The mind-body problem examines the relationship between the human body and the mind. Philosophical positions on this question tend to be predicated on either the reduction of one or the other, or a belief in the discrete coexistence of both.

Philosophical perspectives

If the mind is not seen as a "mysterious" substance, and it is assumed there are only mental events and that "the mind" is no more than a series of mental events, then it is possible to inquire about the relation between mind and body in terms of the relation between mental events and physical events. One can ask: are mental events completely different from physical events, so that you can't explain what mental events are in terms of physical events; or are mental events somehow explainable as being identical with certain physical events? For example, when John feels a pain, a mental event is occurring; is that pain identical to something that is physically occurring in John's brain, such as the firing of some special group of neurons?

The mind-body problem can be introduced more fully with the following example. Suppose John decides to walk across the room, whereupon he does in fact walk across the room. John's decision is a mental event and his walking across the room is a physical event. In addition, there is another physical event involved, namely, something occurs in John's brain that tells John's legs to start walking. This brain event is closely connected with John's decision; the brain event happens at about the same time or right after (or right before!), John decides to walk across the room. One might ask: how is it possible that a decision, a mental event, results in something physical happening in John's brain? If it is claimed that the mental and the physical are completely different, then how can one have any causal impact on the other? How can a mere mental event, a decision, actually 'cause' neurons in one's brain to start firing? (Or, conversely, how can the firing of neurons in one's brain 'cause' a mental event, e.g., a conscious decision.)

A different philosophical view describes the situation thus: John's decision is itself a physical event. When John decides to take a walk across the room, a group of neurons fire in his brain. He is not aware of those neurons; but the firing of those neurons is itself just the same as his decision. There isn't any more to the decision than that physical event. In this view there's no trouble thinking about how a mental event can have a physical effect; mental events are themselves physical. Ultimately, everything is physical.

To many it may sound strange to say that a mental process is no more than a special kind of physical process. Many believe the mind is more spiritual, ethereal, and is simply not the sort of thing that can be physical. Still, there are other reasons for rejecting this reduction of the mental to the physical.

In the past, some philosophers have believed instead that the reduction goes the other way, that the body should be explained in terms of mental events. In this way, the physical is reduced to the mental. In this view, when John walks across the room, really that was only happening in John's mind and perhaps also in each of our minds individually at the same time. There is, in this view, nothing more to John's walking across the room than our having the thought, or the perception, that it happens. This view

The three above-described views about the relationship between the mental and the physical have names:

• Dualism is the view that mental events and physical events are totally different kinds of events.

• Materialism, or physicalism, is the view that mental events are nothing more than a special kind of physical event.

• Phenomenalism, or subjective idealism, is the view that physical events are nothing more than a special kind of mental event.

Materialism and Phenomenalism are two opposite forms of monism since they both assume that only a single kind of substance (respectively, matter or mind) exists.

The mind-body problem, to put it as generically and broadly as possible, is this question: "What is the basic relationship between the mental and the physical?" For the sake of simplicity, we can state the problem in terms of mental and physical events. It could just as well be put in terms of processes, or of consciousness. So the problem restated is: "What is the basic relationship between mental events and physical events?"

There are, then, three basic views: mental and physical events are totally different, and cannot be reduced to each other (dualism); mental events are to be reduced to physical events (materialism); and physical events are to be reduced to mental events (phenomenalism). To put it in terms of what exists "ultimately", we could say that according to dualism, both mental and physical events exist ultimately; according to materialism, only physical events exist ultimately; and according to phenomenalism, only mental events exist ultimately. Materialism and phenomenalism are both varieties of monism, and of monism there is one further variety, namely neutral monism.

Dualism

Dualism is the idea that the mental and the physical are two completely different kinds of things.

Within Western Philosophy, the first major proponent of Dualism was Plato, who put forward a concept that has come to be known as Platonic idealism. Platonic idealism is the theory that the substantive reality around us is only a reflection of a higher truth. That truth, Plato argues, is the abstraction. A particular tree, with a branch or two missing, possibly alive, possibly dead, and initials of two lovers carved into its bark, is distinct from the form of Tree-ness. A Tree is the ideal that each of us holds that allows us to identify the imperfect reflections of trees all around us.

Perhaps the most famous adherent of Dualism was Descartes, who proposed a type of Dualism that has come to be known as Cartesian dualism or Interaction Dualism. Cartesian dualism is the idea that Mind and Body are two fundamentally different types of things, but that they can interact. Physical events can cause mental events—for example, the physical act of hitting your hand with a hammer can cause the mental experience of pain. Conversely, Mental events can cause physical events—for example, the mental decision to speak can cause the physical act of your tongue to move.

Epiphenomenalism is another type of dualism. Like other forms of Dualism, it holds that Mind and Body are two fundamentally different types of things. Epiphenomenalism agrees with Cartesian dualism in saying that physical causes can give rise to mental events—the physical act of hitting your hand with a hammer will create the mental experiene of pain. Unlike Cartesian dualism, Epiphenomenalism argues that mental events cannot under any circumstances give rise to physical effects. So, if my hand touches fire, the physical heat can cause the mental sensation

Parallelism is a form of dualism that argues that both mental and physical events are two fundamentally different types of things which can never interact in any way. This view admits that physical events appear to cause mental effects (Hitting your hand with a hammer appears to cause pain) and that mental events appear to cause physical effects (Deciding to speak appears to cause your tongue to move). Parallelism, however, holds that this correspondence between the mental world and the physical world is simply a correlation, not the result of causation. In this view, the mental world and the physical world are parallel, but separate, never directly interacting.

