So the question it prompts, for me, is how can physicalism, as a philosophical principle, be credibly maintained in light of these conjectures? Given that science now acknowledges that it can account for only a small percentage of what is figured to exist, and that the remainder exists in a form that science can't even comprehend, how can such philosophical principles as 'causal closure' be said to hold? — Wayfarer

Causal closure isn’t so much a hypothesis as it is a definition: anything that has a physical effect is considered a physical thing. Since dark matter has physical effects, as you’ve explained, that classifies it as a physical thing.

That does mean that by definition we could never find anything non-physical, because anything that we could somehow find we would have to count as physical. So “physical” vs “non-physical” is really a meaninglessness distinction in the end. But that suits physicalism just fine: it’s really just saying to treat all stuff the same way we treat ordinary stuff we’re familiar with, consider it with the same scientific method, etc.

So the question it prompts, for me, is how can physicalism, as a philosophical principle, be credibly maintained in light of these conjectures? — Wayfarer

But how is this different in any respect from how we derive our knowledge of "bright" matter?

Dark matter gravitates but doesn't radiate. Regular matter does both. But in both cases, we are imputing a cause that explains the effect.

So it boils down to observing a change of some kind and forming some suitable concept of what is behind the curtain.

If an event looks punctate, we posit a particle. If it looks continuous, we posit a field.

We interpret observed changes as the sign of some metaphysical object.

Since dark matter has physical effects, as you’ve explained, that classifies it as a physical thing. — Pfhorrest

The problem is, this idea is subject to 'Hempel's dilemma'.

Physicalism, in at least one rough sense, is the claim that the entire world may be described and explained using the laws of nature, in other words, that all phenomena are natural phenomena. This leaves open the question of what is 'natural', but one common understanding of the claim is that everything in the world is ultimately explicable in the terms of physics. This is known as reductive physicalism. However, this type of physicalism in its turn leaves open the question of what we are to consider as the proper terms of physics. There seem to be two options here, and these options form the horns of Hempel's dilemma, because neither seems satisfactory.

On the one hand, we may define the physical as whatever is currently explained by our best physical theories, e.g., quantum mechanics, general relativity. Though many would find this definition unsatisfactory, some would accept that we have at least a general understanding of the physical based on these theories, and can use them to assess what is physical and what is not. And therein lies the rub, as a worked-out explanation of mentality currently lies outside the scope of such theories.

On the other hand, if we say that some future, "ideal" physics is what is meant, then the claim is rather empty, for we have no idea of what this means. The "ideal" physics may even come to define what we think of as mental as part of the physical world. In effect, physicalism by this second account becomes the circular claim that all phenomena are explicable in terms of physics because physics properly defined is whatever explains all phenomena. — Wikipedia

So - science doesn't know what dark matter is, what its components are, or even really that it exists, except inferentially. But whatever it is, it must be 'matter', because, ultimately, everything is. So if we have to redefine matter so it 'passes right through the world' and exists in parallel baryonic matter, then it completely upends every prior idea of 'matter'. But, it doesn't matter.

. So “physical” vs “non-physical” is really a meaninglessness distinction in the end. But that suits physicalism just fine: it’s really just saying to treat all stuff the same way we treat ordinary stuff we’re familiar with, consider it with the same scientific method, etc. — Pfhorrest

Right. So all you're really doing is appealing to scientific method. You're not actually dealing with fundamental definitions. Science just goes on its merry way, discovering whatever there is to be discovered, and never mind the anomalies!

But how is this different in any respect from how we derive our knowledge of "bright" matter? — apokrisis

Because there is direct perceptible evidence of regular matter. Its properties are well known - you can see it, dissect it, analyse its components, and so on. As if often observed, it might turn out that dark matter will in the end be like the epicycles of Ptolmaic cosmology - devices introduced to save the appearances, but, in the end, abandoned on account of the reigning paradigm itself being undone.

I think it's noteworthy how sanguine you are about it. I guess it's because you've got a slot for it in your mental model of the world, so it's not a problem.

So - science doesn't know what dark matter is, what its components are, or even really that it exists, except inferentially. — Wayfarer

Nonsense. Science doesn't even claim to "know", only to constrain uncertainty through an epistemology of theory and measurement.

So - as Peirce explained - that is a systematic process of abductive reasoning. Yes, inference from evidence is part of the loop. But so is the free creativity of hypothesis formation and the deductive reasoning used to shape a causal theory.

Your supposed bug is the feature.

So if we have to redefine matter so it 'passes right through the world' and exists in parallel baryonic matter, then it completely upends every prior idea of 'matter'. — Wayfarer

Do electrons respond to the strong force? Did the fact they don't upend our very idea of matter or explain why matter could come in the form of various different fundamental particles?

