The back of the envelope is fundamentally about scale. If details were always crucial, if everything was always a close call, everything about the same size, lots of explanations similarly plausible, then there would be no back of the envelope. Its approximation and estimation would not work. But instead the universe has a broad breadth of scale. There is both the very big, and the very small. Often along side each other, providing contrasts so dramatic, that blurring the pieces doesn't change the picture. Explanations, seemingly plausible, that a moment's rough reflection reveals are not even close to fitting. The rough is often sufficient, and should always be the prelude to worrying about detail.
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