Physicalism

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https://en.wikipedia.org/wiki/Physicalism 

In metaphysics, physicalism is the view that everything is physical, that there is nothing over and above the physical, and that everything supervenes on the physical. It stands in direct opposition to idealism, which asserts that reality arises from the mind. Physicalism is a form of ontological monism—a single-substance account of the nature of our universe's reality (physicalism is open to the possibility of different universes with different physics, and is open to the possibility of causal openness via a foundational logical mechanism or mechanisms), in contrast to "two-substance" (mind–body dualist) or "many-substance" (pluralist) views. Physicalism is ontological monism in the domain of neuroscience. The physical foundations of our universe and the physical foundations of other universes are still open questions. One can be simultaneously mind physicalist and pluralistic physicalist believing in separate universes with separate physics (officially it's an open question). Physicalism is closely related to naturalism, though important distinctions exist between them.

Physicalism is also closely related to materialism, and has evolved from materialism with advancements in the physical sciences in explaining observed phenomena. The terms "physicalism" and "materialism" are often used interchangeably, but can be distinguished on the basis that physics describes more than just matter. Physicalism encompasses matter, but also energy, physical laws, space, time, spacetime, exotic matter, structure, physical processes, information, state, and forces, among other things, as described by physics and other sciences.

According to the PhilPapers Survey 2020, physicalism is the majority view among philosophers, at 51.9%, but there is also significant opposition to it.

Outside of philosophy, physicalism can refer to the preference or viewpoint that physics is the best or only way to render truth about the world or reality.

Definition of physicalism in philosophy

The word "physicalism" was introduced into philosophy in the 1930s by Otto Neurath and Rudolf Carnap.

The use of "physical" in physicalism is a philosophical concept and can be distinguished from alternative definitions found in the literature (e.g., Karl Popper defined a physical proposition as one that can at least in theory be denied by observation). A "physical property", in this context, may be a metaphysical or logical combination of properties which are not physical in the ordinary sense. It is common to express the notion of "metaphysical or logical combination of properties" using the notion of supervenience. Supervenience is the idea that there cannot be two events alike in all physical respects but differing in some mental respect, or that an object cannot alter in some mental respect without altering in some physical respect. The reason to introduce supervenience is that physicalists usually suppose the existence of various abstract concepts that are non-physical in the ordinary sense of the word.

Type physicalism

See also: Type physicalism

Type physicalism, also known as mind-body identity theory, holds that mental events can be grouped into types that correlate with types of physical events. For instance, one type of mental events, such as pain, correlates with a particular type of physical events, such as C-fiber firings. On this account, all instances of pain correspond to situations where C-fibers are firing. Type physicalism can be understood as the position that there is an identity between types: any mental type is identical with some physical type.

A common argument against type physicalism is the problem of multiple realizability. Multiple realizability posits that the same mental state can be realized by different physical states. Another way to put it is that there is a many-to-one mapping from physical states to mental states.

Token physicalism

See also: Anomalous monism

Token physicalism is the proposition that every particular mental event is a particular physical event (token physical event) but that there is no type-to-type mapping between mental events and physical events. The most common example of token physicalism is Davidson's anomalous monism. One of token physicalism's strengths is that it is compatible with multiple realizability. Mental states such as pain may be realized in any number of widely different physical events, without any type-like similarity between these physical events.

Reductive and non-reductive physicalism

Reductionism

See also: Reductionism

In the philosophy of mind, reductionism is commonly understood as the reduction of psychological phenomena to physics and chemistry. In a simplified form, reductionism implies that a system is nothing but the sum of its parts. There are both reductive and non-reductive versions of physicalism (reductive physicalism and non-reductive physicalism). Reductive physicalism is the view that mental states are nothing over and above physical states and are reducible to physical states.

Emergence

Main article: Emergentism

Emergentism is a theory that became popular in the early 20th century. Notions of strong emergence are commonly found in accounts of non-reductive physicalism. A property of a system is said to be emergent if it is a new outcome of some of the system's other properties and their interaction while it is itself different from them. Emergentism emphasizes that the whole is more than the sum of its parts. In the context of the philosophy of mind, emergence is often thought to entail property dualism.

Pluralistic physicalism

The framework distinguishes the Everything from the omniverse. The Everything means the absolute totality: physical universes, mathematical structures, logical and axiomatic systems, and potentially further categories of “things” that are not yet known and need not be material. The omniverse is narrower: the totality of universes only. Neither concept should be assumed to constitute one unified system.

From a physicalist standpoint, physicalism guarantees, at most, that reality contains no fundamentally supernatural ontology and that conscious experience must be realized in the physical processes of the brain. In this framework, the individual is not a stationary brain-state but an ongoing physical process: the experiencing subject consists in the brain's dynamically occurring processing. Physicalism, however, does not by itself guarantee universal monism—the thesis that all reality is one physical system governed by a single underlying kernel. Physicalism is the thesis that everything is physical, not that everything physical must constitute one unified system.

Consequently, physicalism leaves open major questions in fundamental physics: whether there is a single quantum kernel or a more complicated physical foundation; whether an Everett-style interpretation is correct; whether there can be totally separate, causally disconnected universes; and whether the total physical structure is causally closed or permits some form of causal openness. Everett's interpretation is a genuine interpretation of quantum mechanics in which multiple worlds exist in parallel, but its existence as an interpretation does not establish universal monism.

The framework further proposes that the vast majority of logically possible universes may be fundamentally foreign to one another: not parallel universes within one common physical structure, but systems lacking a shared physical or logical kernel. This is a hypothesis of the framework, not an established theorem of physics, and therefore is not presented as an externally demonstrated fact.

