Does the success of physics justify a belief in Physicalism?

1. You define “a property is physical, if it is at least in principle measurable and quantifiable by measurement.”

   The definition links the core concept of physicalism with a certain method how to interact with physical entities. It is an example of an operational definition.

2. Physicalism is a monist worldview. Since the days of Thales philosophers of nature searched for the fundamental substance. It should provide the great unification which facilitates the understanding of all phenomena in our world.

   IMO this approach has failed.

3. I consider the concept of information and the process of information processing a second basic entity and basic process. The process replaces what in the past has been named “mind”.

   The physical aspect of information deals with the carrier of information only. This can be neglected to a large extent when dealing with the content of information.

4. Hence I do not see why mental properties will also be reducible to physical properties in the future.

You are effectively asking several closely related questions here, and they do not all have the same answer, although the answers couple.

I will try to answer:

1. Are physics and physicalism the same thing?
2. What is the definition of physicalism, and what does it exclude (reduction, abstractions, causal mind)
3. How legitimate is it to infer the truth of physicalism from the success of physicalist reduction?

Are physics and physicalism the same thing?

No. Physics is a sub branch of science, which is a family of related methods of doing empiricism reliably. Empiricism, and science, are intrinsically open (there are always unanswered questions), and "perpetual uncertainly" about one's models and conclusions is an essential presumption.

Physicalism assumes a conclusion of science -- that when science is finally done -- there will be only one type of thing revealed to exist in the world -- whatever it is that is studied by physics.

What is the definition of physicalism, and what does it exclude (reduction, abstractions, causal mind)

To answer this, I reference this answer on what modern theoreticians of physicalism think is the definition: https://philosophy.stackexchange.com/a/135151/29339

Stoljar did not think physicalism can be defined, and it should be abandoned as a philosophical view.

Papineau accepts that minds could exist and be emergent from "physics", but not be causal and still hold by physicalism. Stoljar disagreed on emergence, and considered reduction to be central. Kim considered non-physical emergent minds, even if they are non-causal, to contradict physicalism, and thus became an epiphenomenal dualist. Melnyck considers minds to be a delusion, which removes one major type of data from what he has to explain, and commits to reduction.

Kim also rejects the assumption of the existence or causal nature of abstractions as contrary to physicalism, and therefore rejects the most popular current philosophy of mind, functionalism, as a non-physicalist theory (because functions are abstractions, and causal in functionalism). Papineau left the acceptance of functionalism as an open question for physicalism. Melnyck admitted that physics presumes the reality and causal nature of abstractions, and that they do not reduce to matter, but did not then explore what that would mean to his belief in a monistic "physicalism" that has two different base substrates.

As physics fully accepts information as a causal/bounding parameter, and most theoretical physicists seem to think that math sometimes "gets fire in the equations" in the words of Stephen Hawking, and basically creates physics -- it is actually reasonable to treat physics as a dualist matter/abstractions field, and then treat physicalism as a dualist ontology. IF one does this, THEN, contra the four theoreticians I cited, functionalism (where mind is assumed to be identical to the performance of a set of functions or algorithms) could be a "physicalist" philosophy of mind.

Your definition proposed to accept emergence FROM physics, but that that physics remained the sole substrate. If you accept that physics is matter/abstract dualism, then you could concievably accept the emergnece of mind as a new phenomenon from physics -- leading to Popper's triplism. And per your definition, Popper's triplism would be considered a version of physicalism.

Two final points in your definition -- tying the definiton of "physical" to "measurable" is to reject a variety of social sciences. The existence of markets, norms, beliefs, etc. are not "measurable". Similarly, the existence of core principles in physics: spin, quark type, causation -- are not really metricized. Some aspects of most of these can be metricized, but ideas are more fundamental than any neuralization of them. Metricization as a criteria will break, if pushed.

