I don't know why this post is showing, it was deleted because I meant to post it on my lemmygrad account. Federation is broken. I'm not sure I agree with your definition of materialism and idealism.

You can see my lengthier answer below as I do not want to write this all out again:

• https://www.quora.com/Indirect-realism-has-to-be-true-in-some-sense-due-to-the-way-we-process-information-With-that-being-said-what-is-the-best-indirect-realism-theory-that-is-accepted-by-neuroscience-and-philosophy/answer/%E7%9C%9F%E7%90%86zhenli?prompt_topic_bio=1
• https://amihart.medium.com/the-bizarre-metaphysics-of-the-youtube-channel-dialect-c03f60da8ce9
• https://amihart.medium.com/metaphysical-realism-an-overwhelmingly-dominant-philosophy-that-makes-no-sense-at-all-44343a1d8453

But the change from language from "materialism" to "physicalism" was not random. It arose as bad arguments in favor of indirect realism (sometimes also called metaphysical realism) became popular, and the core defining characteristic of indirect realism is a belief in transcendental-indirect knowledge, that you can come to know of the existence of something which is impossible to observe under any conceivable circumstances (chain-indirect) as well as impossible to observe even with a tool (tool-indirect), but you use logical arguments to transcend what you observe to justify the existence of something entirely unobservable.

In Kant's phenomenal-nomenal distinction, the phenomena is defined to be everything you perceive, so by definition the noumena is entirely unobservable under any possible circumstance, not through chain-indirect or tool-indirect knowledge. This creates an explanatory gap of how you can possibly know anything about reality at all and not just devolve into complete idealism, but this is instead justified with arguments that we have some sort of implicit ability to transcend observation through pure logic.

The popularity of this point of view combined with the discovery of fields by Maxwell and Faraday led people to apply the same logic to physics, to argue that we can transcend what we can observe and know that ontic fields really exist as a beable in the world despite us having no possibility to ever have chain-indirect knowledge about them (we cannot conceive of any circumstance where they can possibly be observed) nor tool-indirect knowledge about them (they cannot be observed via tools either). Rather, they are proposed to be an entirely invisible thing which "acts" upon what we observe, and we come to know of its existence through pure logic, almost like how Christians argue God is a supernatural entity which has no observable properties but influences what we observe from outside of nature.

Engels does not merely say reality is just "matter in motion" to specify that merely that only matter changes. He is very clear he is claiming that all that exists is matter in motion, that this is the ultimate reality.

Physics, like astronomy before it, had arrived at a result that necessarily pointed to the eternal cycle of matter in motion as the ultimate reality.

Indeed, Engels is clear that he takes matter and motion to be an abstraction from what we sensuously perceive, and so unless one wants to conclude Engels is an idealist, one has to necessarily conclude Engels is a direct realist. He is clear that matter only feels like it cannot be perceived directly because when we are talking about "matter as such" we are talking about a metaphysical abstraction of matter, but if we are more concrete and specify the context of the matter we are talking about, we can indeed directly sense it.

The two forms of existence of matter are naturally nothing without matter, empty concepts, abstractions which exist only in our minds. But, of course, we are supposed not to know what matter and motion are! Of course not, for matter as such and motion as such have not yet been seen or otherwise experienced by anyone, only the various existing material things and forms of motions. Matter is nothing but the totality of material things from which this concept is abstracted and motion as such nothing but the totality of all sensuously perceptible forms of motion; words like matter and motion are nothing but abbreviations in which we comprehend many different sensuous perceptible things according to their common properties.

Hence matter and motion can be known in no other way than by investigation of the separate material things and forms of motion, and by knowing these, we also pro tanto know matter and motion as such. Consequently, in saying that we do not know what time, space, matter, motion, cause and effect are, Nägeli merely says that first of all we make abstractions of the real world through our minds, and then can not know these self-made abstractions because they are creations of thought and not sensuous objects, while all knowing is sensuous measurement! This is just like the difficulty mentioned by Hegel; we can eat cherries and plums, but not fruit, because no one has so far eaten fruit as such.

Engels is clear throughout the book he is tying his understanding of material reality to observation. There is one section where he criticizes the towers of abstractions built by mathematicians and insists upon them coming back to the real world and see how their mathematics are actually used in practice, and that is the reality of the mathematics, its connection to the world in real-world applications, a very Wittgensteinian approach.

A four-legged furry feline can be called a "cat" or a "猫." The symbol is ultimately not ontological, but what it refers to in empirical reality is ontological. If I told you a room contained a "cat" before you entered it, what you will predict you will see when you enter it would be the same as if I told you it contains a "猫." It would be very strange, then if someone explained that "cat" and "猫" are ontologically different things because the symbols are different. It is a kind of reification. Symbols are socially constructed and are not intrinsic properties of the things they are being used to identify.

Mathematics is merely a language to describe the motion, and thereby the nomology, of matter. All physical theories can be reformulated into different mathematical language, and so the specific choice of mathematical language we use is socially constructed. It is a kind of reification to insist that the mathematical symbols we use have real ontological existence, a bizarre metaphysics of a kind of Platonism.

Fields are optional. Wheeler-Feynman absorber theory captures electrodynamics without introducing the mathematics of fields, but different mathematics. Heisenberg's matrix mechanics makes all the same predictions as Schrodinger's wave mechanics without using the wavefunction. If you believe fields or wavefunctions are ontological entities in the world, then your ontology is entirely ambiguous as it is completely contingent on socially constructed choices of mathematical formalism, which an alien species that thinks differently than we do and finds different mathematical symbols more intuitive would thus believe in an entirely different ontology.

This kind of transcendental-indirectness thus leads into a kind of postmodernism where reality is whatever you want it to be. Indeed, that is the state of current academic literature. Physicalism has been a complete disaster and has caused physicists to reformulate quantum mechanics in dozens of different ways using different mathematical symbols all believing they are discovering new ontology doing so, and the physics community has become completely split with no one agreeing on what the actual ontology of the world is, with many physicists taking the stance that all viewpoints are valid and reasonable, and thus reality can be whatever you want it to be.

This issue all arises because physicalism is a deviation from materialism that adopted indirect realism and thus took the stance that transcendental-indirect statements are valid, and end up making the category error of treating things that should be nomological as ontological, like fields, wavefunctions, and spacetime fabric. All these problems disappear if we return to materialism, if we stick to matter in motion, and we reject transcendental-indirect statements. Chain-indirect and tool-indirect statements are valid as they do not contradict with a belief that we can define reality solely in terms of what we can actually observe, but transcendental-indirect statements, the basis for indirect realism (sometimes also called metaphysical realism) just leads to an incoherent metaphysical abyss.

There is a whole other topic I don't want to write more and get into here as I have already written enough, but physicalists rarely understand what Chalmers' "hard problem" is. They dismiss the problem as not actually a serious problem because they have not actually read the original papers by Chalmers and Nagel and don't comprehend the actual argument.

A lot of people are convinced out of physicalism and become idealists or dualists because they see Chalmers' and Nagel's "hard problem" argument which has no real rebuttal from physicalists and they rarely even understand what the problem is, and others just dismiss it vaguely as something we might solve one day without specifying what a potential solution would even look like.

However, once you recognize that physicalism arose from adopting an indirect realist stance and thus accepting transcendental-indirectness, you can go back and read Chalmers' and Nagel's original paper and see quite clearly their starting assumption to derive the problem is the assumption of indirect realism. The argument is thus only applicable to physicalism and not to materialism.

In other words, the "hard problem" really is a proof by contradiction that indirect realism, and by extension physicalism, is incorrect.