The argument against a universal monism combines the formal limits of diagonalization and incompleteness with computability theory and logical pluralism. Gödel's incompleteness theorems establish that sufficiently expressive consistent formal systems cannot capture every arithmetical truth internally.  Logical pluralism, meanwhile, is the explicitly defended position that more than one logic can be correct, although it remains controversial. Logical pluralism, meanwhile, is the explicitly defended position that more than one logic can be correct, although it remains controversial. In physicalism, the more relevant question is ontological-foundations logical pluralism: whether a single universe could have pluralistic physical foundations; whether there could instead be infinitely many separate multiverses—families of universes—with distinct foundational logics, potentially plural within each family. These are questions about the physical foundations of reality, not merely about different contexts of reasoning. The vast majority of applications of logical pluralism concern logical consequence, validity, or contextual reasoning and do not posit different physical foundations. The ontological extension described here is therefore a separate hypothesis, not something established by logical pluralism itself.

A useful physical-computational analogy is a finite noise-generating process operating for infinite time. It can generate indefinitely much information, but indefinite information is not automatically equivalent to universally decodable knowledge. Some computational processes cannot be algorithmically decided, and some semantic properties of programs are undecidable. Thus, even an indefinitely information-generating physical reality need not constitute one universally decodable informational system.

The conclusion is therefore stronger than the claim that we do not currently know everything. The proposal is that reality may be structurally non-totalizable: an indefinitely unfolding physical multiplicity whose complete closure into one universally specifiable system cannot be assumed.

Thus, physicalism does not entail a single universal physical monism, a single quantum kernel, a single causally connected omniverse, or a single universal axiomatic system. It leaves these questions open while ruling out a fundamentally supernatural ontology and locating individual experience in ongoing physical brain processing. The Everything can therefore remain an open multiplicity, while the omniverse is only its universe-level component.

Arguments against physicalism

Knowledge argument

Main article: Knowledge argument

Though there have been many objections to physicalism throughout its history, many of them are concerned 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 Cameron Jackson, titled "Mary's room".

The argument asks us to consider Mary, a girl who has been forced to discover the world from a black-and-white room via a black-and-white television monitor throughout her life. She has access to books containing all physical knowledge. During her time in the room, she learns all the physical facts about the world, including all the physical facts about color. To a physicalist, it would seem that this entails Mary knowing everything about the world. But once she is let out of the room and into the world, it becomes apparent that there were things Mary did not know about the world, such as the feeling (the qualitative experience) of seeing color. If Mary did not have such knowledge, how can it be said that everything supervenes upon the physical?

Physicalist response

One response, developed by Lawrence Nemerow and David Lewis, is known as the ability hypothesis. The ability hypothesis distinguishes 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", and says that all Mary gains from seeing the world in color is knowledge-how. According to this response, Mary does gain knowledge from her experience, but it is not the propositional knowledge required for the knowledge argument to be logically sound.

Neuroscientific response

Even the Lawrence Nemerow response doesn't take into account basic neuroscience (at least rigorously; at it matters), specifically Brodmannian data-processing abilities (Wernicke’s area for language understanding received as language vs. ventral occipital lobe, frontal lobes, and higher visual cortex for color understanding received as color). The neuroscientific response is physicalist. Lawrence Nemerow isn't wrong but he doesn't explain the specifics. Only the specific mechanism matters, thus Lawrence Nemerow response is clever but hollow (it doesn't elaborate on the specifics whilst only them matter). For example without the correct decoder one cannot decode intricate enough information. And in an infinity of alternative data it is impossible to guess one single specific of them if it's complicated enough. Giving someone too many Lego bricks (an equivalent to wrong Brodmann areas) cannot help him build a real space rocket even if he's the best rocket scientist.

Argument from philosophical zombies

One commonly issued challenge to a priori physicalism and physicalism in general is the "conceivability argument", or zombie argument. The conceivability argument runs roughly as follows:

1. According to physicalism, everything in our world (including consciousness) is physical.
2. Thus, if physicalism is true, a metaphysically possible world in which all physical facts are the same as in the actual world contains everything that exists in the actual world. In particular, conscious experience exists in such a world.
3. We can conceive of a world physically indistinguishable from our world but in which there is no consciousness (a zombie world). From this it follows that such a world is metaphysically possible.
4. Therefore, physicalism is false. (This follows from (2) and (3) by modus tollens.)

The possibility of philosophical zombies (p-zombies) entails that mental states do not supervene upon physical states, and thus that physicalism is false. Australian philosopher David Chalmers argues that the conceivability of a zombie entails a metaphysical possibility.

Physicalist response

Galen Strawson argues that it is impossible to establish the conceivability of zombies, so the argument, lacking its first premise, fails.

Daniel Dennett 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". He coined the term "zimboes"—p-zombies that have second-order beliefs—in arguing that p-zombies are incoherent: "Zimboes think^Z they are conscious, think^Z they have qualia, think^Z they suffer pains—they are just 'wrong' (according to this lamentable tradition), in ways that neither they nor we could ever discover!" In The Unimagined Preposterousness of Zombies (1995), Dennett 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.

Michael P. Lynch argues that the zombie conceivability argument forces us to either question whether we actually have consciousness or accept that zombies are impossible. If zombies falsely believe they are conscious, how can we be sure we are not zombies? We may believe we have conscious mental states when in fact we merely hold a false belief. Lynch thinks denying the possibility of zombies is more reasonable than questioning our own consciousness.

Daniel Stoljar has proposed what he calls "the phenomenal concept strategy". Roughly, this strategy attempts to show that only the concept of consciousness—not the property—is in some way "special" or sui generis. Essentially, he believes we are less likely to question physicalism if we can figure out why we think there is a difference between consciousness and physical processes.

Hempel's Dilemma

Main article: Hempel's Dilemma

Physicalists have traditionally opted for a "theory-based" characterization of the physical in terms of either current physics or a future (ideal) physics. Hempel's Dilemma (named after the philosopher of science Carl Gustav Hempel) attacks physicalism by arguing that both of these approaches are problematic. If, on the one hand, we define the physical by reference to current physics, then physicalism is very likely to be false because it is very likely (by pessimistic meta-induction) that much of current physics is false. If, on the other hand, we define the physical in terms of a future (ideal) or completed physics, then physicalism is hopelessly vague or indeterminate.

