Definable numbers, then?
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Computable numbers?
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"The mass of this rock in kilograms" does not provide a computation, or even a mathematical definition, of the real number to which it refers.
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Rational numbers, then.
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That's what the ancient Greeks said prior to the discovery that the square root of 2 is irrational.
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In practice, measurements are never precise enough to tell whether a number is irrational, though.
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Yes, the rationals are dense and codense in the reals, so it is impossible to tell whether a number is rational by measuring it precisely enough.
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This was likely what led the Greeks to identify the reals with the rationals.
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