by Katalin Balog
The mind-body problem in its current form – an inquiry into how the mind fits into the physical universe – was formulated by René Descartes in the 17th century. In his Meditations, a thin volume of philosophy that had a monumental effect on all later Western philosophy, he famously argued that it is possible to conceive of a mind without extension – a disembodied soul – and a body without thought – a mindless zombie. And since whatever can be clearly and distinctly conceived, can be brought about by God, it is possible for there to be a mind without body and a body without mind. He concludes, based on observations about our concept of mind and body, that they are really distinct. His position is called dualism, which is the view that the world has fundamental ingredients that are not physical. Of course, dualism was not original to Descartes; from Plato to the Doctors of the Church and ordinary folks, most people up until the Enlightenment have advocated it. But his way of arriving at it – examining what he can “clearly and distinctly” conceive – has set the template for all subsequent discussions. Nota bene, he had more empirically based arguments as well; for example, he thought nothing physical could produce something so open ended and creative as human speech. He would have been very surprised by ChatGPT.
According to Descartes, the mind, or soul, is an exalted thing: it is non-spatial, immaterial, immortal, and entirely free in its volition. He also thought – reasonably enough – that it interacts with the body, an extended, spatially located thing. In Descartes’ view, there are sui generis mental causes; purely physical causes cannot explain actions. Descartes held that only the quantity of motion is strictly physically determined, not its directionality. In Leibniz’s telling of the story in The Monadology, Descartes believed that the mind nudges moving particles of matter in the pineal gland, causing them to swerve without losing speed, like the car going around the corner. He had a real, substantive disagreement with his contemporary Hobbes, a materialist who thought that there were only physical causes.
Developments in science have had an enormous impact on this debate. The gist is that advances in the physical and biological sciences ultimately ruled out Descartes’s idea of the mind as a sui generis force – though not necessarily other forms of dualism which we will talk about shortly. Not only is it hard to comprehend Descartes’ idea that a non-material mind can move a material body, but such sui generis non-physical causes are ruled out by what we know about natural processes.
What proved most consequential for this change is what is known as the Causal Closure Principle: that all physical effects can be completely accounted for by physical causes. The principle implies that anything that has a physical effect must itself either be physical, or act in tandem with the physical. If my mind causes me to go to the park, and so moves my body through space, it follows either that the mind is itself physical, or that there is some mechanism that keeps the mind and body in synchrony without the mind being an independent contributor to the causal order. Leibniz, for example, accepted causal closure. He inferred from this not materialism but pre-established harmony, where mind and body never interact, even though their course unfolds due to God’s harmonizing them at the Creation, like two clocks that always show the same time.
Few believe this anymore, but there are other forms of dualism that are compatible with causal closure. Some property dualists who accept causal closure maintain that the mind is epiphenomenal – just going along for the ride. Others hold that it overdetermines its physical effects – it causes exactly those actions that prior physical events would cause anyway. Yet another view that is gaining in popularity is panpsychism. It holds that the intrinsic nature of fundamental properties is mental. This means that even electrons and protons have an intrinsic mental nature; panpsychists think there is something it is like to be an electron, just like there is something it is like to be a conscious human being (of course, the experiences are bound to be vastly different!). This, too, is compatible with the causal closure of the physical.
Some contemporary philosophers hold on to Cartesian dualism by denying the Causal Closure Principle. But this has become an increasingly difficult position to maintain, especially after the scientific progress in physiology made by the middle of the 20th century. Whereas Newton allowed forces other than impact and, in principle, as far as physics was concerned, there was no barrier to the existence of special forces such as chemical, biological, or mental, it became clear by the middle of the 20th century that none of those forces are sui generis, that is, they are all explicable in terms of the forces we are familiar with from physics. During this time, in addition to discoveries in biochemistry, neurophysiological research mapped the body’s neuronal network and analyzed the electrical mechanisms responsible for neuronal activity. If there were sui generis mental forces operating inside living bodies, they should have shown up in some way in our observations of neural activity. But research has failed to uncover evidence of anything except familiar physical forces. By the middle of the last century, belief in an immaterial soul that is a sui generis cause of action had steadily declined, and materialism had become the dominant view in philosophy and increasingly among the wider public.
Dualism nevertheless enjoyed another resurgence in the latter half of the 20th century. The mind’s interaction with the body, and the conceivability of mind without body are the two poles around which modern discussions of the mind-body problem revolve. The mind’s interaction with the body furnishes modern arguments for materialism. The conceivability of body without mind provides an argument for some form of dualism. For all the advances in brain science, it is hard to understand how our minds, especially our conscious experience – like the experience of red – arise from activities in the grey matter of our brain. The most famous modern conceivability argument is David Chalmers’ zombie argument from his book, The Conscious Mind. Philosophical zombies are creatures that are physically exactly like us but have no experiences. Chalmers argues that we can conceive of zombies without incoherence, and he, like Descartes, concludes from this that zombies are possible. But if zombies are possible, materialism must be false since if materialism were true, all processes, including mental processes, would be just very complicated physical ones, which rules out the possibility of zombies. The conclusion is that there is more to reality than just the physical.
However, the debate between materialists and dualists has very different stakes from Descartes’s time. Few dualists today argue for the immortality and spontaneous causal powers of the mind. Materialism and dualism used to disagree on empirical grounds back when Descartes thought the mind had its causal powers independent of the physical. The modern near-consensus on the causal closure of physics changed the nature of the disagreement. If both sides agree that everything in the physical world can be fully explained physically, there is no possible empirical evidence that could count for either materialism or dualism. The opposition between dualism and materialism today is a mere shadow of the opposition between Descartes and Hobbes.
But despite their empirical equivalence, might there be philosophical arguments that settle the question? I think this is not the case either. The materialist can counter the zombie argument successfully by explaining the conceivability of zombies simply as a consequence of the way we think directly and introspectively about our experience. When I think about my present experience of waves splashing against the shore, the experience itself is incorporated into my reflection. Scientific concepts – e.g. concepts in neuroscience – are very different. They have descriptions of neural activities across brain regions associated with them that help identify the referent. The materialist can argue that the conceivability of zombies is an artifact of this mismatch of concepts.
There is a strange dialectic to these views. Both materialism and dualism can defend themselves from attacks from the other position and view the other position as question-begging. Neither seems to offer an overall better theory, and they don’t have empirical consequences that could distinguish between them. It is impossible to know, either on empirical or philosophical grounds, which is true, but strangely, the stakes are very low.
It is even possible that there is no real disagreement between the dualist and the materialist. They might be simply two different descriptions of the same reality in terms of different conceptual schemes, each involving a different basic metaphysical concept of property. The dualist thinks properties are transparent to the concepts we apply to them – so if we can conceive a brain state and an experience as distinct, they are distinct – and the materialist denies this. But if truth is immanent, embedded within a specific theory, and not transcendent – that is, existing independently of a particular theory or framework – then dualism and materialism do not conflict.
The premodern conception of the soul as an independent source of agency and something distinct from the physical turned out to be a projection of how we intuitively think about ourselves. It might be that the premodern conception is more faithful to our experience of who we are as persons and agents. But once we lost it, we lost it, and with it, we lost the substantial mind-body problem.