Philosophy of mind is the philosophical study of the nature of the mind, mental events, mental functions, mental properties, and consciousness. These areas give rise to some very difficult problems and there are many different opinions as to their solutions. But the central issue which overwhelmingly dominates the interest of practitioners of the philosophy of mind is the so-called mind-body problem. It is therefore appropriate to begin our discussion with a brief description of this perennial issue.
Phrenology attempted to correlate mental functions with specific parts of the brain
The Mind-Body Problem
The mind-body problem is essentially the problem of explaining the relationship between minds, or mental processes, and bodily states or processes. It is evident to all of us, for example, that our perceptual experiences depend on stimuli which arrive at our various sensory organs from the external world and that these stimuli affect changes in the states of our brain, ultimately causing us to feel a sensation which may be pleasant or unpleasant. It also seems obvious that someone's desire for a slice of pizza will cause him to move his body in a certain manner in a certain direction until he obtains what he is seeking. But how is it possible that conscious experiences can arise out of an inert lump of gray matter endowed with electrochemical properties? How does someone's desire cause that individual's neurons to fire and his muscles to contract in exactly the right manner? These are some of the essential puzzles that have confronted philosophers of mind at least from the time of René Descartes.
Dualist solutions to the Mind-Body Problem
- Main article: Dualism (philosophy of mind)
Dualism is a set of views about the relationship between mind and matter, which begins with the claim that mental phenomena are, in some respects, non-physical.
The first manifestations of mind-body dualism probably go back thousands of years, to the time when man first began to speculate about the existence of an incorporeal soul (or spirit) which bore the faculties of intelligence and wisdom. We first encounter similar ideas in Western philosophy with the writings of Plato and Aristotle, who maintained, 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. 
However, the best-known version of dualism is due to René Descartes (1641), and holds that the mind is a 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. Hence, he was the first to formulate the mind-body problem in the form in which it still exists today. 
Arguments for dualism
The main argument in favor of dualism is simply that it appeals 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 vague entity, and 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.
Another very important, more modern, argument in favor of dualism consists in 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 obviously do not. For example, what does a burned finger feel like? What does blue sky look like? What does nice music sound 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. And the claim is that qualia seem particularly difficult to reduce to anything physical.
Interactionist dualism, or simply interactionism, is the particular form of dualism first espoused by Descartes in the Meditations. In the 20th century, its major defenders have been Karl Popper and John Eccles. 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 summarized as follows: I have a clear and distinct idea of my mind as a thinking thing which has no spatial extension (i.e., it cannot be measured in terms of length, weight, height, and so on) and I also have a clear and distinct idea of my body as something that is spatially extended, subject to quantification and not able to think. Since Leibniz' law of the identity of indiscernibles says that for any individual entities x and y and for all intrinsic properties F, if x has the property F if and only if y also has the property F, then x and y are identical, it follows that mind and body are not identical because they have radically different properties, according to Descartes.
At the same time, however, it is clear that my mental states (desires, beliefs, etc.) have causal effects on my body and vice-versa: a child touches a hot stove (physical event) which causes pain (mental event) and makes him yell (physical event) which provokes a sense of fear and protectiveness in the mother (mental event) and so on. Descartes' argument obviously depends on the crucial premise that what I believe to be "clear and distinct" ideas in my mind are necessarily true. This idea is rejected by most modern philosophers.
Other forms of dualism
Other important forms of dualism which arose as reactions to, or attempts to salvage, the Cartesian version are:
1) Psycho-physical parallelism, or simply parallelism, is the view that mind and body, while being two distinct substances, do not causally influence one another, but 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 who 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. 
Portrait of Gottfried Wilhelm Leibniz by Bernhard Christoph Francke (circa 1700)
2) Occasionalism is the view espoused by Nicholas Malebranche 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 still different substances on this view, causes (whether mental or physical) are related to their effects by an act of God's intervention on each specific occasion.
3) Epiphenomenalism is a doctrine first formulated by Thomas Henry Huxley. Fundamentally, it consists in the view that mental phenomena are causally inefficacious. 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 (i.e. epiphenomena) of the physical world. The view has been defended most strongly in recent times by Frank Jackson. 
