In contemporary philosophy physicalism is most frequently associated with philosophy of mind, in particular the mind/body problem, in which it holds that the mind is a physical thing in all senses. In other words, all that has been ascribed to "mind" is more correctly ascribed to "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. Some philosophers use the term "materialism" to denote descriptions based on the motions of matter and "physicalism" for descriptions based on matter and world geometry (see: Stoljar 2001).
The ontology of physicalism ultimately includes whatever is described by physics — not just matter but 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 monism.
Supervenience is the most important concept within 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.
It 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, such that one level supervenes on another when there can only be a change in the higher level if there is also a change in 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 in this metaphor is to what extent the levels actually exist independently of their fundamental lowest level - the physical.
Superveniences establish such a relationship between the mental and the physical, so 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, so is the mental dependent upon the physical. Physicalism thus implies (through modal realism) that:
The corresponding conclusion about the mental would be as follows:
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.
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. Another problem is that token physicalism does not capture minimal physicalism, meaning that it does not capture the core commitment of 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.
According to Ullin Place, one of the popularizers of the idea of type-identity in the 1950s and '60s, 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:
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 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 mutilple realizability reigns here as well - any colour has infinite metamers - physical spectral reflectance distributions which can produces indistinguishable colour experiences in the subject. Thus token identity holds between colours and physical/brain states at best.
The physicalist variations discussed above (Token Physicalism aka Property Dualism, and Type Physicalism aka Identity Theory) are all ontologically reductionist, as they reduce mental states and processes into physical states and processes.
Reductive physicalism is not incompatible with eliminativism - the view that psychological states do not exist at all.
All of these 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 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 physical they are not reducible to physical properties. Donald Davidson proposed anomalous monism as a non-reductive physicalism. Supervenience physicalism (also proposed by Donald Davidson) is a non-reductive physicalism, as mental events supervene (i.e. physical properties are identical 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. If we accept reductive physicalism, the pain would be those c-fibres firing.
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.
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 Elliot 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 (see immediately below).
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.
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.
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 that the desire to lift one’s arm as a mental event, and the lifting of one's arm, a physical event. According to the exclusion principle, there must be an event that does not supervene on e while causing e*. This is interpreted as meaning, 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. Yet, this debate has not been settled in the philosophical community..
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. However, this does not seem to have much support. While there are other options when considering the nature of the world, panpsychism in cognitive science, or vitalism in biology, this is irrelevant. The argument merely states that physicalism is the most likely, not that other views are impossible.
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 .
One argument against the conceivability of zombies comes from 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.
However, the previous argument notwithstanding, does the conceivability of zombies entail their possibility? One response 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 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 conceivable. Through this approach to the problem, physicalists can accept that the possibility of zombies is conceivable, while simultaneously denying that zombies are possible.
In response to Davidson's anomalous monism, Kim proposed that one cannot be a physicalist and a non-reductivist. He proposes (using the chart on the right) that M1 causes M2 (these are mental events) and P1 causes P2 (these are physical events). P1 realises M1 and P2 realises M2. However M1 does not causally effect P1 (i.e. M1 is a consequent event of P1). If P1 causes P2, and M1 is a result of P1, then M2 is a result of P2. He says that the only alternatives to this problem is to accept dualism (where the mental events are independent of the physical events) or eliminativism (where the mental events do not exist).
Hempel's Dilemma attacks how physicalism is defined. If, for instance, one defines physicalism as the universe is composed of everything known by physics, one can point out that physics cannot describe how the mind functions. If physicalism is defined as anything which may be described by physics in the future, one is saying nothing.
One possible reply to this dilemma is that over time we see more and more evidence from neurology that mental functions are related to physical neural correlates within the brain. Combined with the observation that phlogiston gave way to thermodynamics, vitalism gave way to cell biology, and other examples of previously dualistic concepts being eroded by continuous scientific progress, it can be argued that the physical basis of the mind will be known sometime in the future.