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I see. We actually failed to 'define a system' for what it means to be a real number. What the set of real numbers contains can't be defined in any strict formal way.

Even for the range [0, 1] saying all numbers including rationals and irrationals is an incomplete cop-out. The rationals are defined. Only some irrationals are/can-be. Saying a 'number that is not rational' is not a definition--it is negative space. Prime numbers are the negative space of composite numbers--they are however countable and computable. The negative space within real numbers is different. There are no possible constructions to reach some/all of them.

Does the same problem arise with any uncountably infinite set or only not-well defined ones? Is "The Set of all Subsets of Natural Numbers" (which is uncountable) also non-mathematical in the same sense? A program (requiring infinite storage and computation time) can be constructed.




"What the set of real numbers contains can't be defined in any strict formal way."

On the contrary. There are multiple ways to rigorously define the Real Numbers. The most popular way is perhaps Cauchy sequence: https://en.wikipedia.org/wiki/Cauchy_sequence

There are also Dedekind cuts: https://en.wikipedia.org/wiki/Dedekind_cut




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