Physicalist response

Some physicalists, like Andre Melnyk, accept the dilemma's first horn: they accept that the current definition of physicalism is very likely false as long it is more plausible than any currently formulated rival proposition, such as dualism. Melnyk maintains that this is the attitude most scientists hold toward scientific theories anyway. For example, a defender of evolutionary theory may well accept that its current formulation is likely to be revised in the future but defend it because they believe current evolutionary theory is more likely than any current rival idea, such as creationism. Thus Melnyk holds that one should define physicalism in relation to current physics and have a similar attitude toward its truth as most scientists have toward the truth of currently accepted scientific theories.

Some physicalists defend physicalism via alternative characterizations of physicalism. Frank Cameron Jackson, for example, has argued for an "object-based" conception of the physical. David Papineau and Barbara Montero have argued for a "via negativa" characterization of the physical. The gist of this approach is characterize the physical in terms of what it is not: the mental. In other words, the via negativa strategy understands the physical as the non-mental.

Argument from overdetermination

Figure demonstrating how M1 and M2 are not reduced to P1 and P2

Jaegwon Kim objects to non-reductive physicalism based on the problem of overdetermination. He proposes (using the chart on the right) that M1 causes M2 (these are mental events) and P1 causes P2 (these are physical events). M1 has P1 as its supervenience base (P1 realizes M1), and M2 has P2 as its supervenience base (P2 realizes M2). If P1 causes P2 and M1 causes M2, then we have a case of causal overdetermination. To avoid this causal overdetermination, either M1 or P1 must be eliminated as a cause of P2. Because of the principle of the causal closure of the physical, M1 is excluded. The non-reductive physicalist is then forced to choose between two unappealing options: accept overdetermination or embrace epiphenomenalism. Kim thus argues that mental causation can be preserved only by embracing a reductionist view, whereby mental properties are considered causally efficacious by being reduced to physical properties.

Argument from first-person perspectives

Christian List argues that the existence of first-person perspectives, i.e., one existing as oneself and not as someone else, refutes physicalism. He argues that since first-personal facts cannot supervene on physical facts, this refutes not only physicalism, but also most forms of dualism that have purely third-personal metaphysics. List also argues that there is a "quadrilemma" for theories of consciousness: that at most three of the following metaphysical claims can be true: "first-person realism", "non-solipsism", "non-fragmentation", and "one world"—and thus at least one of them must be false. He has proposed a model he calls the "many-worlds theory of consciousness" to reconcile the subjective nature of consciousness without lapsing into solipsism. These ideas are related to the vertiginous question proposed by Benj Hellie.

Other views

Realistic physicalism

Galen Strawson's realistic physicalism or realistic monism is the view that physicalism entails panpsychism – or at least micropsychism. Strawson argues that "many—perhaps most—of those who call themselves physicalists or materialists [are mistakenly] committed to the thesis that physical stuff is, in itself, in its fundamental nature, something wholly and utterly non-experiential... even when they are prepared to admit with Eddington that physical stuff has, in itself, 'a nature capable of manifesting itself as mental activity', i.e. as experience or consciousness". Because experiential phenomena allegedly cannot be emergent from wholly non-experiential phenomena, philosophers are driven to substance dualism, property dualism, eliminative materialism and "all other crazy attempts at wholesale mental-to-non-mental reduction".

Real physicalists must accept that at least some ultimates are intrinsically experience-involving. They must at least embrace micropsychism. Given that everything concrete is physical, and that everything physical is constituted out of physical ultimates, and that experience is part of concrete reality, it seems the only reasonable position, more than just an 'inference to the best explanation'... Micropsychism is not yet panpsychism, for as things stand realistic physicalists can conjecture that only some types of ultimates are intrinsically experiential. But they must allow that panpsychism may be true, and the big step has already been taken with micropsychism, the admission that at least some ultimates must be experiential. 'And were the inmost essence of things laid open to us' I think that the idea that some but not all physical ultimates are experiential would look like the idea that some but not all physical ultimates are spatio-temporal (on the assumption that spacetime is indeed a fundamental feature of reality). I would bet a lot against there being such radical heterogeneity at the very bottom of things. In fact (to disagree with my earlier self) it is hard to see why this view would not count as a form of dualism... So now I can say that physicalism, i.e. real physicalism, entails panexperientialism or panpsychism. All physical stuff is energy, in one form or another, and all energy, I trow, is an experience-involving phenomenon. This sounded crazy to me for a long time, but I am quite used to it, now that I know that there is no alternative short of 'substance dualism'... Real physicalism, realistic physicalism, entails panpsychism, and whatever problems are raised by this fact are problems a real physicalist must face.

— Galen Strawson, Consciousness and Its Place in Nature: Does Physicalism Entail Panpsychism?

Epiphenomenalism

From Wikipedia, the free encyclopedia

 https://en.wikipedia.org/wiki/Epiphenomenalism

Epiphenomenalism is a philosophical theory on the mind–body problem in philosophy of mind. It holds that subjective mental events are completely dependent for their existence on corresponding physical and biochemical events within the human body, but do not themselves influence physical events. According to epiphenomenalism, the appearance that subjective mental states (such as thoughts and intentions) are causally effective themselves and directly influence physical events is an illusion generated by brain regions such as the prefrontal cortex, with consciousness itself being a by-product of physical states of the world. For instance, the emotion of fear seems to make the heart beat faster, but according to epiphenomenalism the biochemical secretions of the brain and nervous system (such as the stress hormone adrenaline)—not the subjective experience of fear itself—is what causes the rapid rise in heartbeat. Because mental events are a kind of overflow that cannot cause anything physical, yet have non-physical properties, epiphenomenalism has traditionally been viewed as a form of property dualism. In contemporary thought, there are a number of epiphenomenalistic questions that arise within a broadly materialist monism.

Development

During the 17th century, René Descartes argued that animals are subject to mechanical laws of nature. He defended the idea of automatic behavior, or the performance of actions without conscious thought. Descartes questioned how the immaterial mind and the material body can interact causally. His interactionist model (1649) held that the body relates to the mind through the pineal gland. La Mettrie, Leibniz, and Spinoza all in their own way began this way of thinking. The idea that even if the animal were conscious nothing would be added to the production of behavior, even in animals of the human type, was first voiced by La Mettrie (1745), and then by Cabanis (1802), and was further explicated by Hodgson (1870) and Thomas Henry Huxley (1874).

