"What is not forbidden..."

There's a maxim in particle physics that "whatever is not forbidden is mandatory" -- i.e. that if a process does not violate a conservation law, it will take place with some nonzero probability. The attribution was traditionally to Murray Gell-Mann or to Richard Feynman, with a slight bias towards the latter. (In general, doubtful quotes are eventually attributed to Feynman.) Intriguingly, a similar phrase -- "what is not forbidden is compulsory" -- appears in one of W.H. Auden's late poems. Auden appears to have taken it from T.H. White's Arthurian novel The Once and Future King.

There are three possibilities for how it entered physics. Either Feynman or Gell-Mann: 1. came up with it independently. 2. read T.H. White. or 3. took it from later Auden. While 1. isn't impossible, I'm inclined to prefer 2. or 3. because Gell-Mann was a literary sort who took the word "quark" from Finnegans Wake. (Where it's supposed to be onomatopoeia for the aerial croaking of geese.) The question then is whether it was 2. or 3. I'm inclined to favor 2. Either way, it couldn't possibly have been Feynman.