Why do neutrinos not notice electromagnetic charge? Same again.

I think it's noteworthy how sanguine you are about it. I guess it's because you've got a slot for it in your mental model of the world, so it's not a problem. — Wayfarer

Or maybe the idea that in a world of mammals and reptiles, there could also be egg-laying monotremes and pouched marsupials, might be extraordinary if one had no general biological framework.

Dark matter could demand some sort of new physics of course. But the likelihood is that it is just another category of particles explained by symmetry principles.

There are much more serious challenges in a phenomenon like dark energy, or a positive cosmological constant.

As if often observed, it might turn out that dark matter will in the end be like the epicycles of Ptolmaic cosmology - devices introduced to save the appearances, but, in the end, abandoned on account of the reigning paradigm itself being undone. — Wayfarer

And what do you think the scientists are hoping for?

If physics were a faith, then paradigm shifts would be the end times. But it is a science. And so killing the reigning paradigm is what motivates every new generation.

The problem is keeping a lid on those careerist ambitions.

Dark matter could demand some sort of new physics of course. But the likelihood is that it is just another category of particles explained by symmetry principles. — apokrisis

I think 'hope' as much as 'likelihood'.

Why is it unlikely? Is the current problem some lack of theories or the capability to test between them?

We'll see, but I doubt that it will become any clearer in my lifetime.

So - science doesn't know what dark matter is, what its components are, or even really that it exists, except inferentially.
— Wayfarer

Nonsense. Science doesn't even claim to "know", only to constrain uncertainty through an epistemology of theory and measurement. — apokrisis

Actually I will take issue with this. I watched an excellent PBS documentary series recently, The Mystery of Matter, which contained dramatized episodes from the work of early modern scientists, including a great depiction of Mendeleev's discovery of the periodic table (which he basically accomplished in a weekend.)

That series covered Priestly and Lavoisier's discovery of oxygen, until about the mid 20th century, during which time science went from basically an Aristotelian conception of the 'four elements' to the periodic table and atomic theory. So I don't see how these discoveries don't constitute knowledge.

But the nature and constituents of dark matter, or even whether it consists of particles, and if so, what they are, is still totally unknown. The only thing known, as the galactic mass behaves as if it is subject to the gravity from an unknown source, which is presumed to be a form of matter.

it is believed that the visible universe, comprising baryonic matter - stuff made from atoms - comprises only about 4% of the totality, the remainder comprising dark matter and dark energy — Wayfarer

... and of that 4%, almost all physical matter is in the form of pure radiation and plasma (protons, electrons, helium), leaving only a trace amount for material substance in the form of gas, liquids, or solids. Which might make a difference for the strength of scientific realism that we hold. Should we care only about what is sensible on Earth, or also about a conservative comprehensible Newtonian scientific world, or do we leave our philosophy open to anything rapidly advancing theoretical physics agrees upon at any moment. How far out should we venture while following cutting edge physics?

The structure of the universe emerges from supradimensional fields that were separated into different spectrums by the astronomical force of the big bang, a process which is still ongoing on mostly localized scales, within the parameters of various environments. We have much yet to learn about these spectrum interactions such as electromagnetic radiation, soundwaves, atomic matter, nuclear matter, dark matter, dark energy, gravity, standing waves, and likely much more.

In general, interactions of fields within fields and their various spectral ranges generate a huge assortment of waves by complex interference mechanisms, giving rise to diverse quantization as periodic amplitude. Within specific spectral parameters, which are analogous to aggregated substance such as in the planets of our solar system, interference is such that dynamic equilibrium or fluctuating wave superposition is attained; the waves commonly tend to synthesize as in visible light, electron orbitals, pitches, etc., while continuing to participate in various kinds of relative motion and oscillation. Our sense-perceptual organs are designed to hone in on some of these spectral ranges in an extremely reflexive way as per the laws of classical physics and thermodynamic chemistry. ESPN taps into additional spectral ranges that lie beyond these boundaries. Consciousness is a hybrid of many spectral ranges, some within and amongst biochemical bodies, some moving or perturbed faster than the speed of light so as to conjoin bodies in synchronicity, and some comprising aliens of the spirit world.

Fundamental physics and cosmology are full of the most outrageous discoveries. And yet folk really seem to go for this dark matter mystery. Curious.

It is like setting out to explore the world and thinking the next closest town is it.

