Donne's "A Valediction: Forbidding Mourning" (1611) was written when Donne left for an extended trip to Europe (1611-1612) with Sir Robert Drury, Donne's most important and influential patron. As to its sincerity, we assume from its tone and content that Donne wrote the poem to convince his wife, Anne (who produced twelve children during their marriage), that their love is centered in the spiritual rather than the physical world--a physical separation, then, matters very little. Given the care with which Donne makes his argument--using some of the most well-known metaphorical language in English literature--it would be difficult, if not impossible, to argue against the poem's sincere purpose of assuring Donne's wife that their love cannot be affected by physical separation.
As many scholars of Donne have noted, the poem is essentially an argument, which uses highly unusual, sometimes startling metaphors and similes, a very typical technique for poets who are part of what is known as the metaphysical school. A hallmark of metaphysical poetry is the use of imagery that seems at first to be completely incongruous--Samuel Johnson said, "yoked violently together"--but the unusual comparisons work to startle the reader into understanding. In addition to being a fine poet, Donne is one of the most important and influential preachers of his time, and his preaching voice comes through loudly in this poem.
Because the poem actually frames Donne's central argument, every stanza centers on an image that elevates his love for his wife, and hers for him, to a place that is both private and joyful. In the second stanza, for example, Donne argues that
So let us melt, and make no noise, 5
No tear-floods, nor sigh-tempests move ;
'Twere profanation of our joys
To tell the laity our love.
In other words, their love is so special that it cannot and should not be shared with just anyone ("the laity"), and so, as they part, their goodbyes must be calm and undemonstrative so as not to give away their strong feelings for one another. In addition, Donne's use of laity to refer to the common people is very skillful because the word is usually used to refer to members of a church who are not trained as ministers or preachers. By using laity, Donne subtly elevates their love beyond earthly concerns.
One of the strongest metaphors argues for the strength of the love by comparing it implicitly with a much weaker kind of love:
Dull sublunary lovers' love
—Whose soul is sense—cannot admit
Of absence, 'cause it doth remove 15
The thing which elemented it.
Again, we have a strong argument: people who have a physical rather than spiritual relationship ("whose soul is sense") cannot bear a separation because their love is centered on the physical--their love is created by its physicality, that is, they need to be near each other for the love to continue. Donne implicitly argues here that his and Anne's love is the opposite of love based on "sense" (physicality).
Perhaps the most beautiful example of metaphorical language that reinforces Donne's overall argument that separation in inconsequential comes in the sixth stanza:
Our two souls therefore, which are one,
Though I must go, endure not yet
A breach, but an expansion,
Like gold to aery thinness beat. (ll. 21-25)
Another wonderful metaphysical image is the comparison of their love, during the separation, to gold that is beaten (by the separation) to an "aery thinness," the implication being that their love is so flexible that it can be stretched but never broken. Donne also relies on the fact that gold can be melted, beaten, changed utterly, but never destroyed.
From a practical perspective, one can readily see that the care and skill with with Donne created this poem, an argument attesting to the strength of his love for his wife, create an absolutely sincere expression of love and a masterful argument that their separation cannot break their bond of love