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What do professional philosophers mean by "physicalism" ?

The definition I use is that physicalism is the worldview that the entities that actually exist in the world are those used for physics (entities like, electrons, quarks, mass, charge etc.) Physics has a set of objects/entities like this and it has equations relating them.

Here:

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

we have a distinction between metaphysical physicalism and linguistic physicalism:

Metaphysical physicalism simply asserts that what there is, and all there is, is physical stuff and its relations. Linguistic physicalism is the thesis that everything physical can be expressed or captured in the languages of the basic sciences…Linguistic physicalism is stronger than metaphysical physicalism and less plausible.

Linguistic physicalism has a well defined meaning as far as I see. But what does metaphysical physicalism even mean?

Another quote from the link:

Similarly to Flanagan, Torin Alter contends that Jackson conflates physical facts with "discursively learnable" facts, without justification: ...some facts about conscious experiences of various kinds cannot be learned through purely discursive means. This, however, does not yet license any further conclusions about the nature of the experiences that these discursively unlearnable facts are about. In particular, it does not entitle us to infer that these experiences are not physical events.[10]

So physical doesn't mean "describable via physics" then what does it mean? What makes a physical fact a "physical" fact if it doesn't mean describable by physics?

In the context of the knowledge argument... we're talking about color... the above quotes are stating, Mary does indeed learn something new about color, but this fact is still "physical" even though it's not describable by physics. What does this even mean?

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    @KristianBerry, So I think you're right... they seem to be saying the same type of thing... anomalous monism seems to rely on "token" identity... two things being identical even though they are not really identical... I'm finding it equally puzzling. If two things happen at the same time, and they are not identical why not simply say so instead of introducing this idea of "token" identity. The additional terminology doesn't help imo. Jul 10, 2023 at 3:16
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    @agentsmith's tag-edit is superb: What is physicalism? is a metaphysical question. Ha!
    – Rushi
    Jul 10, 2023 at 18:09
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    In practice, the concept of the physical started with that which is tangible to our senses, and has been extended many times to include anything that is found to be causal within that domain. This pragmatic definition of what is physical is not predicated on the assumption that everything that fits it will be describable in any language.
    – A Raybould
    Jul 11, 2023 at 3:56
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    One concern over 'linguistic physicalism' is whether "the language of the basic sciences" can be defined without circularly (and perhaps tacitly) presupposing a definition of the physical. If it is simply precise or careful language, then a) that also embraces the language of, e.g., the humanities and theology, and b) abstract concepts such as algorithms would be included in the physical. OTOH, it cannot just mean quantitative language: for one thing, science differs from being just arithmetic in that all of the former's numbers have units, and it is this which makes them 'physical' numbers.
    – A Raybould
    Jul 12, 2023 at 11:41
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    ... Furthermore, many important parts of the physical sciences are presented in language that is no more quantitative than the average newspaper. As exhibit 1 for this proposition, consider Darwin's Origin of Species.
    – A Raybould
    Jul 12, 2023 at 11:50

8 Answers 8

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Here's an interesting fact. Most real numbers are not describable by mathematics.

It's true. To be more specific, most real numbers cannot be named by any mathematical formula.

Why? There is an uncountable infinity of real numbers. Cantor proved that. However, there is only a countable infinity of mathematical formulas (in any formal system with a finite alphabet, such as ZFC). The mathematical formulas are countable because you could list them all one by one, in order of increasing length.

The inevitable conclusion is that only a very tiny minority of the real numbers can get their own formulas; there simply aren't enough formulas to go around.

What's the relevance to physicalism? Well, suppose that the universe fundamentally works based on continuous real numbers, such as the position of a particle or the magnitude of a wave function at a particular point. Then almost all of those real numbers would not be describable by physics - linguistic physicalism would be false. There simply would not be enough formulas to describe the real-valued state of anything with perfect precision. But metaphysical physicalism would be true.

Or what if the universe evolves over time in an incomputable way? Then physics would not be able to predict this evolution - no formulas or manipulation of formulas would perfectly yield the system dynamics. But this would still be compatible with metaphysical physicalism.

