If you mean the poem by Robert Graves, he's distinguishing between:
1. nakedness = a natural state of "being without clothes": we are naked when we wash, or when we are examined by a doctor.
2. nudity = an artificial state of "being without clothes": we are nude when we want to use our nakedness for a special purpose, such as attracting someone's attention, or posing for a photograph or a painting.
Not everyone would agree with Graves, however, so it's best not to treat them as if they were dictionary definitions!
(If you did mean Graves, by the way, he was English, not American.)
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