How often studying some "sure physical" phenomenon you were disappointed to find no physical object behind it, but a mathematical relationship causing illusion of "physicality" of the phenomenon? I found it so often (when analysing origin of phenomena in depth, not just staring at it's "physical surface") - that it is like chasing a ghost. "Physics" is just a label we use for what we (personally) don't know the mathematical origin of yet. Example. Yesterday (well, a couple hundred years ago) a "force" was a "solidly physical" phenomenon. Starting with Einstein's general relativity and especially developing quantum mechanics it became obvious that force does not physically exist. Say, in GR a "force" does not exist at all. In quantum mechanics all we have is conservation of momentum during interactions - and this conservation is perceived as "force". Averaging over many interactions we then define macroscopic "force" as the average of rate of change of momentum F=dp/dt. So, there are no "forces" in nature. All there is a mathematical result (say, of of bent space in GR, or of conservation of momentum during interactions in QM) which we perceive as s "physical force". Same with, say, many known "physical" properties (say, rigidness of solid bodies) - at close look they begin losing "physicality" but instead become more and more a mathematical consequence of rules of interaction of their "parts". Rules of interaction are, by the way, strictly mathematical: symmetries, idistinguishability of "parts" or states the parts can occupy, quantization (=indivisibility without radical change of properties), etc. "Parts" themselves (say, atoms) at close look happen to be NOT ultimate "physical" blocks but rather "intermediate mathematical results" of more fundamental "blocks" (and of mathematical rules of play). Today no one can claim anymore that there is anything "surely physical" in our universe. So, when we DON'T know the origin of some object (say, of electron), we call the object "physical". But when we DO know (say, of origin of rigidness of fermions versus softness of bosons) then we no longer call object/phenomenon as "physical" but call it "mathematical consequence" instead (statistics resulting in Pauli exclusion and Bose condensation in this case being the origin of this "repulsion" of fermions and "attraction" of bosons).