John Donne's "A Valediction: Forbidding Mourning Group" contains a metaphysical conceit, an extended metaphor or simile in which the poet draws an ingenious comparison between two very unlike things [enotes]. In this poem, the comparison is made with Donne and his wife, who must part temporarily and two compasses.
The main idea of Donne's poem is that in their parting (he sojourned in France), they should not profane their love. While the love of "sublunary lovers" cannot survive separation, theirs easily can.
For, the love that he shares with his wife is "interassured of the mind" and it surpasses the limitations of such mundane circumstances as their temporary parting.
In this parting, they experience, not an end, but an expansion likened to the "gold to aery thinness beat."
If their souls are separated, Donne contends in his metaphysical conceit that they are like the feet of a compass: "as stiff twin compasses are two." His foot is the part of the compass that moves around the fixed foot, that of his wife:
Thy firmness makes my circle just
And makes me end where I begun.
Donne's love transcends the mere physical in this conceit of the simile of his and his wife's two feet as the points of a compass.