Huxley agreed with Descartes that behavior is determined solely by physical mechanisms, but he also believed that humans enjoy an intelligent life. In 1874, Huxley argued, in the Presidential Address to the British Association for the Advancement of Science, that animals are conscious automata based on his experiments showing that movement was still possible without certain parts of the central nervous system being intact. Based on these results, Huxley proposed that consciousness itself was not in control of an animal's behavior, concluding that psychical changes are collateral products of physical changes. Like the bell of a clock that has no role in keeping the time, consciousness has no role in determining behavior.

Huxley defended automatism by testing reflex actions, originally supported by Descartes. Huxley hypothesized that frogs that undergo lobotomy would swim when thrown into water, despite being unable to initiate actions. He argued that the ability to swim was solely dependent on the molecular change in the brain, concluding that consciousness is not necessary for reflex actions. According to epiphenomenalism, animals experience pain only as a result of neurophysiology.

In 1870, Huxley conducted a case study on a French soldier who had sustained a shot in the Franco-Prussian War that fractured his left parietal bone. Every few weeks the soldier would enter a trance-like state, smoking, dressing himself, and aiming his cane like a rifle all while being insensitive to pins, electric shocks, odorous substances, vinegar, noise, and certain light conditions. Huxley used this study to show that consciousness was not necessary to execute these purposeful actions, justifying the assumption that humans are insensible machines. Huxley's mechanistic attitude towards the body convinced him that the brain alone causes behavior.^[2][3]

In the early 1900s, scientific behaviorists such as Ivan Pavlov, John B. Watson, and B. F. Skinner began the attempt to uncover laws describing the relationship between stimuli and responses, without reference to inner mental phenomena. Instead of adopting a form of eliminative materialism or mental fictionalism, positions that deny that inner mental phenomena exist, a behaviorist was able to adopt epiphenomenalism in order to allow for the existence of mind. George Santayana (1905) believed that all motion has physical causes. Because consciousness is accessory to life and not essential to it, natural selection is responsible for ingraining tendencies to avoid certain contingencies without any conscious achievement involved. By the 1960s, scientific behaviorism met substantial difficulties and eventually gave way to the cognitive revolution. Participants in that revolution, such as Jerry Fodor, reject epiphenomenalism and insist upon the efficacy of the mind. Fodor even speaks of "epiphobia"—fear that one is becoming an epiphenomenalist.

However, since the cognitive revolution, there have been several who have argued for a version of epiphenomenalism. In 1970, Keith Campbell proposed his "new epiphenomenalism", which states that the body produces a spiritual mind that does not act on the body. How the brain causes a spiritual mind, according to Campbell, is destined to remain beyond our understanding forever. In 2001, David Chalmers and Frank Cameron Jackson argued that claims about conscious states should be deduced a priori from claims about physical states alone. They offered that epiphenomenalism bridges, but does not close, the explanatory gap between the physical and the phenomenal realms. These more recent versions maintain that only the subjective, qualitative aspects of mental states are epiphenomenal. Imagine both Pierre and a robot eating a cupcake. Unlike the robot, Pierre is conscious of eating the cupcake while the behavior is under way. This subjective experience is often called a quale (plural qualia), and it describes the private "raw feel" or the subjective "what-it-is-like" that is the inner accompaniment of many mental states. Thus, while Pierre and the robot are both doing the same thing, only Pierre has the inner conscious experience.

Frank Cameron Jackson (1982), for example, once espoused the following view:

I am what is sometimes known as a "qualia freak". I think that there are certain features of bodily sensations especially, but also of certain perceptual experiences, which no amount of purely physical information includes. Tell me everything physical there is to tell about what is going on in a living brain... you won't have told me about the hurtfulness of pains, the itchiness of itches, pangs of jealousy....

Some thinkers draw distinctions between different varieties of epiphenomenalism. In Consciousness Explained, Daniel Dennett distinguishes between a purely metaphysical sense of epiphenomenalism, in which the epiphenomenon has no causal impact at all, and Huxley's "steam whistle" epiphenomenalism, in which effects exist but are not functionally relevant.

Arguments for

Some neurophysiological data has been proffered in support of epiphenomenalism, suggesting that at least some choices and actions appear to be actually controlled by subconscious brain processes for which the conscious mind later takes credit. Some of the oldest such data is the Bereitschaftspotential or "readiness potential" in which electrical activity related to voluntary actions can be recorded up to two seconds before the subject is aware of making a decision to perform the action. More recently Benjamin Libet, et al. (1979) have shown that it can take 0.5 seconds before a stimulus becomes part of conscious experience even though subjects can respond to the stimulus in reaction time tests within 200 milliseconds. The methods and conclusions of this experiment have received much criticism (e.g., see the many critical commentaries in Libet's (1985) target article), including fairly recently by neuroscientists such as Peter Ulric Tse, who claims to show that the readiness potential has nothing to do with consciousness at all.

Arguments against

One argument against epiphenomenalism is that it is self-contradictory: if we have knowledge of epiphenomenalism, then our brains know the mind exists, but if epiphenomenalism were true, our brains should not have any knowledge of the mind, because the mind does not affect anything physical.

However, some philosophers do not accept this as a rigorous refutation. For example, philosopher Victor Argonov states that epiphenomenalism is a questionable, but experimentally falsifiable theory. He argues that the personal mind is not the only source of knowledge about the mind's existence in the world. A creature (such as a philosophical zombie) could have knowledge about the mind and the mind-body problem by virtue of some innate knowledge. The information about the mind (and its problematic properties such as qualia and the hard problem of consciousness) could have been, in principle, implicitly "written" in the material world since its creation. Epiphenomenalists can say that God created an immaterial mind and a detailed "program" of material human behavior that makes it possible to speak about the mind–body problem. That version of epiphenomenalism seems highly exotic, but it cannot be ruled out by pure theory. However, Argonov suggests that experiments could refute epiphenomenalism. In particular, epiphenomenalism could be refuted if neural correlates of consciousness are found in the human brain and it is proven that human speech about consciousness is caused by them.