4) Property dualism asserts that when matter is organized in the appropriate way (i.e. 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 and has undergone something of a renaissance in recent years.
Monist solutions to the Mind-Body Problem
Monism states, in contrast to dualism, that there is only one fundamental substance. Today nearly all forms of monism are physicalistic. 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 are possible. Another form of monism, which is more of historical interest, is that which states that the only existing substance is mental. Such idealistic monism is very uncommon in modern times. A third possibility is to accept the existence of a basic substance which is neither physical nor mental. The mental and physical would both be properties of this neutral substance. Such a position was adopted by Baruch Spinoza and popularized by Ernst Haeckel in the 19th century. This neutral monism, as it is called, resembles property dualism. In the following discussion, only physicalistic monisms are considered.
- Main article: Behaviorism
Behaviorism dominated philosophy of mind for much of the 20th century, especially the first half. In psychology, behaviorism 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 are not generalizable. Without generalizability and the possibility of third-person examination, the behaviorists argued, science is simply not possible. The way out for psychology was to eliminate the idea of an interior mental life altogether and focus instead on the description of observable behavior.
Parallel to these developments in psychology, a philosophical behaviorism (sometimes called logical behaviorism) was developed. This is characterized by a strong verificationism, which generally considers unverifiable statements about interior mental life senseless. But what are mental states if they are not interior states on which one can make introspective reports? The answer of the behaviorist is that mental states are just descriptions of behavior and/or dispositions to behave.
Philosophical behaviorism is considered by most modern philosophers of mind to be outdated. Apart from other problems, behaviorism implausibly maintains, for example, that someone is talking about behavior if she reports that she has a wracking headache.
- Main article: Type physicalism
Type physicalism (or type-identity theory) was developed by John Smart and Ullin Place as a direct reaction to the failure of behaviorism. These philosophers reasoned that, if mental states are something material, but not behavior, 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 Identity theory gained its appeal in the context of increasing nonscientific knowledge. Today, sophisticated imaging procedures, such as fMRI (above), furnish ever more knowledge about the workings of the brain
Despite a certain initial plausibility, the identity theory faces at least one heavy challenge in the form of the thesis of multiple realizability, which was first formulated by Hilary Putnam. It seems clear that not only humans, but also amphibians or Martians, for example, can experience pain. On the other hand, it seems very improbable that all of these diverse organisms with the same pain are in the same brain identical brain state. If this is not the case however, then the pain cannot be identical to a certain brain state. Thus the identity theory is empirically unfounded.
But even if this is the case, 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. Despite the problems faced by the type identity theory, there is a renewed interest in it these days, primarily due to the influence of Jaegwon Kim.
- Main article: Functionalism (philosophy of mind)
Functionalism was formulated by Hilary Putnam and Jerry Fodor as a reaction to the inadequacies of the identity theory.  Putnam and 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 and David Lewis formulated a version of functionalism which analyzed 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 Peter Sellars and Gilbert Harman.
What all these different varieties of functionalism share in common is the thesis that mental states are essentially characterized by their causal relations with other mental states and with sensory inputs and behavioral outputs. That is, functionalism quantifies over, or 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.
Many philosophers hold firmly to two essential convictions with regard to mind-body relations:
1. Physicalism is true and mental states must be physical states.
2. All reductionist proposals are unsatisfactory: mental states cannot be reduced to behavior, brain states or functional states.
Hence, the question arises whether there can still be a non-reductive physicalism. Donald Davidson's anomalous monism is an attempt to formulate such a physicalism.
The idea is often formulated in terms of 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.
- Main article: Eliminative materialism
If one is a materialist but believes that all reductive efforts have failed and that a non-reductive materialism is incoherent, then one can adopt a final, more radical position: eliminative materialism. Eliminative materialists maintain that mental states are fictitious entities introduced by everyday "folk psychology". Should "folk psychology", which eliminativists view as a quasi-scientific theory, be proven wrong in the course of scientific development, then we must also abolish all of the entities postulated by it. Eliminativists such as Patricia and Paul Churchland often invoke the fate of other, erroneous popular theories which have arisen in the course of history. For example, the belief in witchcraft turned out to be wrong and the consequence is that most people no longer believe in the existence of witches.
