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Physicalism is a philosophical theory holding that everything which exists is no more extensive than its physical properties; that is, that there are no kinds of things other than physical things. The term was coined by Otto Neurath in a series of early twentieth century essays on the subject, in which he wrote:
- "According to physicalism, the language of physics is the universal language of science and, consequently, any knowledge can be brought back to the statements on the physical objects."
In contemporary philosophy, physicalism is most frequently associated with the mind-body problem in philosophy of mind, regarding which physicalism holds that all that has been ascribed to "mind" is more correctly ascribed to "brain" or the activity of the brain. Physicalism is also called "materialism", but the term "physicalism" is preferable because it has evolved with the physical sciences to incorporate far more sophisticated notions of physicality than matter, for example wave/particle relationships and non-material forces produced by particles.
The related position of methodological naturalism says that philosophy and science should at least operate under the assumptions of natural sciences (and thus physicalism). Physicalism is a strong form of metaphysical naturalism.
The ontology of physicalism ultimately includes whatever is described by physics — not just matter but also energy, space, time, physical forces, structure, physical processes, information, state, etc. Because it claims that only physical things exist, physicalism is generally a form of ontological monism.
Reductive and non-reductive physicalism
Physicalism can be reductive or non-reductive; reductionism is a concept regarding the relationship between the parts of an object and the whole. There are several important concepts relevant to the distinction between the types of physicalism.
Supervenience is a concept with broad applicability throughout philosophy that has particular importance to physicalism. It describes the relationship between the fundamental objects of physical reality and those of everyday experience as well as those of a more abstract social nature. Subtle differences in the interpretation of the supervenience concept underscore different schools of thought within physicalism.
Supervenience can be seen as the relationship between a higher level and lower level of existence where the higher level is dependent on the lower level. One level supervenes on another when there can only be a change at the higher level if there is also a change at the lower level. (e.g., a set of properties A supervenes upon a set of properties B when there cannot be an A difference without a B difference). The debate about this notion as regards physicalism is to what extent mental phenomena exist independently of their (supposed) fundamental lowest level — the physical.
Supervenience establishes such a relationship between the mental and the physical, that any change in the mental is caused by a change in the physical. Just as a shadow is dependent upon the position of the object causing it, is the mental dependent upon the physical. Physicalism thus implies (through modal realism) that:
- No two worlds could be identical in every physical respect yet differ in some other respect.
The corresponding conclusion about the mental would be as follows:
- No two beings, or things could be identical in every physical respect yet differ in some mental respect.
Another description of supervenience does away with levels altogether and rather pictures reality as a matrix or mosaic, upon which we imply different patterns (the old levels) but emphasising that all patterns are variations of the same implicit reality.
However, supervenience alone is not sufficient to establish the basis of physicalism. It is possible that mental or other non-physical states supervene upon the physical. As this allows for the possibility that the mind is causally inefficacious and only contingently related to the physical, supervenience physicalism is compatible with epiphenomenalism. However, when supervenience physicalism and token physicalism are combined, minimal physicalism is met, as will be detailed in the following sections.
Token and type theories
The type/token distinction is easily illustrated by way of example. In the phrase "yellow is yellow is yellow is yellow", there are only two types of words ("yellow" and "is") but there are seven tokens (four of one and three of the other). The thesis of type physicalism consists in the idea that mental event types (e.g., pain in all individual organisms of all species at all times) are, at least contingently, identical with specific event types in the brain (e.g., C-fibre firings in all individual organisms of all species and at all times).
If type physicalism is true then mental state M1 would be identical to brain state B1. This would imply that a specific mental state of pain, for example, would perfectly correlate to a specific brain state in all organisms at all times. However, some qualify this by saying that some mental states are not always reduced to only one specific brain state (see Putnam's multiple realizability). That is, the same mental state can be produced from many different physical brain states. Token physicalism only states that for every particular occurrence, there is a physical particular with which it is identical. Therefore, while the mental state of pain or happiness is not type-identical to any one specific brain state, it is still physical and identical to a particular brain state. It may be helpful to understand that we often use different sets of vocabulary to describe an identical thing, which arise out of different disciplines. For example, a particular color, say, yellow, is a term that is identical to a particular light wavelength within the visible electro-magnetic spectrum. In this case to describe the actual color yellow and to describe the same as a wavelength, is an example of a type-type identity for they are the same thing.
