Goe, and catche a falling starre,
Get with child a mandrake roote,
Tell me, where all past yeares are,
Or who cleft the Divels foot,
Teach me to heare Mermaides singing,
Or to keep off envies stinging,
And finde
What winde
Serves to advance an honest minde.
If thou beest borne to strange sights,
Things invisible to see,
Ride ten thousand daies and nights,
Till age snow white haires on thee,
Thou, when thou return'st, wilt tell mee
All strange wonders that befell thee,
And sweare
No where
Lives a woman true, and faire.
Donne is saying ‘If you’re used to seeing strange things, then you might want to try finding a true and fair woman.  You will look for her for what can be calculated as 27 years and 130 days. After seeing many strange things, but not an honest and beautiful woman!’ 
 This stanza is used to prove the impossibility of this feat. Interestingly, the subject of this poem, this fictional woman, is first mentioned only at the end of this stanza.
Song