Linguistic Criticism of the Mind-Body Problem
Each attempt to answer the mind-body problem encounters substantial problems. Some philosophers argue that this is because there is an underlying conceptual confusion. Such philosophers reject the mind-body problem as an illusory problem. Such a position is represented in analytic philosophy these days, for the most part, by the followers of Ludwig Wittgenstein and the Wittgensteinian tradition of linguistic criticism. The exponents of this position explain that it is an error to ask how mental and biological states fit together. Rather it should simply be accepted that humans can be described in different ways - for instance, in a mental and in a biological vocabulary. Illusory problems arise if one tries to describe the one in terms of the other's vocabulary or if the mental vocabulary is used in the wrong contexts. This is the case for instance, if one searches for mental states of the brain. The brain is simply the wrong context for the use of mental vocabulary - the search for mental states of the brain is therefore a category error or a pure conceptual confusion.
Today, such a position is often adopted by interpreters of Wittgenstein such as Peter Hacker. However, Hilary Putnam, the inventor of functionalism, has also adopted the position that the mind-body problem is an illusory problem which should be dissolved according to the manner of Wittgenstein. 
Naturalism and its problems
The thesis of physicalism is that the mind is part of the material (or physical) world. Such a position faces the fundamental problem that the mind has certain properties that no material thing possesses. Physicalism must therefore explain how it is possible that these properties can emerge from a material thing nevertheless. The project of providing such an explanation is often referred to as the "naturalization of the mental." What are the crucial problems that this project must attempt to resolve? The most well-known are probably the following two:
- Main article: Qualia
Many mental states have the property of being experienced subjectively in different ways by different individuals. For example, it is obviously characteristic of the mental state of pain that it hurts. Moreover, your sensation of pain may not be identical with mine, since we have no way of measuring how much something hurts or describing exactly how it feels to hurt. Where does such an experience (qualia) come from? Nothing indicates that a neural or functional state can be accompanied by such a pain experience. Often the point is formulated as follows: the existence of cerebral events, in and of themselves, cannot explain why they are accompanied by these corresponding qualitative experiences. Why do many cerebral processes occur with an accompanying experiential aspect in consciousness? It seems impossible to explain.
Yet it also seems to many that science will eventually have to explain such experiences. This follows from the logic of reductive explanations. If I try to explain a phenomenon reductively (e.g., water), I also have to explain why the phenomenon has all of the properties that it has (e.g., fluidity, transparency). In the case of mental states, this means that there needs to be an explanation of why they have the property of being experienced in a certain way.
John Searle - one of the most influential philosophers of mind (Berkeley 2002)
Intentionality is the capacity of mental states to be directed towards (about) or be in relation with something in the external world. This property of mental states entails that they have contents and semantic referents and can therefore be assigned truth values. When one tries to reduce these states to natural processes there arises a problem: natural processes are not true or false, they simply happen. It would not make any sense to say that a natural process is true or false. But mental ideas or judgments are true or false, so how then can mental states (ideas or judgements) be natural processes? The possibility of assigning semantic value to ideas must mean that such ideas are about facts. Thus, for example, the idea that Herodotus was an historian refers to Herodotus and to the fact that he was an historian. If the fact is true, then the idea is true; otherwise, it is false. But where does this relation come from? In the brain, there are only electrochemical processes and these seem not to have anything to do with Herodotus.
Philosophy of mind and science
Humans are corporeal beings and, as such, they are subject to examination and description by the natural sciences. Since mental processes are not independent of bodily processes, the descriptions that the natural sciences furnish of humans beings play an important role in the philosophy of mind. There are many scientific disciplines that study processes related to the mental. The list of such sciences includes: biology, computer science, cognitive science, cybernetics, linguistics, medicine, pharmacology, psychology, etc..
