The Mind-Body Problem concerns the explanation of the relationship, if any, that exists between minds, or mental processes, and bodily states or processes. One of the aims of philosophers who work in this area is to explain how a supposedly non-material mind can influence a material body and vice-versa.

Our perceptual experiences depend on stimuli which arrive at our various sensory organs from the external world and these stimuli cause changes in our mental states; ultimately causing us to feel a sensation, which may be pleasant or unpleasant. Someone's desire for a slice of pizza, for example, will tend to cause that person to move their body in a specific manner and in a specific direction to obtain what they want. The question, then, is how it can be possible for conscious experiences to arise out of a lump of gray matter endowed with nothing but electrochemical properties. A related problem is to explain how someone's propositional attitudes (eg: beliefs and desires) can cause that individual's neurons to fire and his muscles to contract in exactly the correct manner. These comprise some of the puzzles that have confronted epistemologists and philosophers of mind from at least the time of René Descartes.

Dualist solutions to the Mind-Body Problem

Dualism is a set of views about the relationship between mind and matter. It begins with the claim that mental phenomena are, in some respects, non-physical. 3 criteria distinguish mind and body:-

1. ‘Consciousness’ means we think, imagine, perceive and reason
2. Mental states are private and unique to their owner - others can express empathy with a mental state (eg: pain) but they can’t experience it as you experience it
3. Mental states show intentionality - they are directed towards some goal - but physical states do not have this property

One of the earliest known formulations of Mind-Body Dualism was expressed in the eastern Sankhya and Yoga schools of Hindu philosophy (circa 650 BCE), which divided the world into purusha (mind/spirit) and prakrti (material substance). Specifically, the Yoga Sutra of Patanjali presents an analytical approach to the nature of the mind.

In Western Philosophy, the earliest discussions of Dualist ideas are in the writings of Plato and Aristotle. Each of these maintained, but for different reasons, that man's ‘intelligence’ (a faculty of the mind or soul) could not be identified with, or explained in terms of, his physical body. Nonetheless, Plato linked the mind to the brain while Aristotle preferred to associate it with the heart, However, the best-known version of Dualism is due to Descartes (1641), and holds that the mind is a non-extended, non-physical substance. Descartes was the first to clearly identify the mind with consciousness and self-awareness, and to distinguish this from the brain, which was the seat of intelligence. Accordint to Descartes, God somehow - mysteriously? - ensured the mental and the physical came together. He was, therefore, the first to formulate the Mind-Body Problem in the form in which it still exists today.

Arguments for Dualism

The main argument in favour of Dualism is that it seems to appeal to the common-sense intuition of the vast majority of non-philosophically-trained people. If asked what the mind is, the average person will usually respond by identifying it with their self, their personality, their soul, or some other such entity. They will almost certainly deny that the mind simply is the brain, or vice-versa, finding the idea that there is just one ontological entity at play to be too mechanistic, or simply unintelligible. The majority of modern ‘philosophers of mind’ think that these intuitions, like many others, are probably misleading and that we should use our critical faculties, along with empirical evidence from the sciences, to examine these assumptions and determine if there is any real basis to them.

Another important argument in favour of Dualism is the idea that the mental and the physical seem to have quite different, and perhaps irreconcilable, properties. Mental events have a certain subjective quality to them, whereas physical events do not. So, for example, one can reasonably ask what a burnt finger feels like, or what a blue sky looks like, or what nice music sounds like to a person. But it is meaningless, or at least odd, to ask what a surge in the uptake of glutamate in the dorsolateral portion of the hippocampus feels like.

Philosophers of mind call the subjective aspects of mental events qualia (or raw feels). There is something that it is like to feel pain, to see a familiar shade of blue, and so on. There are qualia involved in these mental events that seem particularly difficult to reduce to anything physical.

Interactionist Dualism

Interactionist Dualism, or simply Interactionism, is the particular form of Dualism first espoused by Descarte. In the 20th Century, its major defenders have been Karl Popper & John Carew Eccles.(2002). It is the view that mental states, such as beliefs and desires, causally interact with physical states.

Descartes' famous argument for this position can be summarised as follows: Seth has a clear and distinct idea of his mind as a thinking thing which has no spatial extension (ie: it cannot be measured in terms of length, weight, height, and so on). He also has a clear and distinct idea of his body as something that is spatially extended, subject to quantification and not able to think. It follows that mind and body are not identical because they have radically different properties.

At the same time, however, it is clear that Seth's mental states (desires, beliefs, etc) have causal effects on his body and vice-versa. A child touches a hot stove (physical event) which causes pain (mental event) and makes him yell (physical event), this in turn provokes a sense of fear and protectiveness in the mother (mental event), and so on.