N.B. Though popular, and useful, modern colour science has discredited the view that any colour is identical with any single wavelength. In fact multiple realizability reigns here as well — any colour has infinite metamers — physical spectral reflectance distributions which can produce indistinguishable colour experiences in the subject. Thus token identity holds between colours and physical/brain states at best.
All types of reductive physicalism are grounded in the idea that everything in the world can actually be reduced analytically to its fundamental physical, or material, basis. This is one reason why "physicalism" is often used interchangeably with the word "materialism." Both terms (in these instances) hold that all organic and inorganic processes can be explained by reference to the laws of nature. The general success of physics in explaining a large range of phenomena in terms of a few of these basic natural laws; such as gravity, electricity, composition of mass, has assisted this belief.
The physicalist variation discussed below (type physicalism aka identity theory) is ontologically reductionist, as it reduces mental states and processes into physical states and processes.
Reductive physicalism is compatible with eliminativism - the view that psychological states do not exist at all.
Type physicalism (also known as type identity theory, type-type theory or just identity theory) is the theory, in the philosophy of mind, which asserts that mental events are type-identical to the physical events in the brain with which they are correlated. In other words, that mental states or properties are neurological states or properties. It is called type identity in order to distinguish it from a similar but distinct theory called the token identity theory.
According to Ullin Place, one of the popularizers of the idea of type-identity in the 1950s and 1960s, the idea of type-identical mind/body physicalism originated in the 1930s with the psychologist E. G. Boring and took nearly a quarter of a century to finally catch on and become accepted by the philosophical community. Boring, in a book entitled The Physical Dimensions of Consciousness (1933) wrote that:
To the author a perfect correlation is identity. Two events that always occur together at the same time in the same place, without any temporal or spatial differentiation at all are not two events but the same event. The mind-body correlations as formulated at present, do not admit of spatial correlation, so they reduce to matters of simple correlation in time. The need for identification is no less urgent in this case.
The barrier to the acceptance of any such vision of the mind, according to Place, was that philosophers and logicians had not yet taken a substantial interest in questions of identity and referential identification in general. The dominant epistemology of the logical positivists at that time was phenomenalism, in the guise of the theory of sense-data. Indeed Boring himself subscribed to the phenomenalist creed, attempting to reconcile it with an identity theory and this resulted in a reductio ad absurdum of the identity theory, since brain states would have turned out, on this analysis, to be identical to colors, shapes, tones and other sensory experiences.
The revival of interest in the work of Gottlob Frege and his ideas of sense and reference on the part of Herbert Feigl and J.J.C. Smart, along with the discrediting of phenomenalism through the influence of the later Wittgenstein and J.L. Austin, led to a more tolerant climate toward physicalistic and realist ideas. Logical behaviorism emerged as a serious contender to take the place of the Cartesian "ghost in the machine" and, although not lasting very long as a dominant position on the mind/body problem, its elimination of the whole realm of internal mental events was strongly influential in the formation and acceptance of the thesis of type identity.
The earliest forms of physicalism, growing historically out of materialism, were reductionist. But after Donald Davidson introduced the concept of supervenience to physicalism, non-reductionist physicalism became more popular.
Non-reductive physicalism is the idea that while mental states are caused by physical states they are not reducible to physical properties (i.e. of a different ontological class).
Nonreductive physicalism has been especially popular among philosophers of biology and some biologists, who argue that all biological facts are fixed by physical facts but that biological properties and regularities supervene on so many multiple realizations of macromolecular arrangements that the biological is not reducible to the physical. Prominent exponents of this view are Philip Kitcher and Elliott Sober. Alexander Rosenberg introduced Davidson's notion to the debate in 1978 but thereafter argued against nonreductive physicalism in ways similar to Jaegwon Kim's. There are several varieties of non-reductive physicalism; some of which are opposed to each other.
Supervenience physicalism (proposed by Donald Davidson) is a non-reductive physicalism, as mental events supervene (i.e., physical properties are mapped to mental properties) on physical events rather than mental events reducing to physical events. For example if we accept supervenience physicalism, the pain someone would feel if electrocuted would supervene on the firing of their c-fibres. Whereas if we accept reductive physicalism, the pain would be those c-fibres firing.
