driven mad by a gap in my memory

[x_los]

There was a (Cavalier, I think?) poem that began with something like "Go and." It was instructions to search the world for all manner of impossible things and ended with the poet saying that even if the person he instructed did all this, he'd never be able to find a 'woman true' or 'woman fair and true.' True as in constant- he didn't doubt the veracity of her womanhood. I can't remember the intervening text, or who wrote it, and it REALLY is frustrating me. Anyone know?