Descartes' argument crucially depends on the premise that what Seth believes to be “clear and distinct” ideas in his mind are necessarily true. Many contemporary philosophers doubt this. For example, Joseph Agassi (1975) believes that several scientific discoveries made since the early 20th Century have undermined the idea of privileged access to one's own ideas. Sigmund Freud (1917, 1940) has shown that a psychologically-trained observer can understand a person's unconscious motivations better than she does. Pierre Duhem (1906) has shown that a philosopher of science can know a person's methods of discovery better than he does, while Bronislaw Malinowski (1922) has shown that an anthropologist can know a person's customs and habits better than he does. He also asserts that modern psychological experiments that cause people to see things that are not there provide grounds for rejecting Descartes' argument, because scientists can describe a person's perceptions better than he can.

Other Forms of Dualism

• Psycho-Physical Parallelism, or simply Parallelism, is the view that mind and body, while having distinct ontological statuses, do not causally influence one another. Instead, they run along parallel paths (mind events causally interact with mind events and brain events causally interact with brain events) and only seem to influence each other. This view was most prominently defended by Gottfried Leibniz (1686). Although Leibniz was an ontological Monist who believed that only one type of substance, the monad, exists in the universe, and that everything is reducible to it, he nonetheless maintained that there was an important distinction between ‘the mental’ and ‘the physical’ in terms of causation. He held that God had arranged things in advance so that minds and bodies would be in harmony with each other. This is known as the doctrine of ‘Pre-established Harmony’.
• Occasionalism is the view espoused by Nicholas Malebranche (1688) which asserts that all supposedly causal relations between physical events, or between physical and mental events, are not really causal at all. While body and mind are different substances, causes (whether mental or physical) are related to their effects by an act of God's intervention on each specific occasion. So, for example, if somebody wanted to raise their arm, God would ensure the link was made between a person’s intention to raise their arm and the actual physical event of raising their arm.
• Property Dualism asserts that when matter is organsed in the appropriate way (ie: in the way that living human bodies are organized), mental properties emerge. Hence, it is a sub-branch of Emergent Materialism. These emergent properties have an independent ontological status and cannot be reduced to, or explained in terms of, the physical substrate from which they emerge. This position is espoused by David Chalmers (1996) and has undergone something of a renaissance in recent years - although it was already suggested in the 19th Century by William James (1897).

1. Epiphenomenalism is a doctrine first formulated by Thomas Henry Huxley (1900). It is the view that mental phenomena are causally ineffectual. Physical events can cause other physical events and physical events can cause mental events, but mental events cannot cause anything since they are just causally inert by-products (ie: epiphenomena) of the physical world. This view has been defended strongly in recent times by Frank Jackson (1982).
2. Non-reductive Physicalism is the view that although mental properties form a separate ontological class to physical properties, all mental states are casually reducible to physical states.

Monist solutions to the Mind-Body Problem

In contrast to Dualism, Monism states that there are no fundamental divisions. Today, the most common forms of Monism in Western Philosophy are Physicalist.  Physicalistic Monism asserts that the only existing substance is physical, in some sense of that term to be clarified by our best science. However, a variety of formulations (see below) are possible. Another form of Monism, Idealism, states that the only existing substance is mental. Although pure Idealism, such as that of George Berkeley (1710), is uncommon in contemporary Western Philosophy, a more sophisticated variant called ‘Panpsychism’, according to which mental experience and properties may be at the foundation of physical experience and properties, has been espoused by some philosophers such as William Seager (1991).

Other forms of Monism assert that the mental supervenes on the physical or vice versa. Variants of this type of Monism include arguments that, as the limits of obtainable information are approached, a strict Physicalist understanding and a strict Idealist understanding will supervene on each other. Lastly, there are advocates of Monism who believe that the mental and the physical are both part of a larger whole, which is generally considered to be not directly perceivable and hence not equivalent to the physical world. This last version of Monism is generally associated with Eastern religions and philosophies, including Buddhism, Transcendentalism, and mysticism in general. This is very much the position Ken Wilber (2000) takes, dismissing both Dualism and Physicalist Monisms as products of  reductionism (‘Flatland’).

Phenomenalism is the theory that representations (or sense data) of external objects are all that exist. Such a view was briefly adopted by Bertrand Russell (1910) and many of the Logical Positivists during the early 20th Century. A third possibility is to accept the existence of a basic substance which is neither physical nor mental. The mental and physical would then both be properties of this neutral substance. Such a position was adopted by Baruch Spinoza (1670) and was popularized by Ernst Mach (1897) in the late 19th and early 20th Centuries. This ‘Neutral Monism’, as it is called, resembles Property Dualism.