Some philosophers, such as Daniel Dennett, reject both epiphenomenalism and the traditional conception of qualia, arguing that they involve a category mistake analogous to the one Gilbert Ryle identified in the Cartesian conception of the mind as a "ghost in the machine". On this view, qualia are not intrinsic, private objects of experience that exist independently of the cognitive and behavioral processes in which experience consists; rather, conscious experiences are to be understood in terms of the ways organisms perceive, discriminate, respond to, and otherwise engage with the world.

Functionalists assert that mental states are well described by their overall role, their activity in relation to the organism as a whole. "This doctrine is rooted in Aristotle's conception of the soul, and has antecedents in Hobbes's conception of the mind as a 'calculating machine', but it has become fully articulated (and popularly endorsed) only in the last third of the 20th century." In so far as it mediates stimulus and response, a mental function is analogous to a program that processes input/output in automata theory. In principle, multiple realisability would guarantee platform dependencies can be avoided, whether in terms of hardware and operating system or, ex hypothesi, biology and philosophy. Because a high-level language is a practical requirement for developing the most complex programs, functionalism implies that a non-reductive physicalism would offer a similar advantage over a strictly eliminative materialism.

Eliminative materialists often state that folk psychology is so unscientific that, ultimately, it will be better to eliminate primitive concepts such as mind, desire and belief, in favor of a future neuroscientific account. A more moderate position such as J. L. Mackie's error theory suggests that false beliefs should be stripped away from a mental concept without eliminating the concept itself, the legitimate core meaning being left intact.

Benjamin Libet's results are quoted in favor of epiphenomenalism, but he believes subjects still have a "conscious veto", since the readiness potential does not invariably lead to an action. In Freedom Evolves, Daniel Dennett argues that a no-free-will conclusion is based on dubious assumptions about the location of consciousness, as well as questioning the accuracy and interpretation of Libet's results. Similar criticism of Libet-style research has been made by neuroscientist Adina Roskies and cognitive theorists Tim Bayne and Alfred Mele.

Others have argued that data such as the Bereitschaftspotential undermine epiphenomenalism for the same reason: such experiments rely on a subject reporting the point in time at which a conscious experience and a conscious decision occur, thus requiring the subject to be able to perform an action consciously. That ability would seem to be at odds with early epiphenomenalism, which according to Huxley is the broad claim that consciousness is "completely without any power… as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery". Many mind–body dualists reject epiphenomenalism on the same grounds. Some philosophers have also pointed out how strange it is that the brain expends copious amounts of energy and glucose maintaining the state of consciousness, yet the conscious mind may play no role in making the final decision. It remains difficult to provide a coherent mechanistic explanation of how a non-physical mind could actually influence any part of the physical body or the physical world at large.

Adrian G. Guggisberg and Annaïs Mottaz have also challenged those findings.

A study by Aaron Schurger and colleagues published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) challenged assumptions about the causal nature of the readiness potential itself (and the "pre-movement buildup" of neural activity in general), thus denying the conclusions drawn from studies such as Libet's and Fried's.

In favor of interactionism, Celia Green (2003) argues that epiphenomenalism does not even provide a satisfactory solution to the problem of interaction posed by substance dualism. Although it does not entail substance dualism, according to Green, epiphenomenalism implies a one-way form of interactionism that is just as hard to conceive of as the two-way form embodied in substance dualism. Green suggests that the assumption that it is less of a problem may arise from the unexamined belief that physical events have some primacy over mental ones.

A number of scientists and philosophers, including William James, Karl Popper, John Eccles, and Donald Symons, dismiss epiphenomenalism from an evolutionary perspective. They point out that the view that the mind is an epiphenomenon of brain activity is not consistent with evolutionary theory, because if the mind were functionless, it would have disappeared long ago, as it would not have been favoured by evolution.

Theory of everything

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Theory_of_everything

 

Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons

A theory of everything or final theory is a hypothetical coherent theoretical framework of physics containing all physical principles. The scope of the concept of a "theory of everything" varies. The original technical concept referred to unification of the four fundamental interactions: electromagnetism, strong and weak nuclear forces, and gravity. Finding such a theory is one of the major unsolved problems in physics. Numerous popular books apply the words "theory of everything" to more expansive concepts such as predicting everything in the universe from logic alone, complete with discussions on how this is not possible.

Starting with Isaac Newton's unification of terrestrial gravity, responsible for weight, with celestial gravity, responsible for planetary orbits, concepts in fundamental physics have been successively unified. The phenomena of electricity and magnetism were combined by James Clerk Maxwell's theory of electromagnetism and Albert Einstein's theory of relativity explained how they are connected. By the 1930s, Paul Dirac combined relativity and quantum mechanics and, working with other physicists, developed quantum electrodynamics that combines quantum mechanics and electromagnetism. Work on nuclear and particle physics led to the discovery of the strong nuclear and weak nuclear forces which were combined in the quantum field theory to implement the Standard Model of physics, a unification of all forces except gravity. The lone fundamental force not built into the Standard Model is gravity. General relativity provides a theoretical framework for understanding gravity across scales from the laboratory to planets to the complete universe, but it has not been successfully unified with quantum mechanics.

General relativity and quantum mechanics have been repeatedly validated in their separate fields of relevance. Since the usual domains of applicability of general relativity and quantum mechanics are so different, most situations require that only one of the two theories be used. The two theories are considered incompatible in regions of extremely small scale – the Planck scale – such as those that exist within a black hole or during the beginning stages of the universe (i.e., the moment immediately following the Big Bang). To resolve the incompatibility, a theoretical framework revealing a deeper underlying reality, unifying gravity with the other three interactions, must be discovered to harmoniously integrate the realms of general relativity and quantum mechanics into a seamless whole: a theory of everything may be defined as a comprehensive theory that, in principle, would be capable of describing all physical phenomena in the universe.

