To me, the idea of substitution as a test of a science fiction story (the SF/Western substitution mentioned above) is pretty irrelevant if the story is a good one. Using the Galaxy Test I don't see how The Green Hills of Earth could qualify as SF. It's just (sic) a sea story, with spaceships and atomic piles substituted for tramp steamers and steam tubes, a prose extension of Kipling's McAndrew's Hymn.

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The Devil offered me power. I told him I preferred aperture.