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Physicalism is the ontological theory that what we call "ideas" do not exist by themselves, but only as configurations of physical stuff, what we can see, touch or interact with (in a sense, as we can't really interact with an electron but we can build machines that interact with them).

For example a poem does not exist in and of itself, but as a certain pattern of neuron firings in an author's brain, a listener's brain, ink spots on a paper when written down or air vibrations when vocalized, etc...

It follows that the emotions we feel when listening to a poem could be explained by physical means (not only physics, but it's subdomains like chemistry, etc...): the air vibration strikes our ear, activating a pattern of neurons that reactivates memories associated with emotions, or some primal reaction that appeared through natural evolution.

Linguistical physicalism add to it the claim that the theory of physics can be complete, i.e. that we can explain and model every event by the means of language. For example I could establish a complete theory of human sadness, communicate it to you using a formal language, without ambiguity provided we agreed on the definitions of the terms, and you could conduct your own experiments to independently verify that indeed, there is no sadness that can't be explained using my theory.

As such, linguistical physicalism is more of an epistemological claim. It claims something about the nature of knowledge rather than the nature of being.

Metaphysical physicalism, on the other hand, makes no claim about how much we can know about the physical reality. As stated by David Chalmers with his hard problem of consciousness, there is something more to experiencing life than just the firing of our neurons, and this is the experience itself. You and me can agree on what things are red, by pointing to different objects and saying to each other "this is Red" (it is actually, the way we learned to use the word "red"), but we can't agree on what it is to feel red.

I can't make you feel what I feel when I see red, the communication would always be done by the mediation of some language, or common reference to objects. Even if I had some very advanced device to analyse my own neuron pattern when seeing red and fire your neurons precisely the way I want, it probably wouldn't work because we don't have exactly the same neurons as each brain, while similar to others, is unique.

Since it looks like there are things we can't communicate without ambiguity, it sounds reasonnable to think there are things that, while material, can't be studied by physics.

To build on your example about Mary's room and the qualia of red, i think we can summarize the opinion of non physicalists, metaphisical physicalists and linguistical physicalists as follows:

  • non physicalists: the experience of Mary seeing red is non physical in nature, there is something more to reality than just physical stuff.

  • metaphisical physicalists: the experience of Mary seeing red is physical in nature, we just don't know what it is and assuming dualism would be presomptuous. But it escapes our ability to express it in non ambiguous terms and therefore can't be studied by physics and we might never know.

  • linguistical physicalists: the experience of Mary seeing red is physical in nature, we don't know what it is yet but one day we will be able to know.

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  • "configurations of physical stuff"... Can you elaborate what "physical" stuff is exactly... what makes certain stuff physical as opposed to non-physical? Jul 10, 2023 at 4:23
  • @AmeetSharma as per wikipedia "Physicalism encompasses: matter, but also energy, physical laws, space, time, structure, physical processes, information, state, and forces, among other things, as described by physics and other sciences". The inclusion of physical laws is kind of surprising as they are models we build, but there are, as far as we know, regularities in the behavior of matter so maybe that is what they mean. It's difficult to define positively what is "physical stuff" as our undertsanding of physics is not complete and changes fast.
    – armand
    Jul 10, 2023 at 6:16
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    @armand the inclusion of laws is less surprising if you think of the underlying assumption that all physical things behave according to certain laws. Laws that we know aren't presumed to be the "true" laws, but may be faulty approximations – but the same is true of other ideas about physical reality like particles. We must take care to distinguish the idea of an underlying physical reality from our various descriptions of physical reality.
    – Cubic
    Jul 10, 2023 at 11:09
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    You're right to say "Metaphysical physicalism ... makes no claim about how much we can know about the physical reality", but then you seem to contradict this in the summary by saying metaphysical physicalists say "we will never know". Some metaphysical physicalists may hold to that, while others don't. Their position on metaphysical physicalism doesn't make a claim one way or the other about what we can theoretically know about emotions. Linguistical physicalism seems to be a subset of metaphysical physicalism, rather than being distinct from it, as you're presenting it.
    – NotThatGuy
    Jul 10, 2023 at 14:26
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    @armand That's how I read it, so if you meant it differently, then it demonstrably isn't very clear.
    – NotThatGuy
    Jul 10, 2023 at 16:11
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I think that the other word for it is "materialism". That everything that exists is material and that a thing or notion that is not material does not exist in reality.