In pursuit of this goal, quantum gravity has become one area of active research. One example is string theory, which evolved into a candidate for the theory of everything, but not without drawbacks (most notably, its apparent lack of currently testable predictions) and controversy. String theory posits that at the beginning of the universe (up to 10⁻⁴³ seconds after the Big Bang), the four fundamental forces were once a single fundamental force. According to string theory, every particle in the universe, at its most ultramicroscopic level (Planck length), consists of varying combinations of vibrating strings (or strands) with preferred patterns of vibration. String theory further claims that it is through these specific oscillatory patterns of strings that a particle of unique mass and force charge is created (that is to say, the electron is a type of string that vibrates one way, while the up quark is a type of string vibrating another way, and so forth). String theory/M-theory proposes six or seven dimensions of spacetime in addition to the four common dimensions for a ten- or eleven-dimensional spacetime.

Name

The scientific use of the term theory of everything occurred in the title of an article by physicist John Ellis in 1986 but it was mentioned by John Henry Schwarz in a conference proceedings in 1985.

Historical antecedents

Antiquity to 19th century

Archimedes was possibly the first philosopher to have described nature with axioms (or principles) and then deduce new results from them. Once Isaac Newton proposed his universal law of gravitation, mathematician Pierre-Simon Laplace suggested that such laws could in principle allow deterministic prediction of the future state of the universe. Any "theory of everything" is similarly expected to be based on axioms and to deduce all observable phenomena from them.

In the late 17th century, Isaac Newton's description of the long-distance force of gravity implied that not all forces in nature result from things coming into contact. Newton's work in his Mathematical Principles of Natural Philosophy dealt with this in a further example of unification, in this case unifying Galileo's work on terrestrial gravity, Kepler's laws of planetary motion and the phenomenon of tides by explaining these apparent actions at a distance under one single law: the law of universal gravitation. Newton achieved the first great unification in physics, and he further is credited with laying the foundations of future endeavors for a grand unified theory.

An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.

— Essai philosophique sur les probabilités by Pierre-Simon Laplace, Introduction. 1814

Modern quantum mechanics implies that uncertainty is inescapable, and thus that Laplace's vision has to be amended: a theory of everything must include gravitation and quantum mechanics. Even ignoring quantum mechanics, chaos theory is sufficient to guarantee that the future of any sufficiently complex mechanical or astronomical system is unpredictable.

In 1820, Hans Christian Ørsted discovered a connection between electricity and magnetism, triggering decades of work that culminated in 1865, in James Clerk Maxwell's theory of electromagnetism, which achieved the second great unification in physics. During the 19th and early 20th centuries, it gradually became apparent that many common examples of forces – contact forces, elasticity, viscosity, friction, and pressure – result from electrical interactions between the smallest particles of matter.

In his experiments of 1849–1850, Michael Faraday was the first to search for a unification of gravity with electricity and magnetism. However, he found no connection.

Early 20th century

In the late 1920s, the then new quantum mechanics showed that the chemical bonds between atoms were examples of (quantum) electrical forces, justifying Dirac's boast that "the underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known".

After 1915, when Albert Einstein published the theory of gravity (general relativity), the search for a unified field theory combining gravity with electromagnetism began with a renewed interest. In Einstein's day, the strong and the weak forces had not yet been discovered, yet he found the potential existence of two other distinct forces, gravity and electromagnetism, far more alluring. This launched his 40-year voyage in search of the so-called "unified field theory" that he hoped would show that these two forces are really manifestations of one grand, underlying principle. During the last few decades of his life, this ambition alienated Einstein from the rest of mainstream of physics, as the mainstream was instead far more excited about the emerging framework of quantum mechanics. Einstein wrote to a friend in the early 1940s, "I have become a lonely old chap who is mainly known because he doesn't wear socks and who is exhibited as a curiosity on special occasions." Prominent contributors were Gunnar Nordström, Hermann Weyl, Arthur Eddington, David Hilbert, Theodor Kaluza, Oskar Klein (see Kaluza–Klein theory), and most notably, Albert Einstein and his collaborators. Einstein searched in earnest for, but ultimately failed to find, a unifying theory.

Late 20th century and the nuclear interactions

In the 20th century, the search for a unifying theory was interrupted by the discovery of the strong and weak nuclear forces, which differ both from gravity and from electromagnetism. A further hurdle was the acceptance that in a theory of everything, quantum mechanics had to be incorporated from the outset, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped.

Gravity and electromagnetism are able to coexist as entries in a list of classical forces, but for many years it seemed that gravity could not be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason, work on unification, for much of the 20th century, focused on understanding the three forces described by quantum mechanics: electromagnetism and the weak and strong forces. The first two were combined in 1967–1968 by Sheldon Glashow, Steven Weinberg, and Abdus Salam into the electroweak force. Electroweak unification is a broken symmetry: the electromagnetic and weak forces appear distinct at low energies because the particles carrying the weak force, the W and Z bosons, have non-zero masses (80.4 GeV/c² and 91.2 GeV/c², respectively), whereas the photon, which carries the electromagnetic force, is massless. At higher energies W bosons and Z bosons can be created easily and the unified nature of the force becomes apparent.

While the strong and electroweak forces coexist under the Standard Model of particle physics, they remain distinct. Thus, the pursuit of a theory of everything remained unsuccessful: neither a unification of the strong and electroweak forces – which Laplace would have called 'contact forces' – nor a unification of these forces with gravitation had been achieved.

Modern physics

A depiction of the cGh cube

Depicted as a Venn diagram

Conventional sequence of theories

A theory of everything would unify all the fundamental interactions of nature: gravitation, the strong interaction, the weak interaction, and electromagnetism. Because the weak interaction can transform elementary particles from one kind into another, the theory of everything should also predict all the different kinds of particles possible. The usual assumed path of theories is given in the following graph, where each unification step leads one level up on the graph.