Donald Davidson proposed anomalous monism as a non-reductive physicalism. Anomalous monism is a philosophical thesis about the mind-body relationship. It was first proposed by Donald Davidson in his 1970 paper "Mental Events". The theory is twofold and states that mental events are identical with physical events, and that the mental is anomalous, i.e. under their mental descriptions, relationships between these mental events are not describable by strict physical laws. Hence, Davidson proposes an identity theory of mind without the reductive bridge laws associated with the type-identity theory. Since the publication of his paper, Davidson has refined his thesis and both critics and supporters of anomalous monism have come up with their own characterizations of the thesis, many of which appear to differ from Davidson's.
Token physicalism states "for every actual particular (i.e., object, event or process) x, there is some physical particular y such that x=y". This does not entail nor is entailed by supervenience, although if supervenience is true, it does not necessarily rule out token physicalism. The difference between supervenience and token physicalism is simple; token physicalism states that for every mental particular there is a physical particular to which it is identical, while supervenience physicalism states that set A (e.g., mental properties) cannot change unless set B (e.g., physical properties) changes as well. (i.e., A supervenes on B). While nominally a form of physicalism, token physicalism neither affirms nor denies the possibility that physical properties may also have non-supervened mental properties. Supervenience physicalism does deny this possibility.
Still, token physicalism presents at least two problems. It requires that for social, moral, and psychological particulars there must be a physical particular identical with them. Consider the Supreme Court of the United States. The Supreme Court exists, but according to token physicalism, there is a physical object that is identical to the Supreme Court. However, this physical particular does not necessarily exist in any conventional use of the word 'physical'. Supervenience escapes this problem as the social, moral, and psychological particulars are said to supervene on the physical particulars that compose them. Token physicalism does not capture reductive physicalism, i.e., that everything is physical. Simply because every particular has a physical property does not rule out the possibility that some particulars have non-supervenient mental properties.
Emergentism is a theory which came to popularity in the early twentieth century. It is a form of non-reductive supervenience, but one where reality is considered to supervene in a manner more akin to layers, rather than patterns within a single layer, as per later physicalism. These layers are said to be genuinely novel from each other (i.e., the psychological vs. the physical), and is thus a type of dualism. Physicalism is essentially monistic.
Epiphenomenalism isn't exactly a variety of non-reductive physicalism, but it is compatible with some forms of it. Epiphenomenalism is about a one-way causal interaction; it sees mental states as being the byproducts of brain states; with mental states lacking any causal effects on brain states(some types of epiphenomenalism do allow that some mental states can cause other mental states). It was initially an attempt to solve the problem posed by the question 'what is the causal factor, the brain state or the mental state?' in relation to substance dualism; however it is also relevant to the causal significance of qualia; and as such has implications for property dualists and some non-reductionist physicalist theories. According to epiphenomenalism, the physical (brain state) is the causal factor.
Physicalism and materialism are often supposed to be close cousins. But this needn't be the case. On the contrary, one can be both a physicalist and a panpsychist - or even a monistic idealist. Strawsonian physicalists acknowledge the world is exhaustively described by the equations of physics. There is no "element of reality", as Einstein puts it, that is not captured in the formalism of theoretical physics — the quantum-field theoretic equations and their solutions. However, physics gives us no insight into the intrinsic nature of the stuff of the world — what "breathes fire into the equations and makes there a world for us to describe" as arch-materialist Stephen Hawking poetically laments. Key terms in theoretical physics like "field" are defined purely mathematically.
So is the intrinsic nature of the physical, the "fire" in the equations, a wholly metaphysical question? Kant claimed famously that we would never understand the noumenal essence of the world, simply phenomena as structured by the mind. Strawson, drawing upon arguments made by Oxford philosopher Michael Lockwood but anticipated by Russell, turns Kant on his head. Actually, there is one part of the natural world that we do know as it is in itself, and not at one remove, so to speak — and its intrinsic nature is disclosed by subjective properties of one's own conscious mind. Thus it transpires that the "fire" in the equations is utterly different from what one's naive materialist intuitions would suppose.