The only thing known, as the galactic mass behaves as if it is subject to the gravity from an unknown source, which is presumed to be a form of matter. — Wayfarer

It's more than just that. Galaxies' star rotation velocities were the original reason for positing dark matter, but there are numerous other lines of evidence at this point; from gravitational lensing (both inside and between galaxies), the CMB and models of the distribution of visible matter, analysis of redshift and so on.

And of course, ironically, one of the best evidence we've found that dark matter must be some kind of discrete material, are the galaxies we've found without it.
It's very rare, but there are a few galaxies that contain stars with rotation velocities exactly what we'd expect from a Newtonian model of star rotation. If dark matter is discrete material, explaining a few galaxies that lack it is no problem. But it's a big problem for alternative hypotheses, like that our understanding of gravity is flawed.

And yet folk really seem to go for this dark matter mystery. Curious. — apokrisis

Well, the weird stuff in most of theoretical physics is beyond the layman's comprehension; you really need at least higher mathematics but also probably to have actually studied physics beyond the high school level to even understand the claims.

Dark matter on the other hand, sounds like a straightforward theory. Plus it's often reported as if it's just scientists taking a wild guess. So people think their own wild guesses can be on an equal footing.

I used to dislike the name "dark matter", but now I think that if it had been called something unique or cool sounding we'd have easily 10x the amount of woo and misconception as we do right now. "Dark matter" is actually a pretty prosaic name all told.

Dark matter on the other hand, sounds like a straightforward theory. — Mijin

What about the idea that the mass/energy of 96% of the universe is of an unknown type if ‘straightforward’? Ought not that be considered mystifying or surprising? Or is it just, you know, ‘business as usual’?

people think their own wild guesses can be on an equal footing. — Mijin

I am not hazarding any guesses here. What I’ve said is that ‘dark matter’ undermines the philosophical idea of the ‘causal closure of the physical’. It does this by showing that our ideas of ‘the physical’ must be radically deficient in some way.

On Hempel's dilemma: there's really more of a trilemma there, which illustrates why one fork of the supposed dilemma is clearly superior, even though it still has the effect of making the distinction between physical and nonphysical basically meaningless.

On one horn of the trilemma, we can assume that we already have everything figured out. This is the first horn of Hempel's dilemma, with its obvious problems.

On the third horn, we can assume that there are some things we will never figure out, that are forever beyond being comprehended together with the stuff we already have figured out in a way that makes sense of it all together. I hope the problems of this are just as obvious, but I can elaborate if necessary.

Meanwhile back on the second horn, we could instead assume neither of those, or in other words assume that we haven't yet figured everything out, but that there is nothing that we can never figure out. That leaves things categorized into either "things we have already figured out", and "things we haven't figured out yet". Meanwhile both of those sets, and everything we could possibly conceive of, falls together under the set of "things we could eventually figure out", which makes the distinction between that and its complementary set meaningless, in a way, because there is nothing in the complementary set; the set of things we could eventually figure out is the entire universe. Which is basically the thesis of physicalism.

Yeah well the basic problem with that is that it is essentially meaningless because it is so open-ended.

What about the idea that the mass/energy of 96% of the universe is of an unknown type if ‘straightforward’? Ought not that be considered mystifying or surprising? — Wayfarer

I meant straightforward in terms of comparative ease of understanding. I mean, you just summarized the theory in 14 words, you can't do that with all areas of modern physics.

In terms of it being mystifying or surprising, sure, it could be those too if you like.

What I’ve said is that ‘dark matter’ undermines the philosophical idea of the ‘causal closure of the physical’. It does this by showing that our ideas of ‘the physical’ must be radically deficient in some way. — Wayfarer

Really? Why? We have no reason to consider dark matter as non-physical; it has physical effects and it's in turn affected by physical forces.

If your point is that it shows we don't know everything about the physical universe, well scientists have never thought that (there were a couple of times where the consensus was we were close).
So any version of materialism that assumed absolute knowledge was flawed from the very start.

I think an issue here is with the 96%. This is just a measure of how much of the universe's total mass/energy is accounted for by each phenomenon.
A number of pop sci articles phrase it as some version of "We don't understand 96% of the universe". It makes for eye-catching headlines, but it's a very misleading way of putting it.
The "size" of our understanding is not correlated with the relative proportions of these phenomena.

So any version of materialism that assumed absolute knowledge was flawed from the very start. — Mijin

Don’t you think that covers a lot of what goes by the name ‘philosophical materialism’?

This is just a measure of how much of the universe's total mass/energy is accounted for by each phenomenon. — Mijin

I thought it was ‘accounted for by the known laws of physics’. ‘Phenomenon’ is ‘what appears’ and is only ever a subset of scientific knowledge, as that also covers scientific laws, which are not ‘phenomena’, but that which determines phenomena.