Theory of everything

Quantum gravity

Space Curvature

Electronuclear force (Grand Unified Theory)

Standard model of cosmology

Standard Model of particle physics

Strong interaction SU(3)

Electroweak interaction SU(2) x U(1)Y

Weak interaction SU(2)

Electromagnetism U(1)EM

Electricity

Magnetism

In this graph, electroweak unification occurs at around 100 GeV, grand unification is predicted to occur at 10¹⁶ GeV, and unification of the GUT force with gravity is expected at the Planck energy, roughly 10¹⁹ GeV.

Several Grand Unified Theories (GUTs) have been proposed to unify electromagnetism and the weak and strong forces. Grand unification would imply the existence of an electronuclear force; it is expected to set in at energies of the order of 10¹⁶ GeV, far greater than could be reached by any currently feasible particle accelerator. Although the simplest grand unified theories have been experimentally ruled out, the idea of a grand unified theory, especially when linked with supersymmetry, remains a favorite candidate in the theoretical physics community. Supersymmetric grand unified theories seem plausible not only for their theoretical "beauty", but because they naturally produce large quantities of dark matter, and because the inflationary force may be related to grand unified theory physics (although it does not seem to form an inevitable part of the theory). Yet grand unified theories are clearly not the final answer; both the current Standard Model and all proposed GUTs are quantum field theories which require the problematic technique of renormalization to yield sensible answers. This is usually regarded as a sign that these are only effective field theories, omitting crucial phenomena relevant only at very high energies.

The final step in the graph requires resolving the separation between quantum mechanics and gravitation, often equated with general relativity. Numerous researchers concentrate their efforts on this specific step; nevertheless, no accepted theory of quantum gravity, and thus no accepted theory of everything, has emerged with observational evidence. It is usually assumed that the theory of everything will also solve the remaining problems of grand unified theories.

In addition to explaining the forces listed in the graph, a theory of everything may also explain the status of at least two candidate forces suggested by modern cosmology: an inflationary force and dark energy. Furthermore, cosmological experiments also suggest the existence of dark matter, supposedly composed of fundamental particles outside the scheme of the Standard Model. However, the existence of these forces and particles has not been proven.

String theory and M-theory

Since the 1990s, some physicists such as Edward Witten believe that 11-dimensional M-theory, which is described in some limits by one of the five perturbative superstring theories, and in another by the maximally-supersymmetric eleven-dimensional supergravity, is the theory of everything. There is no widespread consensus on this issue.

One remarkable property of string/M-theory is that seven extra dimensions are required for the theory's consistency, on top of the four dimensions in our universe. In this regard, string theory can be seen as building on the insights of the Kaluza–Klein theory, in which it was realized that applying general relativity to a 5-dimensional universe, with one space dimension small and curled up, looks from the 4-dimensional perspective like the usual general relativity together with Maxwell's electrodynamics. This lent credence to the idea of unifying gauge and gravity interactions, and to extra dimensions, but did not address the detailed experimental requirements. Another important property of string theory is its supersymmetry, which together with extra dimensions are the two main proposals for resolving the hierarchy problem of the Standard Model, which is (roughly) the question of why gravity is so much weaker than any other force. The extra-dimensional solution involves allowing gravity to propagate into the other dimensions while keeping other forces confined to a 4-dimensional spacetime, an idea that has been realized with explicit stringy mechanisms.

Research into string theory has been encouraged by a variety of theoretical and experimental factors. On the experimental side, the particle content of the Standard Model supplemented with neutrino masses fits into a spinor representation of SO(10), a subgroup of E₈ that routinely emerges in string theory, such as in heterotic string theory or (sometimes equivalently) in F-theory. String theory has mechanisms that may explain why fermions come in three hierarchical generations, and explain the mixing rates between quark generations. On the theoretical side, it has begun to address some of the key questions in quantum gravity, such as resolving the black hole information paradox, counting the correct entropy of black holes and allowing for topology-changing processes. It has also led to many insights in pure mathematics and in ordinary, strongly-coupled gauge theory due to the Gauge/String duality.

In the late 1990s, it was noted that one major hurdle in this endeavor is that the number of possible 4-dimensional universes is incredibly large. The small, "curled up" extra dimensions can be compactified in an enormous number of different ways (one estimate is 10⁵⁰⁰) each of which leads to different properties for the low-energy particles and forces. This array of models is known as the string theory landscape.

One proposed solution is that many or all of these possibilities are realized in one or another of a huge number of universes, but that only a small number of them are habitable. Hence what we normally conceive as the fundamental constants of the universe are ultimately the result of the anthropic principle rather than dictated by theory. This has led to criticism of string theory, arguing that it cannot make useful (i.e., original, falsifiable, and verifiable) predictions and regarding it as a pseudoscience/philosophy. Others disagree, and string theory remains an active topic of investigation in theoretical physics.

Loop quantum gravity

Current research on loop quantum gravity may eventually play a fundamental role in a theory of everything, but that is not its primary aim. Loop quantum gravity also introduces a lower bound on the possible length scales.

There have been recent claims that loop quantum gravity may be able to reproduce features resembling the Standard Model. So far only the first generation of fermions (leptons and quarks) with correct parity properties have been modelled by Sundance Bilson-Thompson using preons constituted of braids of spacetime as the building blocks. However, there is no derivation of the Lagrangian that would describe the interactions of such particles, nor is it possible to show that such particles are fermions, nor that the gauge groups or interactions of the Standard Model are realised. Use of quantum computing concepts made it possible to demonstrate that the particles are able to survive quantum fluctuations.

This model leads to an interpretation of electric and color charge as topological quantities (electric as number and chirality of twists carried on the individual ribbons and colour as variants of such twisting for fixed electric charge).

Bilson-Thompson's original paper suggested that the higher-generation fermions could be represented by more complicated braidings, although explicit constructions of these structures were not given. The electric charge, color, and parity properties of such fermions would arise in the same way as for the first generation. The model was expressly generalized for an infinite number of generations and for the weak force bosons (but not for photons or gluons) in a 2008 paper by Bilson-Thompson, Hackett, Kauffman and Smolin.