Additionally, trying to define a "property" as physical or not -- can quickly tie one into knots. Is the color of a stop sign, or its hardness, physical? If I imagine a stop sign, and knocking on it, is its color or hardness actually "physical"? One is far better assigning properties to a physical thing, or a mental thing, rather than trying to define "physical" with abstract massless and locationless "properties".

None of the four I cited, you, or the emergent physicalists who would accept Popper's emergent consciousness as physical, have converged on a specific shared definition. Physicalism is a broad research programme, per Imre Lakatos' Research Programme approach to characterizing science. I recognize it as a family of beliefs, even if it is not explicitly definable.

How legitimate is it to infer the truth of physicalism from the success of physicalist reduction?

Papineau made this an explicit claim in his paper The Rise of Physicalism. He argued that YES, most scientists and philosophers observed the significant success of reducing about half of chemistry (valence theory, bonding theory, periodic table of the elements) to physics, and of reducing some of the most mysterious aspects of cellular biology to biochemistry in the early decades of the 20th century were astounding. Under Lakatosian principles of accepting a research program which is progressive, and abandoning one which is regressive, a huge number of major thinkers abandoned idealism and dualism, and embraced physicalism.

However, Papineau stopped his discussion at the conclusion of the "rise" period of physicalism. The subsequent 3/4 of a century has not shown the same sort of progressivity as the first half of the 20th century did. The inability to reduce abstractions to matter, the stalling out of chemistry at roughly 50% reduction, the failure to reduce basically any other science, the failure of physicalist theories of consciousness which is called the "hard problem of consciousness", and the discovery in systems theory that the same "systems phenomena" appear in a diverse range of substrates (economics, computer networks, traffic patterns, monetary systems, language usage, etc.) tend to be characteristic of a research programme that is becoming regressive.

The answer to your ultimate question is -- yes it was reasonable at one time, but maybe no longer is.

There are good reasons that there is a decreasing level of acceptance of physicalism, and a revival of multiple rival ontologies.

Physics models the universe using mathematical formalisms. It is our best way of understanding how things work at the most fundamental empirical level.

Does this imply physicalism? It strongly points towards it, since we see no evidence of supernatural or preternatural phenomena. It is however not logically entailed by explanations from physics. Just because supernatural phenomena were not to exist this does not also entail that physicalism is true. Dualism or panpsychism could be competing naturalistic explanations. A panpsychist believes consciousness is in everything and it’s not reducible to physical laws. A dualist believes that the mind is a separate substance to the body, and also not reducible to it.

With respect to the question of consciousness and physicalism, this is hotly contested. Does consciousness strongly emerge from physical matter or weakly emerge? How do we explain subjective first person experience, the mind, etc. under a physicalist interpretation of reality? Physics gives causal closure of the phenomena we observe which puts heavy pressure on non-physicalist explanations.

Is consciousness emergent in the same way that say wetness, temperature, or biological processes are? For consciousness to be strongly emergent, ruling out physicalism and replacing it with property dualism for example, there would have to be a causal mechanism that is over and above the physical, ie the mind itself can have an effect on reality. More precisely, there is no independent empirical evidence of non-physical causal powers beyond those already described by physics.

Physics describes how the world behaves; physicalism claims what the world is made of.

Causal closure where at a fundamental level physical processes explain the change in or give rise to any empirical phenomena and its causes, is a strong argument for why physicalism is true. It puts pressure on dualism because a mind-body ontology isn’t required for explaining what we see in the world.

Even if consciousness is correlated with brain processes, it remains unclear why those processes give rise to subjective experience at all.

First person subjective experience seems to be just a fact of emergence. There is no why, much the same there is no why an electron has a certain mass. It’s just a brute fact of our universe.

The hard problem (David Chalmers) posits that consciousness is strongly emergent or at least that the why of how consciousness emerges from non-conscious physical matter is a mystery at this point. Strong emergence would imply new causal laws and properties beyond the physical but can interact with the physical. These properties are irreducible to physical laws.

