In the first stanza of this short poem, Arnold compares humans to islands, to suggest how distant we are from one another. He paints an image of a vast sea between the islands (people), and emphasizing their separation through the line "We mortal millions live alone."
And yet these islands are drawn to one another, through the lovely sounds of birds singing, sounds which drift between the islands. The speaker expresses his desire for connection, which modern society lacks. He suggests that we must have once been together - all the "islands" must have once been one "continent." He desperately wishes that the water between the islands would recede so that the landforms might meet again.
In the final stanza, he asks what power could possibly keep lovers apart like this, and "render vain their deep desire." The answer, he states, is God — the God of the modern world does not provide the same hope and connection that He once did, since much of faith is tainted by science.
It is likely that this poem was Arnold's response to the famous line from John Donne's Devotions opon Emergent Occasions published in 1624. The line read, "No man is an Iland, intire of it selfe; every man is a peece of the Continent, a part of the maine." (Translated to contemporary parlance, the most famous part of the line is "No man is an island.") Donne wished us to believe that none of us are entirely alone - instead, we are all interdependent, reliant on one another. Every piece of land survives and thrives as part of a greater community, or "continent."
More than 200 years later, Arnold pessimistically argues that the opposite is true. The poem suggests that every man is an island, separated by water from those around him, even though they may seemingly be close. The real tragedy, however, is our awareness of others. Each island can hear the nightingales sing from other islands, a beautiful sound that is nevertheless too distant to reach. We know that there is joy in connection, but cannot achieve that.
The undercurrent of the poem is a skepticism in scientific discovery. The basic premise - that the continent has broken apart and drifted into separate islands - is based on a rational theory that reflects Enlightenment thought. This rational, scientific reading might have a basis in fact, but it for Arnold makes us spiritually distant from one another. We have traded faith - in the great community engendered by shared religious faith - for separation.
The line "We mortal millions live alone" is one of Arnold's most famous. It is so effective for a number of reasons; first, the juxtaposition of "millions" and "alone" is eerie and unsettling; surrounded by so many people all the time, it is hard for any of us to imagine feeling entirely alone. The word alone is italicized to stress that fact; in Victorian England, human isolation was extremely prominent, and being alone was a realistic fear. The terminal punctuation at the end of this line also adds to its potency. There are very few lines that end terminally in this poem; most are halted by commas, which is a subtle form of enjambment (when a sentence continues from one line to another). The period at the end of this sentence allows it to resonate with a reader all the more strongly, since the next line is a whole new idea rather than a continuation of this one. The overall effect is one of unease, which naturally aligns with the sadness and anger at the poem's center.
This poem highlights in particular the isolation brought on by romantic and sexual feelings. In stanza 2, Arnold uses metaphors of nightingales, starry nights, and "lovely notes" to illustrate the connection between people. Naturally, these are romantic images. However, he then negates the potential to connect with these sounds in the subsequent two stanzas, suggesting the impossibility of such intimate connection. Yet again, the awareness of love is worse than ignorance of it, since the former eventually leads to "despair." Overall, the poem espouses an extremely pessimistic worldview, one that acknowledges the potential for human connection, but then express frustration that "longing's fire/Should be, as soon as kindled, cool'd."