The theoretical background of biology, as is the case with modern natural sciences in general, is fundamentally materialistic. The objects of study are are, in the first place, physical processes, which are considered to be the foundations of mental activity and behavior. The increasing success of biology in the explanation of mental phenomena can be seen by the absence of any empirical refutation of its fundamental presupposition: "there can be no change in the mental states of a person without a change in brain states."
Within the field of neurobiology, there are many subdisciplines which are concerned with the relations between mental and physical states and processes:
The methodological breakthroughs of the neurosciences, in particular the introduction of high-tech neuroimaging procedures, has propelled scientists toward the elaboration of increasingly ambitious research programs: one of the main goals is to describe and comprehend the neural processes which correspond to mental functions (see: neural correlate). A very small number of neurobiologists, such as Emil du Bois-Reymond and John Eccles have denied the possibility of a "reduction" of mental phenomena to cerebral processes, partly for religious reasons. However, the contemporary neurobiologist and philosopher Gerhard Roth continues to defend a form of "non-reductive materialism." 
Computer science concerns itself with the automatic processing of information (or at least with physical systems of symbols to which information is assigned) by means of such things as computers. From the beginning, computer programmers have been able to develop programs which permit computers to carry out tasks for which organic beings need a mind. A simple example is multiplication. But it is clear that computers do not use a mind to multiply. Could they, someday, come to have what we call a mind? This question has been propelled into the forefront of much philosophical debate because of investigations in the field of artificial intelligence ("AI").
Within AI, it is common to distinguish between a modest research program and a more ambitious one: this distinction was coined by John Searle in terms of a weak AI and a strong AI. The exclusive objective of "weak AI", according to Searle, is the successful simulation of mental states, with no attempt to make computers become conscious or aware, etc.. The objective of strong AI, on the contrary, is a computer with consciousness similar to that of human beings. The program of strong AI goes back to one of the pioneers of computation Alan Turing. As an answer to the question "Can computers think?", he formulated the famous Turing test. Turing believed that a computer could be said to "think" when, if placed in a room by itself next to another room which contained a human being and with the same questions being asked of both the computer and the human being by a third party human being, the computer's responses turned out be to indistinguishable from those of the human. The Turing test has received many criticisms, among which the most famous is probably the Chinese room thought experiment formulated by Searle. 
The question about the possible sensitivity (qualia) of computers or robots still remains open. However, at this point in time, most computer scientists are not very optimistic. Some computer scientists believe that the specialty of AI can still make new contributions to the resolution of the mind-body problem. They suggest that based on the reciprocal influences between software and hardware that takes place in all computers, it is possible that someday theories can be discovered that help us to understand the reciprocal influences between the human mind and the brain.
Psychology is the science that investigates mental states directly. It uses generally empirical methods to investigate concrete mental states like joy, fear or obsessions. Psychology investigates the laws that bind these mental states to each other or with inputs and outputs to the human organism.
An example of this is the psychology of perception. Scientists working in this field have discovered general principles of the perception of forms. A law of the psychology of forms says that objects that move in the same direction are perceived as related to each other. This law describes a relation between visual input and mental perceptual states. However, it does not suggest anything about the nature of perceptual states. The laws discovered by psychology are compatible with all the answers to the mind-body problem already described.
Philosophy of mind in the continental tradition
Most of the discussion in this article has focused on the predominant school (or style) of philosophy in modern Western culture, usually called analytic philosophy (sometimes also inaccurately described as Anglo-American philosophy). Other schools of thought exist, however, which are sometimes (also misleadingly) subsumed under the broad label of continental philosophy. In any case, the various schools that fall under this label (phenomenology, existentialism, etc.) tend to differ from the analytic school in that they focus less on language and logical analysis and more on directly understanding human existence and experience. With reference specifically to the discussion of the mind, this tends to translate into attempts to grasp the concepts of thought and perceptual experience in some direct sense that does not involve the analysis of linguistic forms.