In the case of weak emergence consciousness is fully determined by the physical, does not introduce any new fundamental laws, and is compatible with physicalism.

The explanatory gap between consciousness emerging from physical matter remains unexplained but weak emergence is better supported by the evidence.

If you are asking whether physics favours physicalism to idealism, then yes. Physics provides detailed descriptions, with strong predictive powers, for an increasingly broad range of phenomena. It seems self-evident, to me at least, that the best explanation is that there is a mind-independent reality that physics is describing.

If you are asking whether consciousness will be explained by physics, I am not so sure.

To use a poor analogy, physics can tell you how the electronics in a computer works, but it has nothing to say about software. Physics can tell you why the pixels on a screen are lit up in a certain way, but it has nothing to say about whether the resulting pattern displayed is a picture of Paris or Grimsby.

When I use the words 'physics has nothing to say' I have a specific meaning in mind. Physics uses mathematics to model reality, representing physical quantities with numbers and variables. Physics models reality with equations, and a key feature of equations is that the terms on either side of the equals sign must be of the same kind. If you have energy on one side of the equation, you must have energy on the other, and so on. Physics therefore concerns itself with matters that can be expressed that way. There are no physical quantities or units associated with Paris, Grimsby, redness, beauty and so on, so I would not expect physics ever to provide the tools to explain those ideas. I would not expect ever to see an equation with physical units on one side and X on the other, where X represents 'picture of Paris'.

None of the forgoing necessarily takes us into substance dualism- it just makes the point that physics doesn't address certain types of emergent phenomena. However, if we did find some new thing that accounted for consciousness, presumably it would get added to the list of other stuff we consider physical and therefore would end up being covered by a rescoped use of the term physicalism.

OP: "Should we assume that because physics has been successful in predicting and explaining observations, that this is a good reason that all of reality will be explainable in terms of physical properties?
Is the fact that biological and chemical properties, have so far been reducible to physical properties, a good reason to belief that mental properties will also be reducible?

Explanatory observations actually do constitute basic reality. These explanations are where 'mental properties' form as one with the phenomena in question. The idea is demonstrated in John Wheeler's famous statement:

No phenomenon is a real phenomenon until it is an observed phenomenon.
– Quantum Theory and Measurement (1983).

While this is (apparently) 'mathematically' true in quantum physics, it is also true generally. The articulated perception and comprehension of the phenomenon and the phenomenon's intelligibility result in the same thing and constitute the reality of the real. That is to say, the 'mental properties' concerning the thing – e.g. the understanding of the atom – are the same as the known 'physical properties'. Phenomena exist as 'explained observations'.

The reason this is the basic material reality is because it is the already the limit of rational articulation. One may ask why this is so, but this simply refers back to rational articulation, as it reaches its limit.

Nevertheless, it should be mentioned, while this all constitutes 'actuality' in Kantian terms – that which is actual – from a finite viewpoint planning the future, it is necessary to weigh possibilities, so for example 'reality' as the sum of possibilities may involve keeping an umbrella handy just in case of the actuality of it raining.

Yes, fortunately/unfortunately, "the success of physics" puts a constraint on ontology, what (can) exist. Thinking, for instance, is reducible to physics (electricity) via chemistry (biomolecular activity).

A belief in physicalism is justified by:

• The success of physics (and science in general)
• AND the lack of success of attempts to gain knowledge about things outside of that.

At this point, we seem to only have reliable knowledge of things we can directly or indirectly observe, consistently and repeatedly.

Physicalism rejects the existence of everything outside of that, which can be supported by the lack of justification for such things. There are infinitely many things that might potentially exist outside of what is directly or indirectly observable - if we don't want to believe false things, we shouldn't accept the existence of any such things until we have justification for it. How do we get justification for it? Beats me. We can't, by definition, consistently observe it, directly or indirectly. And observing things seems to be the foundation of all our knowledge. Whereas consistent observation is typically necessary to distinguish things that actually exist from hallucination, measurement error, false cause inference, etc. If a consciousness existed exclusively as floating in a void somewhere (if that were possible), it'd likely be unable to even form a coherent thought, because we can only do that because our brain learnt language from observation, and we built models of what things are and how reality works through observation.

