Imagine an exact replica of you, down to the last molecule and neuron. Now imagine that your twin moves, talks and reacts like you but has no mental life, no consciousness at all. Your twin is a philosophical zombie. Is such a creature logically possible?
The zombie thought experiment is an attempt to clarify the exact relationship between mind and body, and any response to the experiment entails a commitment to one or other metaphysical position. If you think such a creature is logically possible, then you are a form of dualist. If you are a physicalist, you must reject the possibility.
Of course, the idea of philosophical zombies is not the only thing at stake in this discussion. The mind-body problem exists because our minds seem distinct from the physical world around us (including our own bodies) and we have no satisfactory vocabulary to describe in a unified way a creature possessing both mental and physical properties. Moreover other metaphysical questions, such as survival after death and the freedom of the will, depend in part upon the way we come to understand our minds.
The notion that body and mind are distinct substances, and can in principle be separated, is known as dualism. The classical statement of this theory appears in Plato’s Phaedo, where Socrates welcomes death as the release of the soul from the prison of the body. Unlike the body, the soul is non-physical and indestructible, and for Plato it is our means of accessing the realm of universal Forms. A more modern dualist account was famously developed by René Descartes, and the theory is therefore often referred to as Cartesian dualism.
Through a simple process of introspection, Descartes recognised the very different natures of mind and body:
Next I examined attentively what I was. I saw that while I could pretend that I had no body and that there was no world and no place for me to be in, I could not for all that pretend that I did not exist. I saw on the contrary that from the mere fact that I thought of doubting the truth of other things, it followed quite evidently and certainly that I existed; whereas if I had merely ceased thinking, even if everything else I had ever imagined had been true, I should have had no reason to believe that I existed. From this I knew I was a substance whose whole essence or nature is solely to think, and which does not require any place, or depend on any material thing, in order to exist. Accordingly this ‘I’ – that is, the soul by which I am what I am – is entirely distinct from the body, and indeed is easier to know than the body, and would not fail to be whatever it is, even if the body did not exist. Discourse on Method IV
This accords with common sense. People appear to have bodies and minds. Like Descartes we seem to be able to imagine ourselves existing without a body, or moving our mind into another body or even, perhaps, a computer. The philosophical form of this intuition is known as Leibniz’s Law, one version of which states that two things are identical only if they share exactly the same properties. Since the mind and body each seem to have qualities not shared by the other, they must be different things.
There are several properties that seem unique to either the mind or the body, and by an application of Leibniz’s Law they represent a strong argument for dualism. For example, bodies are extended: they have a definable size and shape and can be measured and located. The mind, however, appears not to have extension or a precise location in space. A peculiar property of mind is privacy: other people can observe my behaviour, but my mental life is directly accessible only to myself. If I am thirsty and want a drink, only I can know that immediately. My actions might reveal my thoughts to others, if I start panting and reach for the tap; but I can also hide my thirst, or pretend to be thirsty when I am not. Mental states also exhibit intentionality, that is, they are ‘about’ something. If I desire, perceive, remember or trust, I am desiring, perceiving, remembering or trusting something: I may desire a pizza, perceive a rainbow, remember Paris or trust the government. This intentionality is completely absent from any physical picture: a pattern of neurons firing in my brain is not ‘about’ anything. Mental experiences also have a uniquely subjective aspect: an after-image may appear green to the subject, but there is no greenness in the corresponding brain state. By Leibniz’s Law, therefore, mind and body must be distinct.
Most philosophers feel that there is something wrong with this argument. Such a blanket application of Leibniz’s Law seems to miss the subtle ways in which we use language to describe the world. When we consider the same thing from different perspectives, we pick out different aspects, and the perspectives we adopt are often determined by our state of mind. Consider the following argument:
This argument is not true, because Oedipus did not want to marry his mother. The crucial factor here is that Oedipus did not realise that Jocasta was his mother, and therefore the third statement points not to a fact in the world, but to a belief in Oedipus’ mind. Our language often reflects different aspects of the world and allows for the same thing to be described in various ways, a fact which undermines Leibniz’s Law. It seems possible, therefore, that mind and body are identical, and the differences that strike us are different aspects of the same thing.
Philosophers have generally rejected substance dualism as a theory, despite its intuitive appeal, because of seemingly intractable problems and contradictions in the dualist position. One of the most important is the problem of interaction. Physics gives us a full account of how matter affects matter, but offers no clue as to how a non-physical substance can interact with a physical one. A non-physical mind influencing matter sounds suspiciously like paranormal activity and would appear to violate the law of the conservation of energy. Some philosophers have claimed that the causal relation only goes in one direction: mental events are caused by physical events, but cannot themselves influence matter. The mind is an epiphenomenon, like the whistle on a steam train: caused by physical events but not affecting them. Epiphenomenalism resolves the problem of how the mental affects the physical (although not how the physical affects the mental, which still requires explanation), but is deeply counterintuitive. It seems clear that mental events can cause physical events: the pain I feel makes me cry out, the sight of a wild animal makes me run, my desires determine my choices, and so on. Dualist thinkers have gone to great lengths to resolve the problem of interaction. Leibniz himself argued that mind and body do not interact, but run in parallel like two accurate clocks which always tell the same time. Malebranche argued that God intervened on each occasion that interaction was required. Such contorted solutions suggest that the problem of interaction is not easy to overcome.
