John Donne's “A Valediction: Forbidding Mourning” is a prime example of a metaphysical poem, for it contains many metaphysical features. Let's examine some of these.

First, the poem focuses on profound subject matter, especially death and love. Lovers, the speaker claims, can never truly be separated, for their souls are joined as one, and not even death can break that bond.

Getting that message from the poem, however, isn't especially easy, for the poem is complex and not overly clear, which is another metaphysical trait. Donne's poem, like other metaphysical poems, requires time and effort to figure out, as readers have to untangle the interwoven threads of the language and images. The poet, for instance, begins with an image of “virtuous men” passing away, whispering to their souls to leave while their friends debate whether they have truly died. But the speaker and his beloved will never admit such an absence or any fear of death, not with their souls so united.

To creatively illustrate his point, Donne employs metaphysical conceits, unusual (even strange) extended comparisons that juxtapose two highly dissimilar things for emphasis and interest. In this poem, the speaker says that he and his beloved are “so much refined” that they are like “gold to airy thinness beat.” This is an odd comparison indeed, but Donne is showing how the souls of the speaker and his beloved are expanded so much by their love that they can never be separated. They simply expand as much as they need to so that they remain united.

Donne also uses the conceit of a compass (not the kind that points north but the kind that draws a perfect circle). As the two legs of a compass, the souls of the speaker and his beloved are joined so that no matter how far one travels, they are always united and will always return to one another.

Metaphysical poetry is also both intellectually driven and passionately emotional. Donne's poem, too, is both, for its conceits and complex images require intellectual proficiency and creativity, but the poem is still about the passion of a lover who wants to remain eternally united with his beloved.

"Metaphysical" is an adjective that describes something that transcends the physical world. Metaphysical poets often use complex and concrete metaphors to explore complicated metaphysical ideas—such as death, the subject of Donne's poem. 

Readers and critics characterize metaphysical poetry as explorations of complex, highly intellectual thought. The transition and relationship between life and death is certainly complex, perhaps our most complex subject matter as humans. How does Donne resolve the complexity of dealing with death?

For one, Donne offers multiple ways of thinking about death. Perhaps it is a great conjunction, all things living and dying becoming one, as our souls "endure not yet / A breach, but an expansion, / Like gold to airy thinness beat." Or, if we remain as individual units when we die, our relationship to the living is a parallel one, "such as stiff twin compasses are two." To offer competing descriptions of what death may be like does not undermine any one idea but conveys the many nuances of death, the many possibilities as to what it is and how it relates to life. 

Aside from its intellectual complexity, this poem encapsulates metaphysical poetry in other ways. For instance, metaphysical poets often use wit or humor. While a metaphysical poem may address such lofty subjects as love, life, and death, its authors frequently reach for levity to ground their poems. In Donne's poem, the entire first stanza comically makes light of death itself: 

"As virtuous men pass mildly away, 

   And whisper to their souls to go, 

Whilst some of their sad friends do say 

   The breath goes now, and some say, No...."

One feature in "A Valediction:  Forbidding Mourning" that reflects what we, today, label metaphysical poetry is stretched metaphors or conceits.  Four of the stretched metaphors, with explanations, follow:

1. Separation of death compared with separation when one lover leaves another (stanzas one & two).  Let we two lovers not cry or sigh, but keep our separation to ourselves.  The idea is that to speak loosely about their feelings is to lose them.
2. Movement of the earth draws attention to itself, yet movement among the stars, which is movement of far more importance, goes unnoticed (stanzas three-five).  Their love is like the movement of the stars.  It doesn't need to draw attention to itself to be monumental.  They don't need to cry or make a show of their separation.
3. Their love does not suffer a breach, or break, but experiences an expansion:  like gold that is beaten to airy thinness (stanza six).
4. Their love is like two legs of a compass.  One leg travels around, but is always connected to and anchored by the other.  Two legs of a compass cannot be fully separated, just as the two lovers can never really be separated. 

The most famous of the conceits is the final one.  The metaphor is stretched in the sense that two things not usually thought to have any thing in common are compared--the legs of a compass are compared to two lovers.  The metaphor is highly artificial and witty, artsy, if you will.  This is one of the marks of metaphysical poetry.