In John Donne's song "Go and Catch a Falling Star" the speaker demands many impossible things of the reader. The reader is told to "catch a falling star," bring the past back to the present, and "things invisible see." The last impossible instruction is to find a woman who is both "true and fair."
Donne's speaker's message here becomes clear: it is impossible for a beautiful woman to be faithful. The speaker even entertains the possibility of this impossibility. He warns the reader that even if such a woman was found, by the time the speaker would walk "next door" to meet her, she would have already been unfaithful "to two, or three" other men.
The message, although misogynist and sexist to a more current reader, can still be considered positive in a way. The poem suggests that all its impossibilities are, in fact, possible if one works hard enough. The speaker does not advise the reader against such impossible things, but instead demands that the reader achieves them. The final message can be summarized as the following: although I do not believe a beautiful woman could ever be faithful, I encourage you to search far and wide until you find a woman who can prove me wrong.