Note that the above also means not accepting the existence of "physical" things for which we don't have evidence, i.e. distant planets we haven't yet observed. The difference between that and "non-physical" things is that, if not-yet-observed physical things exist, we can theoretically observe them, consistently and reliably, and people can agree when we're observing them. Non-physical and non-existent things both cannot be observed, and I can't tell the difference - all non-physical things seem to also be non-existent (or improperly labelled physical things).

What does "non-physical" mean?

I'm still waiting for anyone to explain what "non-physical" even means or to justify the existence of any proposed non-physical thing (and justify it not reducing to things accepted as physical). Just one single concrete example of that is all that would be required to "falsify" physicalism. Yet people primarily argue in the abstract against physicalism (and not very well at that). Consciousness, for one thing, can only really be said to be something we don't yet fully understand. But not understanding something doesn't mean we actually do understand it, and it's some vaguely-defined entirely-separate category of things.

Any way I can think of slicing things up into "physical" and "non-physical" ends up with lines that are little more than an arbitrary classification of things, with no robust criteria supporting that classification. Or it ends up with fuzzy lines that doesn't suggest a difference in the fundamental nature of thing, and it that classifies typically-considered-physical things as non-physical, or vice versa. See the answer linked above for more on that.

Without a justification for the existence of, and the line between, the 2 proposed distinct categories of things (physical and non-physical), without a justification for a single "non-physical" thing, I'll stick to not accepting the existence of any such things, and I'll reject the "non-physical" categorisation itself as poorly defined.

Hempel's dilemma?

Related: The coherence of physicalism: are there any solutions to Hempel's dilemma?. Hempel's dilemma might not be a dilemma at all, and it's arguably shifting the burden of proof onto physicalists to define something they don't propose and don't accept (the non-physical, and how to differentiate that from the physical).

Is physicalism dualist? Has there been a "failure of reduction"?

Physicalism is not dualism, at least not in its most common forms (e.g. reductive physicalism or functionalism). Dualism specifically says there's the physical and another thing (usually the mind, in some form). But physicalism, by definition, only consider there to be the physical. One could say that any form of dualism is, by definition, not physicalism, but it's possible that some physicalists may disagree with that.

More specifically, physicalism doesn't accept a matter/abstractions dualism on account of not accepting that abstractions objectively exist (other than as e.g. brain chemistry). The existence of mental objects (including numbers and such) is mostly a Platonic idea. I guess a dualist might see it as most "reasonable" for physicalism and physics to be such. But that's somewhat just presupposing that physicalism is false, which would be an odd thing to "treat physicalism as" (at least if one is trying to present it most strongly, in a way physicalists would accept). It's odd to try to reduce physicalism down to dualism, when it stands in strong contrast to it.

Side note: One probably shouldn't trust opponents of a position (myself included) to give an accurate account (and/or the strongest possible version) of that position and how justified it is, even if some might present such positions more accurately than others.

Related: Has reductive physicalism been falsified / has there been a failure of reduction? [No.]

Does physics justify Physicalism?

Just to clarify, to "justify" means to give adequate reasons to make it rationally acceptable to believe something. It does not mean the topic of the belief is "true" in any objective sense. We'll come back to that in the last paragraph. (N.B. I notice I am being lax in some parts further down and mixing up Justification and Trueness. Nobody is perfect, I hope you get my meaning anyways.)

Also to clarify, the topic of Physicalism is, like always in Philosophy, complicated, diverse, and often, frankly, opinion-based. At the core believing Physicalism means that one believes that everything is physical. Circular? Maybe. The SEP has pages upon pages on that.