I think it is a refutation of dualism and certainly of any notion of supernaturalism or of theism.

What I am not sure of is how materialists consider consciousness and notions of human emotion. Like love and hate. Aspiration. Wonder. Awe. Disgust. Feelings. Do these things really exist or are they illusions? Are we, like Daniel Dennett might suggest, only automatons that think we have consciousness?

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  • But I don't understand what "materialism" means either. Jul 10, 2023 at 2:38
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    What don't you understand? It is the notion that everything that exists in reality is material or physical. That things we might consider deeper than physical, like consciousness, emotion, even morality and ethics, are illusions in reality. Materialists might disagree, but I think if you take the notion to its end, that right and wrong do not really exist. That suffering and joy do not really exist. They are illusory. Jul 10, 2023 at 2:43
  • I don't understand what it means to say something is material or physical... other than something like "describable by physics". But my puzzlement is that some philosophers are saying some thing are not describable by physics and yet still "physical". I don't understand what they mean. Jul 10, 2023 at 2:58
  • Consciousness is an emergent property of the brain. I'll leave it to others to philosophise about whether emergent properties "really exist". Emotion (which other animals seem to experience too) is just your brain doing brain stuff: triggering neurons and such. There is probably some known evolutionary explanation for most emotions, like if you see some food that is harmful to you, it aids survival for your brain to make you feel bad, to keep you from consuming it, and it also helps to become nauseous in case you consumed it, so you can "unconsume" it. Therefore, disgust.
    – NotThatGuy
    Jul 10, 2023 at 14:15
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    Physicalists typically wouldn't describe emotion as any more "illusionary" than "wetness" or "brightness" or "sweetness" or "car". None of these are properties that 'exist' on small scales (nobody in their right mind would argue that any single atom is a car – yet, somehow, cars seem to exist! fancy that.)
    – Cubic
    Jul 10, 2023 at 14:33
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The word physis is the ancient Greek counterpart of the Latin natura, so it is profitable to compare physicalism with naturalism. The SEP article on naturalism then mentions the rise of physicalism proper:

... the nineteenth-century discovery of the conservation of energy continued to allow that sui generis non-physical forces can interact with the physical world, but required that they be governed by strict force laws. Sui generis mental and vital forces were still widely accepted, but an extensive philosophical debate about the significance of the conservation of energy led to a widespread recognition that any such forces would need to be law-governed and thus amenable to scientific investigation. We might usefully view this as a species of ontological naturalism that falls short of full physicalism. Mental and other special forces were still sui generis and non-physical, but even so they fell within the realm of scientific law and so could not operate spontaneously. ... In the final twentieth-century phase, the acceptance of the causal closure of the physical led to full-fledged physicalism. The causal closure thesis implied that, if mental and other special causes are to produce physical effects, they must themselves be physically constituted. It thus gave rise to the strong physicalist doctrine that anything that has physical effects must itself be physical.

Consider then also scientific reductionism and priority monism: if the fundamental level of reality on the one hand, and the most all-encompassing one on the other, are the ones studied by physicists (particle physics and cosmology respectively), one can parse the discrepancy between a denial of linguistic physicalism and an assertion of metaphysical physicalism as a claim that, for linguistic-epistemic reasons, humans are not in a position to show how every intermediary level is grounded in, and sums to, the physical levels, but we might assume that this grounding and summation obtain apart from our subjective knowledge of reduction and composition, then.

In other words, one might speak of metaphysical physicalism alongside linguistic naturalism, where nature generally is understood as a bit abstracted over physics generally.

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I think that your understanding is better than the answers posted so far:

"The definition I use is that physicalism is the worldview that the entities that actually exist in the world are those used for physics"

It is the most cautious way of thinking about it, even if it leaves a lot of questions unanswered. At least it avoids vicious circles.

For example, it is certainly not true that "materialism" is a synonym for "physicalism", as it might be that matter (as we usually understand it) is not the fundamental element in Physics.