In Georg Wilhelm Friedrich Hegel's Phenomenology of Mind, Hegel discusses three distinct types of mind: the subjective mind, the mind of an individual; the objective mind, the mind of society and of the State; and the Absolute mind, a unity of all concepts.
In modern times, the two main schools that have developed in response or opposition to this Hegelian tradition are Phenomenology and Existentialism. Phenomenology, founded by Edmund Husserl, focuses on the contents of the human mind (see noema) and how phenomenological processes shape our experiences.  Existentialism, a school of thought led by Jean-Paul Sartre, focuses on the content of experiences and how the mind deals with such experiences.
An important, though not very well-known, example of a philosopher of mind and cognitive scientist who tries to synthesize ideas from both traditions is Ron McClamrock. Borrowing from Herbert Simon and also influenced by the ideas of existential phenomenologists such as Maurice Merleau-Ponty and Martin Heidegger, McClamrock suggests that man's condition of being-in-the-world ("Da Sein") makes it impossible for him to understand himself by abstracting away from it and examining it as if it were a detached experimental object of which he himself is not an integral part. 
Consequences of Philosophy of Mind
There are countless subjects that are affected by the ideas developed in the philosophy of mind. Clear examples of this are the nature of death and its definitive character, the nature of emotion, of perception and of memory. Questions about what a person is and in what her identity consists also have much to do with the philosophy of mind. There are two subjects that, in connection with the philosophy of the mind, have aroused special attention: free will and the self.
In the context of the philosophy of mind, the question about the freedom of the will takes on a renewed intensity. This is certainly the case, at least, for materialistic determinists. According to this position, natural laws completely determine the course of the material world. Mental states, and therefore the will as well, would be material states. So human behavior and decisions would be completely determined by natural laws. Some take this argumentation a step further: people cannot determine by themselves what they want and what they do. Consequently, they are not free.
Immanuel Kant rejected determinism and defended free will
This argumentation is rejected, on the one hand, by the compatibilists. Those who adopt this position suggest that the question "Are we free?" can only be answered once we have determined what the term "free" means. The opposite of "free" is not "caused" but "compelled" or "coerced". It is not appropriate to identify freedom with indetermination. A free act is one where the agent could have done otherwise if she had chosen otherwise. In this sense a person can be free even though determinism is true. The most important compatibilist in the history of the philosophy was David Hume. Nowadays, this position is defended, for example, by Daniel Dennett.
On the other hand, there are also many incompatibilists who reject the argument because they believe that the will is free in a stronger sense called originationism. These philosophers affirm that the course of the world is not completely determined by natural laws: the will at least does not have to be and, therefore, it is potentially free. The most prominent incompatibilist in the history of philosophy was Immanuel Kant. Critics of this position accuse the incompatibilists of using an incoherent concept of freedom. They argue as follows: if our will is not determined by anything, then we desire what we desire by pure chance. And if what we desire is purely accidental, we are not free. So if our will is not determined by anything, we are not free.
The philosophy of mind also has important consequences for the concept of self. If by "self" or "I" one refers to an essential, immutable nucleus of the person, most modern philosophers of mind will affirm that no such thing exists. The idea of a self as an immutable essential nucleus arises from the Christian idea of an immaterial soul. Such an idea is unacceptable to most contemporary philosophers, due to their physicalistic orientations. However, in the light of empirical results from developmental psychology, developmental biology and the neurosciences, the idea of an essential constant material nucleus - located, for example, in an single, unchanging area of the brain - seems reasonable.
In view of this problem, some philosophers affirm that we should abandon the idea of a self. But this is a minority position. More common is the view that we should redefine the concept: by "self" we would not be referring to some immutable and essential nucleus, but to something that is in permanent change. A well-known defender of this position is Daniel Dennett. It is surprising to see how the reflections of modern philosophy of mind agree on this point with the traditional perspective of some non-European philosophies such as Buddhism.
- For more information and links about topics discussed in the article, see: Portal:Mind and Brain
- For more information about scientific research related to topics discussed in the article, see: Cognitive science
The structure and some of the contents of this article are borrowed from the German language wikipedia article.
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