The scientific picture tends to make life difficult for the dualist. While evolutionary theory accounts for physical change, it offers no explanation for the appearance of mind in the evolutionary timeline. A similar question arises at the developmental level: at what point between fertilisation and birth does the mind arise in a human foetus? Moreover, while it is admittedly difficult to see how matter can come to display properties like privacy and intentionality, the problem is not immediately solved by positing a second substance, mind. How can any substance, of any sort, display intentionality? In this respect, dualism offers no explanatory advantage but merely shifts the problem elsewhere.
There is a large amount of empirical evidence indicating correlation between physical and mental states. If my brain is damaged in a certain way, my memory is affected; excite a part of my cortex and I experience hallucinations; inject a drug into my system and I lose consciousness. While such examples of correlation do not rule out dualism, they do suggest that the mind is best understood as part of the physical world.
Materialism, or more properly physicalism, is the theory that mind and body are not separate substances but are in fact both part of the physical world. Although this is the standard philosophical position, it is not easy to define exactly how the mental and the physical are related. The starting-point for most discussions is the notion that mental states are simply identical to physical states.
Identity theories of mind insist that mental events just are brain processes. My sensation of colour, my emotional state, my memories – all are identical to the physical state of my brain and nervous system that accompanies them. Descriptions of mental events and physical events may suggest different substances, but in fact the same thing is being represented in two different ways – like salt and sodium chloride. The strongest form of identity theory is Type Identity which claims that certain kinds of mental state are identical with, and reducible to, a corresponding physical state. So pain is, in fact, just a particular stimulation of c-fibres; and excitement is just an increase of adrenaline in the blood.
However, this identity between types seems too strict. It is unlikely that every person who remembers Paris will possess the same brain state, or that the rich and subtle varieties of human experience can be so easily grouped into types. Hilary Putnam has argued that it is plausible that two creatures could experience a particular mental state without sharing a corresponding brain state. The very different physiologies of humans, birds and molluscs may not allow for identical brain states, but may still allow for the similar experience of pain. In Putnam’s phrase, mental events may be multiply realised: that is, they may exist without necessarily sharing the same physical basis.
A weaker form of identity theory is Token Identity. This theory claims that any particular (token) mental event is identical to a physical event, but not of a particular type. Today’s memory of Paris is identical with a particular arrangement of neurons; but yesterday’s memory of Paris may have had a different corresponding brain state. This avoids the problems raised by Putnam’s hypothesis of multiple realisability, but introduces new questions. Token identity seems to allow for the possibility that two persons who are physically identical (down to the last neuron) could have different mental experiences. This suggests that mind and body are so contingently related that the nature of that relationship is as mysterious as dualism. Philosophers have therefore used the notion of supervenience to more rigorously anchor the mental to the physical. Supervenient properties depend on other properties to exist. Imagine a beautiful painting. If an exact replica of that painting exists, then the replica must be beautiful too because the property of beauty is supervenient on the physical organisation of the painting. The same is true of mental properties: they supervene on physical properties, so two physically identical brain states must share the same mental state. This is compatible with the theory of Token Identity, but it also allows for multiple realisability since supervenience only works in one direction. Two people with the same mental experience could be physically different, just as two different paintings can both be beautiful.
These physicalist theories offer a sophisticated account of the identity of mind and body, but they are also open to objections. Thomas Nagel famously argued that even if we know all the physical facts about a bat’s use of sonar, we still cannot know what it is like to be a bat. The subjective character of experience seems to be omitted in even the fullest scientific (physicalist) account, although it is immediately accessible to the conscious subject. I just know what it is like to be me. These distinct, subjective aspects of consciousness which philosophers call qualia represent a problem for physicalism. Frank Jackson proposed the knowledge argument (or ‘Mary’s Room’) to clarify this issue:
Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal chords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’... What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? It seems just obvious that she will learn something about the world and our visual experience of it. But then it is inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false.
There has been a great deal of discussion about the knowledge argument. Physicalists argue that it is unclear that what Mary learns after leaving the room is knowledge of the world (rather than an ability or a form of acquaintance), or that before leaving she could in fact have all the relevant physical information. It is worth noting that Jackson himself has recently rejected his own argument and come to embrace physicalism. Nevertheless, the discussion does reveal how difficult it is to capture the subjective aspects of experience in a physicalist account of the mind.
Recent developments in this area have tended to involve the theory of Functionalism. This approach does not ask what mental states are, but what they do. Take the example of a key: what makes it a key is its function, not its substance. A key could be made of any number of metals or plastics or even wood; it could take any number of forms, from a skeleton key to a smart card; but it remains a key because it opens a lock. That is what keys are: things that perform a particular job. The idea is common in Biology, where (for example) we refer to an organ as an eye because of what it does, despite the very different eyes of a fly, a squid, a human, and so on. By contrast, we define diamonds not by their function (other things sparkle and cut glass) but by their physical structure. Other crystals may be hard, transparent and rare, but they are not diamonds unless they have a diamond’s constitution. So according to functionalism, the identity of a mental state is determined by its function – or more precisely, its relation to sensory input, other mental states and behaviour.
Functionalism allows for Putnam’s claim that mental states can be multiply realised, since any mental event is described without reference to substance (the theory is therefore officially neutral between dualism and physicalism, although its development has generally been in a physicalist direction). Accordingly pain, assuming it performs the same function (as an effect of injury, a cause of anxiety and discomfort, and so on), can be realised by different physical states in different kinds of creatures. One consequence of this is the possibility of artificial intelligence. If conscious mental states can be fully defined by their function, and the role they play within a system, then there is no reason to think that these states cannot be replicated by a machine.