In "To Marguerite," Arnold writes of estrangement and loneliness, imagining that the gulf that separates each of us from others to be like a sea. In contrast to Donne's contention that "no man is island," Arnold envisions us all as little islands. We long for deep communion with others, but we are cut off. Arnold envisions humankind as once a single continent and writes that we have a "longing like despair" (he uses the word "longing" twice in the poem) that once again our "marges" or edges might meet.
In saying we were once one continent, once a true community of people who were not isolated and estranged, Arnold looks back to a better time, implying that the isolated way we now live is not how life necessarily has to be. As he looks around, trying to understand what caused the current separateness, he fixes on God as the answer:
Who renders vain their deep desire?—
A God, a God, their severance ruled.
Arnold leaves it an open question which God—he says "a" God, not "God"—caused this or why this God rules for separation, perhaps suggesting the situation could change. He leaves the reader with a lonely final image: that of us cut off by the "salt, estranging sea."