"Metaphysical physicalism simply asserts that what there is, and all there is, is physical stuff and its relations."

Ok, but this is not a very satisfying answer: what is "physical stuff"? The basic elements/concepts of Physics? What are these basic elements? Waves? Particles? Energy? Matter? Fields? Hard to say as Physics evolves continuously and the emphasis is put on different concepts.

"Linguistic physicalism is the thesis that everything physical can be expressed or captured in the languages of the basic sciences"

This is again not so satisfying: it says that there is a physical world and that humans can describe it with their language(s). By contrast, there might be a physical world that we cannot entirely express in our language. But what is a "physical" world?

You have the best answer to your question. Others may imply that you know what "physical" means, or what Physics is all about. These are already new questions...

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  • The best answer so far. I'd have put it stronger...
    – Rushi
    Jul 10, 2023 at 15:58
  • "Hard to say as Physics evolves continuously". So? It's good that our understanding of the physical world keeps progressing.
    – RonJohn
    Jul 10, 2023 at 17:29
  • Note, though, that even though "the basic elements/concepts of Physics" have evolved, it's been MANY decades since physicists' fundamental concept of what matter is has changed. There have only been "relativity of wrong" expansions.
    – RonJohn
    Jul 10, 2023 at 17:35
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    It is indeed part of the life cycle of Scientific Theories. That it is a progress is somehow debatable (though I also think it is): nothing proves that Physics is anywhere close to understanding reality. If reality is 4.2 light-years away and you walk all of your life towards it, you'll be closer to it when you die, but somehow as far as when you started...
    – Gospadi
    Jul 10, 2023 at 20:50
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    Also note that a (way too proud) famous physicist once said: "There is nothing new to be discovered in Physics now, all that remains is more and more precise measurements". He was dead wrong since the General Theory of Relativity and Quantum Physics emerged afterwards. Where are the "fundamental elements" of today relative to reality? It is an even more annoying question since the two aforementioned theories have not been unified, and since the right way of interpreting Quantum Physics is a topic for raging debates between philosophers of science.
    – Gospadi
    Jul 10, 2023 at 20:55
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Physicalism means that everything there is, is physical, i.e. matter or energy. Physicalism denies the existence of qualia, the subjective experience of physical phenomena, like colour.

Besides qualia, physicalism denies all other mental phenomena, emotions, imagination, memories and knowledge. What is knowledge and understanding, if not a subjective experience of information?

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Since the basic definition has already been covered, I've got a slightly different take on the question:

Metaphysical physicalism is a response to supernatural claims.

There needs to be some proposed existence outside of physical reality for metaphysical physicalism to make sense, otherwise it would basically just be saying "all there is is all there is". But there are plenty of such claims.

People make all sorts of claims about the "spiritual", things that exist outside of space and time, and so forth. But they can't offer a concrete definition of what exactly that means and how such an existence would even work or interact with our observable physical reality, nor do they provide a reliable means to detect, measure, test or verify it.

Metaphysical physicalism is a rejection of all of that, until such a time when those questions have been sufficiently addressed for any given claim (at which point what it's claiming would arguably be considered part of physical reality).


There are plenty of people who reject supernatural claims, but also reject metaphysical physicalism.

Metaphysical physicalism says that non-demonstrable things don't exist, whereas others may say that such things could exist, but we just don't have a good reason to believe that they do.

I don't necessarily consider these to be fundamentally different, but rather it's just different levels of certainty of the existence of some supernatural thing (or at least that's what it seems like to me; some people may disagree).

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Physicalism is the thesis that everything is physical. That’s it.

Your confusion seems to stem from the word “physical” and what it means, not physicalism. What exactly defines “physical” and whether or not materialism = physicalism is still a contentious issue in philosophy. The quote you posted is an argument that something being physical does not imply it must be explainable through physics and in language. But this is an argument and is at the very root of the issue. Note that the existence of consciousness and qualia for example are traditional arguments against physicalism.

Note that there is another philosophy called source physicalism. This acknowledges the existence of the non physical, such as supposed mental states, but claims that these non physical states ultimately always come from and after the physical. An example being consciousness emerging from the physical such as the brain.

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