Physics is not a philosophical topic but a formalism, or a job description, of how we -- powered by the Scientific Method -- systematically try to figure out reality by trial and error (or more high-brow, by theory-crafting, experiment and falsification). It is a craft or art, not a philosophical concept. Religious people can and do work as Physicists; as do people believing in the Mind as a non-physical entity.

Should we assume that because physics has been successful in predicting and explaining observations, that this is a good reason that all of reality will be explainable in terms of physical properties?

No. The success of physics justifies a belief in the overall structure or "algorithm" of the field - i.e., the scientific approach and so on and forth seem to be working mostly. This is in fact a real-world indicator or litmus test of the validity of a physical statement: a great many physical theories have stood the test of time and could not be falsified for centuries. It is very likely that belief in their explanation about how objective reality works is justified. (But not a single one of them gives justification to believe that the facts they speak about are true - i.e., every single one of them comes, by definition, with a way for falsification, and every single scientific fact could, in fact, be falsified every single day and hence be completely false. Otherwise it would not be a scientific fact.)

But on the other hand, especially in theoretical physics, there are a great many papers and theories that could not be experimentally tackled so far (think about everything regarding String Theory, Dark Matter, Dark Energy and so on and forth). Therefore there is no justification whatsoever to believe that they represent reality (and in fact many current Physicists lament this problem mightily).

all of reality

All of that said, you are asking whether the success of Physics leads to the conclusion that all of reality is purely physical. It says nothing at all about that. Thought experiment: assume there is a non-physical thing (God, Spirits, Self, Mind ...). That would change nothing about the facts laid out above (i.e., we do have physical theories, and they clearly do match reality until/unless falsified, by definition). So from the existence of non-physical things (which we by definition cannot tackle with physics theories) you would not conclude that the "physics works" relationship breaks down. So Physics does in fact (still) work. If your asked-about proposal "Physics works -> therefore everything is physical" were true, then everything were physical, and nothing non-physical would exist. This contradicts the original assumption that something non-physical exists.

Be sure to note that this argument makes no assumption about whether non-physical things do in fact exist, or what they could possible be (beyond being, well, non-physical). It is a purely logic argument which leads the proposal you are asking about ("Phyisics works great, therefore Physicalism is true") ad absurdum.

explainable in terms of physical properties

And whether all of reality will be explainable in terms of physical properties (and thus forcibly in terms of physical theories)? The answer to this is clearly also "no" for some interesting questions. For example, no "why" question can ever be fundamentally answered by physics ("Why are there exactly X fundamental forces?", "Why is particle Y massing exactly Z gEV?" and so on and forth). Reason: Even if we can at some point answer these particular examples, in these possible future answers we would by necessity make reference to even more fundamental aspects of reality, and the problem would then shift to those (i.e., "Why is more-fundamental-aspect ABC what it is?"). The root cause for these being unexplainable is that they are simply not up to the Scientific Method, unless we become able to create new universes and play around with different sets of fundamental properties (in which case I will happily retract this answer and stand corrected).

And for other interesting questions ("What happened before the Big Bang or inside a Black Hole?") it -- at least at this moment -- is conceivable that there will never be an answer since we cannot go back in time to witness the Big Bang, and seemingly cannot enter a Black Hole to run experiments and then come out to report on them. Big Bang and Black Holes being real is not inconsistent with them never being open to scientific study.

Note that I am not saying that these particular examples will never be solved. Who knows what happens in the future. But it could be that they are hard limits; and in this case not all of physicality is explainable even though there would possibly be no non-physical aspects to reality.

Is the fact that biological and chemical properties, have so far been reducible to physical properties, a good reason to belief that mental properties will also be reducible?

Not related. The one is totally separate from the other.