Physicalism

Physicalism is the idea that everything in the universe can be explained by physical entities such as matter and energy. In this view, while mental entities (such as thoughts and feelings) might at first appear to be a completely novel type of thing, in reality, the mental is completely explainable by the physical. For example, the software of a computer isn't some "magic" mental substance—rather, the software is entirely explainable by looking at the computer's physical hardware. A computer's behavior is ultimately governed by physics. Physicalism argues that ultimately the physical world and its laws explain the behavior of everything in the universe, including the behavior of humans.

In colloquial speech, Physicalism is sometimes simply called Materialism. Technicially, however, Materialism is a very specific subtype of Physicalism which claims everything in the universe is matter. So Physicalism accepts non-material physical entities such as energy, space, and time; technically speaking, Materialism does not accept the independent existence of anything but matter.

Phenomenalism

Phenomenalism is the view that physical objects, properties, events (whatever is physical) are reducible to mental objects, properties, events. Ultimately, only mental objects exist. According to phenomenalism, the material world is not unlike a dream. When you have a vivid dream, you find yourself in a dream world that appears to be composed of material objects. In reality, however, everything in your dreamworld is a creation of your dream. If you dream you are riding a bicycle, the bicycle certainly feels real. In reality, however, the bicycle does not have an independent existence outside of your own mind. When you awaken, the bicycle will cease to be. Phenomenalism holds that the entire "real world" of our waking lives is fundementally a mental creation, not unlike the dream world.

Neutral monism

A fourth position is called neutral monism. This view denies that the Mental and the Physical are two fundamentally different things. Rather, Neutral monism claims the universe consists of only one kind of primal stuff, which in itself is neither mental nor physical, but is capable of both mental and physical aspects or attributes. This view was introduced by the 17th century Dutch philosopher Baruch Spinoza.

New Mysterianism

Another philosophical viewpoint, known as New Mysterianism, holds that the solution to the mind-body problem is unsolvable, particularly by humans. Just as a mouse could never understand human speech, perhaps humans simply lack the capacity to understand the solution to the mind-body problem.

Theological perspectives

Many atheists endorse physicalism (or materialism). To most atheists the physical world is all that is real. Souls, spirits, and minds either are completely mythical, or else they are explainable by physical events. Some atheists, such as Colin McGinn, reject physicalism but argue that ultimately we cannot know or understand the correct alternative answer.

Mainstream Christianity has generally adhered to substance dualism, also known as Cartesian dualism. In their view, both the material world and the spiritual world are real, and the two worlds can interact. Each person has an immaterial soul which inhabits a physical body and will survive the death of the physical body. Souls can affect the physical world (through free will) and God can affect the physical world (through miracles). Jesus was an immortal spirit that took on physical form (the "Word made flesh", see Logos).

Occasionalism is a theological form of parallelism—it argues that mental events can never cause physical effects, and that physical events can never cause mental effects. Rather, Occasionalism holds that all events are ultimately caused directly by God. For a variety of reasons, most mainstream religions do not subscribe to Occasionalism. See Occasionalism for a wider discussion.

Idealism also has its theological versions: Several modern religious movements and texts, for example the organisations within the New Thought Movement (especially the Unity Church) and the book, A Course in Miracles, may be said to have a particularly Idealism-inspired orientation. The theology of Christian Science is explicitly idealist.

Some religions view the mental world as superior, while regarding the physical world as inherently flawed, inferior, or painful. Theologies with this view include Gnostic Christianity, Christian Science, and subsets of Hinduism and Buddhism.

Scientific perspectives

Most neuroscientists believe in the identity of mind and brain, a position that may be considered related to materialism and physicalism, though there is a subtle difference; namely, that postulating an identity between mind and brain (or more specifically, particular types of neuronal interactions) does not necessarily imply that mental events are 'nothing more' than physical events, but rather is more akin to saying that physical events and mental events are different aspects of a more fundamental mental-physical substratum which can be perceived as both mental and physical, depending on perspective.

Since most neuroscientists believe in the identity of mind and brain, it may not be surprising to hear that the search for the neural correlate of consciousness (NCC) has become something of a Holy Grail in the neuroscientific community.

A major shift in the neurosciences occurred in the 1990s: the topic of consciousness and its relation to brain function has become a respectable topic that many neuroscientists take seriously. Prior to the 1990s, few neuroscientists spoke of consciousness, and even fewer would be bold enough to try to approach the topic scientifically. Consciousness was not considered a topic that was amenable to the methods of science. The tide change in the neuroscientific community of the 1990s is largely due to outspoken scientists such as Nobel-laureate Francis Crick, and philosophers such as David Chalmers. While neuroscience has not yet solved the mind-brain problem in terms of coming up with an NCC, to many in the field, the next decade looks promising. Future research may soon reveal how far science can go in addressing and solving the question of the mind-brain problem.

A research team at Caltech discovered in 2005 that individual brain cells can fire in response to visual stimuli of individual people or places, which conflicted with previous conjectures that visual recognition was an emergent phenomenon of large networks of neurons. This development gives weight to Roger Penrose's suggestion in his 1994 book Shadows of the Mind: A Search for the Missing Science of Consciousness that the activity giving rise to consciousness occurs inside neurons at the scale where quantum phenomena transitions to classical physics.

See also

• consciousness
• Daniel Dennett
• dualism (philosophy of mind)
• functionalism (philosophy of mind)
• materialism
• Mental body
• Mind
• monism
• Odic force
• Thomas Nagel
• panpsychism
• Philosophy of mind
• physicalism
• pluralism (philosophy of mind)
• qualia
• solipsism
• supervenience
• Theory of mind

Problème corps-esprit הבעיה הפסיכופיזית 心身問題の哲学