Arguments for physicalism
One argument is the exclusion principle, which states that if an event e causes event e*, then there is no event e# such that e# is non-supervenient on e and e# causes e*. This comes when one poses this scenario; One usually considers the desire to lift one’s arm a mental event, and the lifting of one's arm, a physical event. According to the exclusion principle, there must be no event that does not supervene on e while causing e*. This is interpreted as meaning that mental events supervene upon the physical. However, some philosophers accept epiphenomenalism, which states mental events are caused by physical events, but physical events are not caused by mental events. However, If e# does not cause e, then there is no way to verify that e# exists. This issue continues to be debated in the philosophical community.
Argument from methodological naturalism
The argument from methodological naturalism has two premises. First, it is rational to form one's metaphysical beliefs based on the methods of natural science. Secondly, the metaphysical world view is one that is led to by the methods of natural science, which is physicalism. Thus, it is most likely that physicalism is true. One reply to this argument is to reject the second premise and state that one is not led to physicalism by the natural sciences.
Arguments against physicalism
Though there have been many objections to physicalism throughout its history, many of these arguments concern themselves with the apparent contradiction of the existence of qualia in an entirely physical world. The most popular argument of this kind is the so-called knowledge argument as formulated by Frank Jackson, titled Mary's room.
The argument asks us to consider Mary, a young girl who has been forced to investigate the world from a black and white room via a black and white television monitor throughout her life. However, she is allowed access to a large number of books, containing all physical knowledge within them. During her time in the room, she eventually comes to know all of the physical facts about the world, including all of the physical facts about color. Now, to the physicalist, it would seem that this would entail Mary knowing everything about the world. However, once she is let out of her room and into the world, it becomes apparent that Mary does not know everything about the world, such as the feeling or experience of seeing color. If Mary did not have such knowledge, how can it be said that everything supervenes upon the physical?
One way the physicalist may respond to this argument is through the ability hypothesis, developed by Lawrence Nemerow and David Lewis. The ability hypothesis draws a distinction between propositional knowledge, such as 'Mary knows that the sky is typically blue during the day', and knowledge-how, such as 'Mary knows how to climb a mountain'. It then states that all that Mary gains from her experience is knowledge-how. This argument shows that while Mary does gain knowledge from her experience, it is not the propositional knowledge which would need to be obtained if the knowledge argument were to be logically sound.
Perhaps one could argue that the circumstances of Mary's room do not include all physical knowledge because it does not include particular wavelengths of light. In other words, for Mary to have the feeling or experience of seeing color, she must be exposed to color. Then because color is a wavelength of light, it too is physical knowledge, and thus must be included in the books that Mary has. She would therefore be exposed to these wavelengths of light while still in the room, and would thus have the feeling or experience of seeing color. But this response seems implausible since Mary is supposed to have all physical knowledge about color, including knowledge about wavelengths, etcetera. The force of the thought experiment is exactly that the "feeling" or "experience" of seeing color is something that cannot be reduced to physical facts or information.
Argument from philosophical zombies
The zombie argument is a thought experiment that states "there is a possible world in which there exist zombies". Zombies are organisms that appear to have consciousness and qualia, but in reality do not. Also, in this case the zombies have to be identical copies of organisms in the actual or other possible world.
It has been argued under reductive physicalism, that one must either believe that anyone including oneself might be a zombie, or that no one can be a zombie — following from the assertion that one's own conviction about being (or not being) a zombie is a product of the physical world and is therefore no different from anyone else's. Daniel Dennett, a critic of the argument, remarks that "Zombies thinkZ they are conscious, thinkZ they have qualia, thinkZ they suffer pains — they are just 'wrong' (according to this lamentable tradition), in ways that neither they nor we could ever discover!" 
The possibility of zombies would also entail that mental states do not supervene upon physical states, a claim that the reductionist physicalist is committed to, and Australian Philosopher David Chalmers argues that the coherent conceivability of a zombie entails a metaphysical possibility. It has also been explained by arguing that the zombie argument rests on the concept of the nature of qualia. If certain non-physical properties exist which match our conception of qualia, then such non-physical properties would be qualia, and zombies would be conceivable and metaphysically possible. However, if there are no non-physical properties, then what we think of as qualia are the physical properties which perform the functional tasks of what we conceive of as qualia. In this scenario, zombies would not be metaphysically possible.
