Constantinos
"For me the physical universe is "what is" for Physics. But we cannot know "what is". We can only know our measurements and observations and understanding of "what is".
This clearly exposes the flaw in your thinking. What we know, given that this is generated on the basis of valid presumptions and due process, must be deemed to be what is (with the caveat 'as at this time', but that is irrelevant to the point, just a reflection of how knowledge must accumulate). Indeed, we will know it is what is when, after some appropriate duration, no new knowledge emerges. That is, we can only know 'what is' on the basis of what we are enabled to know. That may or may not be 'what is', but it is what is for us. Because we are trapped in an existentially closed system. So the question becomes, not what existence might be, which is an unanswerable question, but what form of existence is potentially knowable to us. And since this is determined by a definable physical process, that, which is what physics should be analysing, has a definitive physical form.
You are creating a false differentiation between different forms of knowledge, on the basis of a confusion between what existence possibly could be, and what it is to us. Then declaring one of those forms to be valid on the basis of that flawed differentiation. Knowledge is either valid or not valid, it is not a function of its form, but of its correspondence with the proper reference. And anyway, declaring one form of knowledge valid begs the question as to on what reference is this validity being deemed. Which then comes back to the point that we can only know what we are enabled to know, ie what must, logically, be presumed to be just one possible form of existence. But since we cannot know any other, this is irrelevant, for us, it is what is.
You then invoke some conditions on 'what is'. That is, you start the very process I keep referring to, ie we can only know one possible form of existence. You state: "For me, something "physically exists" if it occupies "physical space" and takes "physical time".
Of itself, there is nothing wrong with this statement, but it is a derivative of the fundamental point, because the question arises as to what, physically, is 'it', 'space', 'time', and how do we know of it. What physically exists (ie is what is for us) is what is physically received by the sensory systems, and what existed which physically created that. In other words, what is, as far as we can know, what is independently 'out there'. Independent being independent of the process which upon physical receipt, then processes what is received. That is, we, as physically existent entities, are as much 'out there' as any other entity. The reference for validity of knowledge is correspondence with that physically independent existence. Since the physical processes whereby sentient organisms are caused to be aware of it (ie receive physical input) are not perfect, then there is a need to hypothesise in some circumstances, ie calculate, on the basis of knowledge as to how the processes function, what would have been received had there not been some identifiable impediment to that. Also, obviously, 'individualism' has to be eradicated from the results of the processing. But these 'adjustments' should not be confused with the generation of beliefs, ie attempts to create knowledge which we cannot know.
What can be investigated has a definitive form, and the process of accumulating knowledge of it must respect that, not presume it is an abstract concept, nor invoke unproven presumtions about how it occurs.
Paul