The theme of John Donne's poem "Goe, and catche a falling starre" is bitterness at a love betrayed. The speaker invites the reader to try doing impossible things, saying that even if the reader succeeds, there is one impossible thing they will never do: meet a faithful woman. Even a woman who appears faithful will show herself to be false, given enough time.

To catch a falling star, or hear the singing of mermaids, is as unlikely to the speaker as "keep[ing] off envies stinging" or discovering the means to "advance an honest mind." The speaker feels he has been poorly treated, despite being "honest," and not even someone gifted with psychic powers could give him proof that his bitterness is unwarranted:

If thou beest borne to strange sights,
Things invisible to see,
Ride ten thousand daies and nights . . .
Thou . . . wilt sweare
No where
Loves a woman true, and faire.

A faithful woman is more mythical than the powers of the mandrake root. Should the reader succeed in finding one, the speaker would not go to meet her, for all women are only faithful for a time, before giving their love to others.