Tom,
With capitalized Relativity in contrast to logically founded relativity I meant the hypothetical explanation of Michelson's 1881/87 null result which was ad hoc enforced by Lorentz, Poincaré, Einstein, and others. This Relativity led Poincaré 1904 and later Minkowski to a spacetime that is based on Parmenides. Physics suffers from such illusory Relativity up to now.
Among items that are not concrete and real are mere mathematical objects. For instance, the potential length of a way around a circle has no end to both sides. Bolzano considered "I think that I think, that I think, ... " as evidence for the existence of infinity. Such path is therefore not commensurable with any measure of that circle, e.g. its circumference. Infinity is no measure, no quantity but a quality. It is neither an even nor an odd number.
Likewise, nothing tangible in nature has the measure zero because in contrast to mathematics, natural items are not endlessly divisible. Non-existence is not a quantity but a quality. It is also not commensurable with anything.
Eckard