness.


I said at the beginning that what’s really at stake in debates about the mind-body problem is not so much who is right (because it’s too soon to tell), but whose explanatory project is most worthy of pursuit. Hopefully it’s now a little clearer what I meant.

If you think about materialism, dualism and panpsychism as free-floating metaphysical theses, detached from real people and their projects, it’s not so easy to distinguish them. You start to wonder, for example, what on Earth “physical” actually means and whether or not panprotopsychism can be a form of materialism. It’s easy to get exasperated by the whole debate.

But in truth, these views are attached to real people and their projects, and the projects are strikingly different. You can spend your time trying to explain away anti-materialist intuitions, or looking for entry points for mental causation, or trying to solve the combination problem, but you can’t (easily) do all three things at once. In that sense, you do face a choice.

Yet I’m inclined to say that, at the present time, all of these projects are worth pursuing. They can’t all be pursued by the same people at the same time, but it’s a good thing if they are all pursued by someone. We should let the materialist project, the interactionist project and the panpsychist project run in parallel, take stock in a hundred years or so, and see which has made the most progress.

By Jonathan Birch


Dr Jonathan Birch is an Associate Professor of Philosophy at the LSE and Principal Investigator (PI) on the Foundations of Animal Sentience project. In addition to his interest in animal sentience, cognition and welfare, he also has a longstanding interest in the evolution of altruism and social behaviour.


Further reading


This work is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, Grant No. 851145.