Physicalistic Monisms

• Behaviourism dominated the Philosophy of Mind for much of the 20th Century, especially the first half. In Psychology, Behaviourism developed as a reaction to the inadequacies of Introspectionism.  Introspective reports on one's own interior mental life are not subject to careful examination for accuracy and can not be used to form predictive generalisations. Without generalisability and the possibility of third-person examination, the Behaviourists argued, Psychology cannot be scientific. The way out, therefore, was to eliminate the idea of an interior mental life (and hence an ontologically independent mind) altogether and focus instead on the description of observable behavior.
  Parallel to these developments in Psychology, a philosophical Behaviourism (sometimes called ‘Logical Behaviourism’) was developed. This is characterised by a strong verificationism, which generally considers unverifiable statements about interior mental life senseless. For the Behaviorist, mental states are not interior states on which one can make introspective reports. As Gilbert Ryle (1963) states, they are just descriptions of behavior or dispositions to behave in certain ways, made by third parties to explain and predict others' behaviour.
  Philosophical Behaviourism, notably held by Ludwig Wittgenstein,(1953)  has fallen out of favor since the latter half of the 20th Century, coinciding with the rise of Cognitivism. Cognitivists reject Behaviourism due to several perceived problems. For example, Behaviourism could be said to be counter-intuitive when it maintains that someone is talking about behavior in the event that a person is experiencing a painful headache.
• Identity Theory was developed by John Smart (1956) and Ullin Place (1956) as a direct reaction to the failure of Behaviourism. These philosophers reasoned that, if mental states are something material, but not behaviour, then mental states are probably identical to internal states of the brain. In very simplified terms: a mental state M is nothing other than brain state B. The mental state “desire for a cup of coffee” would thus be nothing more than the “firing of certain neurons in certain brain regions”.
  The fact that chemicals (physical) - eg: SSRIs - can affect the state of mind (mental) or that there are verified accounts of meditative techniques (mental) seeming to have had a healing effect on illnesses such as cancer (physical) would fit with Identity Theory. The fact that damage to certain areas of the brain can result in changes to mental states, measurable using electroencephalographs, also provides support for the Identity Theory concept - as does the fact that people who afe ‘brain-dead’ show none of the normal electrical activity associated with the brain of a ‘healthy’ person.
  Despite its initial plausibility, the Identity Theory faces a strong challenge in the form of the thesis of multiple realisability, first formulated by Hilary Putnam (1967).  It is obvious that not only humans, but many different species of animal can, for example, experience pain. However, it seems highly unlikely that all of these diverse organisms with the same pain experience are in the same identical brain state. And if the latter is the case, then pain cannot be identical to a specific brain state. The Identity Theory is thus empirically unfounded.
  On the other hand, even granted all above, it does not follow that identity theories of all types must be abandoned. According to Token Identity theories, the fact that a certain brain state is connected with only one mental state of a person does not have to mean that there is an absolute correlation between types of mental states and types of brain state. The type-token distinction can be illustrated by a simple example: the word green contains four types of letters (g, r, e, n) with two tokens (occurrences) of the letter e along with one each of the others. The idea of Token Identity is that only particular occurrences of mental events are identical with particular occurrences or tokenings of physical events. Anomalous Monism (see below) and most other non-reductive physicalisms are Token-Identity theories. Despite these problems, there is a renewed interest in the Type Identity Theory today, primarily due to the influence of Jaegwon Kim (1984).

4 varieties of Dualism - the arrows indicate the direction of the causal interactions [M=Mental; P=Physical]. Occasionalism is not shown. Graphic courtesy of Richard Bruce Baxter.

The classic Identity Theory and Anomalous Monism in contrast. For the Identity Theory, every token instantiation of a single mental type corresponds (as indicated by the arrows) to a physical token of a single physical type. For anomalous monism, the token-token correspondences can fall outside of the type-type correspondences. The result is token identity. Graphic courtesy of PhidiasNL.

• Functionalism was formulated by Hilary Putnam & Jerry Fodor (1967) as a reaction to the inadequacies of the Identity Theory. Putnam & Fodor saw mental states in terms of an empirical computational theory of the mind. At about the same time or slightly after, D M Armstrong & David Kellogg Lewis (1968) formulated a version of Functionalism which analysed the mental concepts of folk psychology in terms of functional roles. Finally, Wittgenstein's idea of meaning as use led to a version of Functionalism as a theory of meaning, further developed by Wilfrid Sellars & Gilbert Harman (1970).
  What all these different varieties of Functionalism share in common is the thesis that mental states are characterized by their causal relations with other mental states and with sensory inputs and behavioural outputs. That is, Functionalism abstracts away from the details of the physical implementation of a mental state by characterizing it in terms of non-mental functional properties. For example, a kidney is characterized scientifically by its functional role in filtering blood and maintaining certain chemical balances. From this point of view, it does not really matter whether the kidney be made up of organic tissue, plastic nanotubes or silicon chips: it is the role that it plays and its relations to other organs that define it as a kidney.
• Non-reductive Physicalism - many philosophers hold firmly to 2 essential convictions with regard to mind-body relations:-

1. Physicalism is true and mental states must be physical states - but
2. All reductionist proposals are unsatisfactory: mental states cannot be reduced to behaviour, brain states or functional states.
   Hence, the question arises whether there can still be a Non-reductive Physicalism. Donald Davidson's (1980) Anomalous Monism is an attempt to formulate such a physicalism.

A basic idea which all non-reductive physicalists share in common is the thesis of supervenience: mental states supervene on physical states, but are

not reducible to them. Supervenience therefore describes a functional dependence: there can be no change in the mental without some change in the physical.