Yet another iteration of "science doesn't know everything there is to know, therefore physicalism is false."

Move along, folks, nothing to see here.

Don’t you think that covers a lot of what goes by the name ‘philosophical materialism’? — Wayfarer

No, I don't think that. Do you?
And if so, on what basis?

Yet another iteration of "science doesn't know everything there is to know, therefore physicalism is false." — SophistiCat

Yep. Hate to see it.

Don’t you think that covers a lot of what goes by the name ‘philosophical materialism’?
— Wayfarer

No, I don't think that. Do you?
And if so, on what basis? — Mijin

'Causal closure of the physical' is precisely that 'every event has a physical cause'. If, however, 'the physical' can't be defined, then how do is that claim sustained? I mean, presently, it is presumed that some unknown substance, provisionally titled 'dark matter', has observable effects on the cosmological observations, but it's nature is unknown. So how can it be known that it is physical? Or, as Pfhorrest says, 'the physical' means 'whatever can be discovered by some future science', then the definition is so broad as to be meaningless.

I would have thought that for the concept of causal closure to be meaningful, then there has to be some assurance that the domain of physics is circumscribed in some way. If it's so open as to include putative substances, the nature of which is unknown, then in what sense can it be referred to as 'closed?'

science doesn't know everything there is to know — SophistiCat

Not what I'm arguing, not that I expect you to be bothered.

Nice article in Vox though, and the thrust of it is, if anything, the opposite of Wayfarer's perennial pitch.

As a slightly nerdier companion piece I would recommend this podcast in which cosmologist Sean Carroll interviews astrophysicist Lina Necib on What and Where The Dark Matter Is.

So the question it prompts, for me, is how can physicalism, as a philosophical principle, be credibly maintained in light of these conjectures? — Wayfarer

Well you can define physical things so as to include it. That's usually how physicalism continues. At first "physical things" were rocks and such, then they became the less intuitive waves, then the non-inuitive "Probability functions" and now "physical things" pass through each other apparently.

I never got the split between physicalism and idealism for this reason, it seems physicalists are playing dirty by changing what counts as "physical" every few decades, leaving no room for something to be "non-physical". Eventually we're going to say that consciousness is a "Physical thing". But at that point the word "Physical" becomes meaningless and redundant, as it should, and so will "Idealism". We'll just have "thingism"

I mean, presently, it is presumed that some unknown substance, provisionally titled 'dark matter', has observable effects on the cosmological observations, but it's nature is unknown. So how can it be known that it is physical? — Wayfarer

Let's go back a step first.
You've now agreed with me that physicalism does *not* make the claim that we know everything about the physical universe.
So what it actually is, is just the proposition that physical causes and effects are all that exists, even while we don't yet know of, or fully understand, all physical events and mechanisms.

Now let's come back to dark matter.
The reason we even started to suspect such a thing exists is because of physical events we see in the universe. We think it itself is a physical thing, and that's been backed up with empirical data such as seeing gravitational lensing.

Why would you think this presents a problem for physicalism?

Do you feel the same way about other potential additions to the standard model, like gravitons or sterile neutrinos?

You've now agreed with me that physicalism does *not* make the claim that we know everything about the physical universe.

So what it actually is, is just the proposition that physical causes and effects are all that exists, even while we don't yet know of, or fully understand, all physical events and mechanisms. — Mijin

You're talking science, I'm talking philosophy.

Let me explain a distinction: methodological naturalism, as opposed to metaphysical naturalism. Methodological naturalism is to put aside any kind of non-naturalist explanation for observed events. It's a perfectly valid working premise for every scientist. No objection. Metaphysical naturalism, on the other hand, limits the scope of what is considered real to what can be validated by physics or by the sciences generally. That is not a scientific theory but a metaphysical attitude. That's what I'm arguing against, on the basis that physics itself is currently so open-ended that it can't be considered 'closed' in the sense that 'the causal closure' argument wants to appeal to. Sure, you can keep changing the definition of what constitutes 'the physical', but then, how is that 'closed'? It amounts to unending ad hoc extensions to your basic theory.

Well you can define physical things so as to include it. That's usually how physicalism continues. At first "physical things" were rocks and such, then they became the less intuitive waves, then the non-inuitive "Probability functions" and now "physical things" pass through each other apparently. — khaled

I'm talking about old-school 'arguments for materialism'. When I studied undergrad philosophy, the professor's famous book was called 'A Materialist Theory of Mind'. That's the target of my criticism. I'm saying, science is now so open-ended, that it's impossible to appeal to ideas like 'the causal closure of physics', when the actual objects of physics have become so abstract.

