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Materialism and Physicalism are two very different notions edit
Materialism is ancient classical, and it refers to things that can be grasped by touch. Things that are only seen are not necessarily material !
Physicalism is both ancient and modern and has evolved with the cultural and technological advancement of physics. It refers to what is evident in public, empirical science. That means that "energy" which is a purely mathematical construct is physically "real" or "evident" but due to its multiplicity of 'forms', all of which express 'potential' energy, it cannot coherently be said to "exist".
Galilean relativity and Galilean invariance edit
Galilean invariance is fundamentally incoherent without an understanding of Galilean relativity.
Galilean *relativity* is exemplified by a sleeping man who is both not moving in his bed and is, at the same time, moving around the Earth and moving around the Sun at different velocities. If this single example is true, then Galilean relativity which says that all things are both moving AND not moving at the same time is necessarily implied. This is a fundamental philosophical insight that underlies all modern science.
This quite simple and absolutely incontrovertible principle appears to be very difficult to accept, or just to understand, even for highly intelligent and educated people. The reason for this turns out not to be the principle, but the rigidity of our psychological habits.
Galilean *invariance* is a necessary 'inner' Galilean/Newtonian principle of physics. It says that the laws of physics are invariant in each of the above three and all other Galilean/Newtonian inertial frames. However, without an infinite number of potential points of view, or origins for potential frames of reference, this scientific invariance would be meaningless.
We live in our *personal* immediate material world, the one we can touch, taste, and smell, and this world is relative to the 'I' that each of us is. We are the origin of our unique 'frame of reference', so that 'I am here, now' at all times in all places as long as I live.
Contrast this to Galilean relativity I described above. In the Galilean *scientific* world, as is different from my world, or my 'I', everything is relative to some arbitrary frame of reference, rather than to my 'I'. That arbitrary frame is set, for practical purposes, such as for the sake of conceptual or calculational simplicity, to be anywhere, and at any time, in the universe.
Let's switch from the philosopher's perspective to the physicist's. The physicist must accept without question and further thought the principles of the physical universe. A principle is different than a hypothesis or a law in this respect. Philosophical principles are unquestionable assumptions in physics. Once the physicist does question one of the philosophical principles of physics, then he has stepped 'outside' of physics, back up to philosophy.
On the 'outside', Galilean invariance is not a principle needed by the philosopher. I don't even think it can possibly be derived from the broader concept of Galilean relativity. Galilean invariance is a *practical* principle of physics, assumed just so that physics, as a universal science with universal laws, can coherently do its job. Just as Euclidean space was needed by Newton for his work.
In any case, searching the internet for "Galilean relativity", most physics class notes will quickly skip to "Galilean invariance" because that is their practical need. But fortunately not all. There are some few physicists out there who do understand the relativity of Galileo's ship and the associated thought experiments.