"Nearing Forty" features a dedication and an epigraph before the main text. The poem is "for John Figueroa," a friend, fellow poet, and important mentor in Walcott's life. Throughout much of the poem, the speaker seems to address him directly.
The epigraph comes from Samuel Johnson, one of the major critics and poets of the 18th century. It appears in Johnson's preface to Shakespeare's collected works, which praises Shakespeare for writing authentically about life as it is—"hold[ing] up to his readers a faithful mirror of manners and of life." From the epigraph on, then, the poem contrasts writing that indulges in "fanciful invention" with writing that confronts the truth. The speaker, now approaching middle age, goes on to describe a kind of split between his earlier style—which was full of "fanciful invention"—and his newfound appreciation for humble, hard-won honesty.
The poem itself opens with the speaker unable to sleep. He's been "Insomniac since four," which could mean either that he's had trouble sleeping since he was four years old or that he's been awake since 4 A.M. Either way, he's anxious. The combination of insomnia and approaching middle age has troubled his mind. He describes the rain outside as "early-rising" (occurring early in the morning) and "rigidly metred": that is, its rhythmic, repetitive sound reminds him of metrical poetry.
Already, then, it seems the speaker is a version of Walcott himself. As the rain's "coolness numbs the marrow"—chills his bones—it reminds him that he's "nearing forty." It seems that the rain's temperature evokes the chilly reality of age and death, while its tempo, like a ticking clock, evokes the passage of time.
In lines 4-5, the speaker imagines how growing older will bring him "nearer [to] the weak / vision thickening to a frosted pane." This metaphor refers to the effects of aging and can be read in two ways. As people age, their sight can worsen and even fail: the clear window "panes" of the eyes might turn cloudy or opaque. But the speaker is also referring to the inner vision or imagination, which poets depend on for their work. The speaker worries that aging will sap the imaginative vigor of his poetry, diminishing its quality.
These opening lines also establish the form of "Nearing Forty." The poem uses rhyme (e.g., "narrow"/"marrow" in lines 1 and 3), but never settles into a consistent rhyme scheme. Likewise, it establishes a meter (iambic pentameter), but this meter loosens a bit rather than remaining "rigid[]" as the poem continues. Finally, the poem consists of a single meandering sentence!
All of these formal choices reflect the speaker's restlessness. He longs for a sense of resolution—much as one line-ending word might seek a rhyming partner—but he never quite finds it or settles into any kind of steady pattern. Frequent enjambment, as in lines 2-3 and 4-5 ("rain / recounting"; "weak / vision"), makes the poem's flow even more unpredictable.