The short answer to this question would be to say Whitman's theme is himself, but the trouble begins when we consider what "himself" might mean. To put it another way, the poem is about the spiritual interconnectedness of all things. This takes a number of different shapes in the poem. Some of the themes Whitman touches on include:
- He himself, the poet, as a living individual ("thirty-seven years old and in perfect health")
- The poet as transcendent, omniscient being, aware of everything, everywhere
- The body, and sex, as a means to experience the transcendence of experience
- Reality is not bounded by life or death
- Poetry (and in particular this poem) as a way of articulating ultimate truth, or as a way of experiencing a reality beyond poetry
Section Six, in which a child asks the poet, "What is the grass," can serve as a gloss on all these themes. The question itself (and the poet's reaction -- "I do not know what it is any more than he") can be understood as the origin of all poetry, and the poet's many answers reflect the multi-sided "truth" or "reality" Whitman wishes to convey. The grass could be "the flag of his disposition," or a symbol of his poetic sentiment; or it could be "the handkerchief of the lord," the physical presence of the divine on earth; or it could be "itself a child," the same as the person who wants to know what it is; or it could be a "uniform hieroglyphic," a kind of truth that is unknowable to all, black or white, a kind of democratic mystery; or it could be "the beautiful uncut hair of graves," an image that fuses life with death, and suggests the life and death are not opposites but equal expressions of the same reality (the dead are "alive and well somewhere") and that, in fact, to die "is luckier" than we know.
It is hard to track down all the "themes" of the poem. Whitman himself, I think, would object to such an approach. He would say, no doubt, that there are "innumerable" themes in the poem, as in life itself, and the only way to know them is through direct experience, or, in other words, by reading the poem!