Although it has been argued that zombies in an observed world are indistinguishable from the observer (and therefore non-existent) under the assumption of reductionist physicalism, it has also been argued that zombies are not conceivable. It has been claimed under reductive physicalism, that when a distinction is made in ones mind between a hypothetical zombie and oneself (assumed not to be a zombie), and noting that the concept of oneself under reductive physicalism may ever only correspond to physical reality, the concept of the hypothetical zombie can only be a subset of the concept of oneself and will in this nature also entail a deficit in observables (cognitive systems) thereby contradicting the original definition of a zombie. This argument has been expressed by Daniel Dennett who argues that, "when philosophers claim that zombies are conceivable, they invariably underestimate the task of conception (or imagination), and end up imagining something that violates their own definition". Dennett, in The Unimagined Preposterousness of Zombies (1995) compares consciousness to health.
Supposing that by an act of stipulative imagination you can remove consciousness while leaving all cognitive systems intact — a quite standard but entirely bogus feat of imagination — is like supposing that by an act of stipulative imagination, you can remove health while leaving all bodily functions and powers intact. ... Health isn't that sort of thing, and neither is consciousness.
Hempel's Dilemma attacks how physicalism is defined. One could either define the physical with respect to the entities stipulated by contemporary physics or with respect to some future complete physics. Both options seem problematic. In the first case, it seems reasonable to assume (based on the history of science) that current physical theories will very probably be refined by future scientific discoveries. Therefore, it is very likely that any definition of "the physical" based on the current state of physics would end up being ultimately false. If on the other hand, we define the notion of what is physical based some future idealized physics, we have not effectively defined anything at all because nobody knows what entities a future physical theory might postulate.
A scientifically minded physicalist may, following Andrew Melnyk, accept the first horn of the dilemma; that is, a physicalist could live with the idea that the current definition of physicalism may very likely end up being false as long as he believes the proposition is more plausible than any currently formulated rival proposition, such as dualism. Melnyk maintains that this is precisely the attitude most scientists hold towards current scientific theories anyway. For example, a defender of evolution may well accept that the current formulation of evolutionary theory is likely to be revised in the future, but she defends evolution because she believes current evolutionary theory is more likely than any current rival idea, such as Creationism. Thus, Melnyk's rebuttal, stated very crudely, is that one should define physicalism in relation to current physics, and hold a similar attitude towards its truth as most scientists hold towards the truth of currently accepted scientific theories.
Jaegwon Kim's argument against non-reductivism
Kim proposed that one cannot be a physicalist and a non-reductivist. Physicalism, according to Kim, has a principle of causal closure according to which every physical event is fully accountable in terms of physical causes. Claims that bodily movements are caused by the preceding state of the body and decisions/intentions are overdetermined. In other words, if the preceding state of the body is both necessary and sufficient, additional conditions are irrelevant.
Kim also attributes to physicalism the principle of causal exclusion according to which if one event has a sufficient cause at one time, no other event can be the cause. This then poses a problem for mental causation if one believes in the supervenience thesis of physicalism.
Further divisions of physicalism
A priori and a posteriori physicalism
Physicalism is then further divided depending on whether it can be known a priori or a posteriori that: If physicalism is true, S is the statement that describes the entire physical nature of the world collectively, and S* is the statement that describes the entire nature of the world, then S entails S*.
A priori physicalism holds that the above can be known without observation, i.e., independently from experience. Originally, it was assumed that physicalism was a priori, until Kripke argued in Naming and Necessity for the existence of necessary a posteriori truths. However, an underexplored approach is that of Helmuth Plessner, whose idea of a material a priori based in our biology and environmental interactions is only now being recognized as relevant to a hermeneutics of nature, for example, in the works of Petran Kokelkoren.
A posteriori physicalism holds physicalism as a necessary truth known a posteriori, i.e., known through empirical observation. There are two main interpretations of a posteriori physicalism which exist today. One is that a posteriori truth can be reached a priori by contingent a posteriori truths. The other holds that there are a posteriori truths that are taken from non-contingent (i.e., necessary) truths. A problem arises when the former is combined with "S entails S*", leading to a contradiction. This view remains controversial within analytic philosophy.
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