This is making me insane. I could swear it was W. H. Auden, but it's not in the only book of his poems I possess, and I can't find it anywhere on the net. . .does anyone here recognize it?

Nevermind. It's from "Leap Before You Look"--but I was remembering it incorrectly. The actual wording is "Look if you like but you will have to leap."

Yes, it is Auden.
http://pages.prodigy.net/maecooper/poetry/leap.html

Thank you--that was a much more organized presentation of the poem than the one I finally found. Odd--I'm not sure where I encountered it (and, oh, jeepers, now I'm gonna have to embark on a voyage to pin THAT down!) but it seems it was as a truncated quotation somewhere, because I recognize only the first stanza. . .and, to my mind, that is really the only bit worth knowing. Sad. . .at his best, Auden was unbeatable.