We explain why and how the Hilbert space comes about in quantum theory. The axiomatic structures of a vector space, of scalar product, of orthogonality, and of the linear functional are derivable from the statistical description of quantum micro-events and from Hilbertian sum of squares |a 1 | 2 + |a 2 | 2 + • • •. The latter leads (non-axiomatically) to the standard writing of the Born formula f = | ψ|ϕ | 2 . As a corollary, the status of Pythagorean theorem, the concept of a length, and the 6-th Hilbert problem undergo a quantum 'revision'. An issue of deriving the normed topology is likely solvable in the affirmative and has been stated as a mathematical problem.
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