The Diminished Fifth Is a Mutated Fifth

If we were to represent 13 in the major scale, we'd end up with a diminished fifth. The tonus diabolicn is the diabolic tone between notes of the major scale. So if you omit number 13, there is no such dissonance. Whether this was somehow understood intuitively through musical experience, or whether conscious knowledge of these things at some time in some place in history far exceeded that which we imagined was possessed, may never know.