Now don't get me wrong. I'm a Physicalist myself, and from my Physicalist world-view there follows that the mind, the perception of Self and all that is purely physical. Of course this is just an opinion, man. If anyone had proof, we would not be having this discussion. But this does unfortunately not follow from our successes of reducing biological and chemical phenomena to physical properties.

The argument is similar to before:

Assume that the Mind is non-physical. From this it does not follow that biology and chemistry cannot be reduced to physics (we are, after all, happily doing just that). Hence biology and chemistry are still reducible to physics. If it were true that from this fact you can conclude that the Mind is physical, then you would therefore conclude that the Mind is physical, contradicting the assumption. q.e.d.

And just to hammer it in a third time: this does not imply that the Mind is non-physical. It just lays rest to the argument you are asking about.

To slowly ramble my way to the end...

In a way, "Physicalism" has everything to do with "physical" but not much with "physics". The names are just a little similar. Physicalism is a mindset, a worldview, a typical philosophical opinion or what the SEP hilariously calls an Attitude. Physics is a hands-on way to perform experiments and process the results and as far removed from Physicalism as can be.

Also as final nail in the coffin: just think about how deep our Physics know-how goes these days. It is absolutely mind-boggling, astounding, world-shattering. How many religious people, as a percentage, has this ever convinced? I dare say very, very, very few. How many of the enlightened and rational people here in the Philosophy.SE are not Physicalist? Plenty! And there is a fine list of professional non-physicalist philosophers as well. So to get back to the original quip about what "to justify" means; not only does the success of Physics not logically justify Physicalism, but obviously it also simply does not work in practice to get people on board with Physicalism. I daresay we could snip all except the first and last paragraph of this answer and end up with the same result.

Should we assume that because physics has been successful in predicting and explaining observations, that this is a good reason that all of reality will be explainable in terms of physical properties?

I’ve always thought that this reasoning hides a non-trivial problem (perhaps not irresolvable, but still worthy of careful and deep analysis).

You start with the description of an empirical phenomenon; something you claim is ontologically happening, processes/events that exist in reality. Namely: physics being successful. Predictions working. Observations. We are introducing into our “model of reality” concepts such as usefulness, working, knowing, having success, explaining, physics/science as human practices, etc. In a very fundamental sense, since we are treating them both as ontological empirical phenomena, happening and existing in reality, and as the criteria through which entire worldviews are evaluated.

Now… if all of reality can be explained in terms of physical properties, can those phenomena (observing, understanding, working, having success, doing science, your own “knowing/experiencing/evaluating” this pragmatic feedback, etc.), all what they epistemologically and ontologically requires and entails, be explained in turn in terms of (and only of) physical properties?

I think that is very very hard. It might be done, but only in a pluralist and emergentist context. I don’t see how it could be done in an eliminativist context (using only fundamental particles + laws of physics, sot to speak).

You can go as far as you want toward physicalism and reductionism (by virtue of their pragmatic success), provided you manage to preserve a serious, robust ontological status and meaning for whatever “the success of physics” imply, require and stands for, and you are able to fully explain what it is about in strictly and exclusively physical descriptions.

In other terms: *If your main argument for physicalism is inductive-pragmatic (“physics works incredibly well → therefore everything is physical”), then you have an obligation to give a purely physical account of what it even means for a theory to “work incredibly well”, of what counts as rational persuasion, of what explanatory satisfaction consists in, of what should constitute evidence or good reasons… otherwise you are covertly using extra-physical concepts to justify the claim that no extra-physical concepts are needed. Otherwise you are helping yourself (in the very justification of physicalism!) to a set of normative, intentional, first-personal, practice-laden concepts that you then declare (in the conclusion) to be either illusory or fully reducible to third-personal physical goings-on (but without being able to perform this reduction).

So yes, I think that your argument for Physicalism can be done. But it is an argument that commit you only to certain types of Physicalism.

No. There are properties that cannot be measured even up to probability. This means that regardless of a physical theory which describes the world, there are non-physical propertis (at least, in your definition).