Present status

At present, there is no candidate theory of everything that includes the Standard Model of particle physics and general relativity and that, at the same time, is able to calculate the fine-structure constant or the mass of the electron. Most particle physicists expect that the outcome of ongoing experiments – the search for new particles at the large particle accelerators and for dark matter – are needed in order to provide further input for a theory of everything.

Other proposals

The search for a Theory of Everything is hindered by fundamental incompatibility between the noncommutative and discrete operator algebra structures underlying quantum mechanics and the commutative continuous geometric nature of classical spacetime in general relativity. Reconciling the background-independent, diffeomorphism-invariant formulation of gravity with the fixed-background, time-ordered framework of quantum theory raises profound conceptual issues such as the problem of time and quantum measurement. While a fully successful and experimentally confirmed unified field theory remains elusive, several recent proposals have been advanced, each employing distinct mathematical structures and physical assumptions.

Twistor theory, developed by Roger Penrose, reinterprets the structure of spacetime and fundamental particles through complex geometric objects called twistors. Instead of treating spacetime points as fundamental, twistor theory encodes physical fields and particles into complex projective spaces, aiming to unify quantum theory and general relativity in a geometric framework. Twistors provide potential descriptions of massless fields and scattering amplitudes and have influenced modern approaches in mathematical physics and quantum field theory, including advances in scattering amplitude calculations. Twistor theory has not yet yielded a complete unified field theory.

Alain Connes developed a geometric framework known as noncommutative geometry in which spacetime is extended via noncommutative operator algebras. When combined with spectral triples, this approach can reproduce features of the Standard Model, including the Higgs field, from purely geometric data.

Asymptotic safety, a concept developed by Steven Weinberg in 1976 and also known as Quantum Einstein Gravity and nonperturbative renormalizability, suggests that gravity could find a role in quantum theory if its behavior at very high energies becomes stabilized into a nontrivial ultraviolet (UV) fixed point. This form has been studied through functional renormalization group methods and on the lattice, and applied in cosmology, particle physics, black hole physics, and quantum gravity. Whereas overwhelming numerical evidence does exist that such a fixed point does occur in lower-dimensional constructions and in the numerics, a rigorous proof even for four-dimensional spacetime remains to be found.

Arguments against

In parallel to the intense search for a theory of everything, various scholars have debated the possibility of its discovery.

Gödel's incompleteness theorem

A number of scholars claim that Gödel's incompleteness theorem suggests that attempts to construct a theory of everything are bound to fail. Gödel's theorem, informally stated, asserts that any formal theory sufficient to express elementary arithmetical facts and strong enough for them to be proved is either inconsistent (both a statement and its denial can be derived from its axioms) or incomplete, in the sense that there is a true statement that can't be derived in the formal theory.

The Benedictine priest and science writer Stanley Jaki, in his 1966 book The Relevance of Physics, suggested that Gödel's theorem dooms searches for a deterministic "theory of everything" at least as a consistent non-trivial mathematical theory.

Freeman Dyson has stated that "Gödel's theorem implies that pure mathematics is inexhaustible. No matter how many problems we solve, there will always be other problems that cannot be solved within the existing rules. […] Because of Gödel's theorem, physics is inexhaustible too. The laws of physics are a finite set of rules, and include the rules for doing mathematics, so that Gödel's theorem applies to them."

Stephen Hawking originally believed that a theory of everything could be found, but after considering Gödel's Theorem, he concluded that one was not obtainable: "Some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles. I used to belong to that camp, but I have changed my mind."

Jürgen Schmidhuber (1997) has argued against this view; he asserts that Gödel's theorems are irrelevant for computable physics. In 2000, Schmidhuber explicitly constructed limit-computable, deterministic universes whose pseudo-randomness based on undecidable, Gödel-like halting problems is extremely hard to detect but does not prevent formal theories of everything describable by very few bits of information.

Related critique was offered by Solomon Feferman and others. Douglas S. Robertson offers Conway's game of life as an example: The underlying rules are simple and complete, but there are formally undecidable questions about the game's behaviors. Analogously, it may (or may not) be possible to completely state the underlying rules of physics with a finite number of well-defined laws, but there is little doubt that there are questions about the behavior of physical systems which are formally undecidable on the basis of those underlying laws.

Fundamental limits in accuracy

No physical theory to date is believed to be precisely accurate. Instead, physics has proceeded by a series of "successive approximations" allowing more and more accurate predictions over a wider and wider range of phenomena. Some physicists believe that it is therefore a mistake to confuse theoretical models with the true nature of reality, and hold that the series of approximations will never terminate in the "truth". Einstein himself expressed this view on occasions.

Definition of fundamental laws

There is a philosophical debate within the physics community as to whether a theory of everything deserves to be called the fundamental law of the universe. One view is the hard reductionist position that the theory of everything is the fundamental law and that all other theories that apply within the universe are a consequence of the theory of everything. Another view is that emergent laws, which govern the behavior of complex systems, should be seen as equally fundamental. Examples of emergent laws are the second law of thermodynamics and the theory of natural selection. The advocates of emergence argue that emergent laws, especially those describing complex or living systems are independent of the low-level, microscopic laws. In this view, emergent laws are as fundamental as a theory of everything.

Impossibility of calculation

Weinberg points out that calculating the precise motion of an actual projectile in the Earth's atmosphere is impossible. So how can we know we have an adequate theory for describing the motion of projectiles? Weinberg suggests that we know principles (Newton's laws of motion and gravitation) that work "well enough" for simple examples, like the motion of planets in empty space. These principles have worked so well on simple examples that we can be reasonably confident they will work for more complex examples. For example, although general relativity includes equations that do not have exact solutions, it is widely accepted as a valid theory because all of its equations with exact solutions have been experimentally verified. Likewise, a theory of everything must work for a wide range of simple examples in such a way that we can be reasonably confident it will work for every situation in physics. Difficulties in creating a theory of everything often begin to appear when combining quantum mechanics with the theory of general relativity, as the equations of quantum mechanics begin to falter when the force of gravity is applied to them.