Eventually we're going to say that consciousness is a "Physical thing". — khaled

Eventually there'll be peace on earth and no child in poverty.

I never got the split between physicalism and idealism for this reason, it seems physicalists are playing dirty by changing what counts as "physical" every few decades, leaving no room for something to be "non-physical" — khaled

I'd actually say it's the other way round.
Alternatives to physicalism generally suggest that there are mental or spiritual aspects of the universe itself.
Do you think probability waves count as evidence for *that*?

That said, on consciousness specifically, I don't want to see any handwaves of it being purely physical until we have a model with explanatory power of subjective states. You can find many posts of me arguing this on this forum.

I don't want to see any handwaves of it being purely physical until we have a model with explanatory power of subjective states — Mijin

:up:

Nice article in Vox though, and the thrust of it is, if anything, the opposite of Wayfarer's perennial pitch. — SophistiCat

Care to explain in what sense it's 'opposite'?

Take for example this paragraph:

Inherent to the nature of science is the fact that whatever we know is provisional,” Natarajan says. “It is apt to change. So I think what motivates people like me to continue doing science is the fact that it keeps opening up more and more questions. Nothing is ultimately resolved.

I suggest that this is incompatible with 'the argument from causal closure'.

Alternatives to physicalism generally suggest that there are mental or spiritual aspects of the universe itself.
Do you think probability waves count as evidence for *that*? — Mijin

You'd first have to tell me what a "mental aspect" is because everyone seems to be using it but I don't get what it is. If you mean something like consciousness, then there are some interpretations of quantum mechanics that emphasize consciousness as required for wave function collapse.

I don't want to see any handwaves of it being purely physical until we have a model with explanatory power of subjective states. — Mijin

Me neither, but it won't be handwavy. I suspect it will be some form of panpsychism given actual mathematical equations. Someone will propose a theory, which will include some form of awareness or proto-awareness as a fundamental part of the universe and explain in great mathematical detail how our consciousness arises from it and what role it plays similar to how string theory attempts to reconcile quantum mechanics with general relativity. Then show that somehow by some uncertainty principle or other we will never be able to compare other people's subjective experience with our own for instance. Basically, that theory will not change our predictions, but will account for consciousness so will be preferred.

The way I think of it is that whatever role consciousness plays right now (if any) has been lost in our physical theories, but eventually someone might come up with some mathematical model or other that has a well defined concept of "consciousness" that matches what we know about it (ineffable, private, etc)

My point is, whenever something was considered "beyond physics" physicists proposed some mathematical structure or other that accounts for what we know about it and proceeded to define that "beyond physics" thing as something physical. Look at dark matter for instance, matter that just seems to phase through other matter. Pretty sure if you asked a physicist a couple hundred years ago whether two masses can occupy the exact same space they would have said no but apparently now they can. Maybe if you ask a physicist whether or not our subjective experiences can be accounted for physically a couple hundred years from now they will say "yes" but at that point "physical" doesn't mean anything really.

It's late where I am so forgive me if I just wrote nonsense.

But it's a big problem for alternative hypotheses, like that our understanding of gravity is flawed. — Mijin

The idea that our understanding of gravity is flawed ought to be taken as a given, rather than rejected and argued against. The commonly employed representation of a center of mass, or center of gravity is so ridiculously primitive, and cannot provide anything close to a real representation of the relationship between a massive object and its gravity.

Care to explain in what sense it's 'opposite'? — Wayfarer

The article tells how much we actually do know about "dark matter." What we don't know may well turn out to be something pretty boring, like a WIMP. Or it could turn out to be something more exotic (which would be that much more exciting, as far as physicists are concerned). Or it could remain forever out of reach of our instruments and our models. But none of this implies or suggests that something "non-physical" is going on (whatever that means).

Well you can define physical things so as to include it. That's usually how physicalism continues. At first "physical things" were rocks and such, then they became the less intuitive waves, then the non-inuitive "Probability functions" and now "physical things" pass through each other apparently.

I never got the split between physicalism and idealism for this reason, it seems physicalists are playing dirty by changing what counts as "physical" every few decades, leaving no room for something to be "non-physical". Eventually we're going to say that consciousness is a "Physical thing". But at that point the word "Physical" becomes meaningless and redundant, as it should, and so will "Idealism". We'll just have "thingism" — khaled

That's a problem if you define "physical" as what current physics posits - see Hempel's dilemma, etc. (SEP article on Physicalism goes into gory details if you are interested.) I personally don't see a satisfactory definition of "physicalism" in terms of an ontological commitment.