According to the speaker in section 1, celebrating oneself is equivalent to celebrating what?
In the first section of this poem, Whitman draws an equivalence between his celebration of himself and his celebration of the person to whom the poem is addressed. He celebrates himself, and therefore he prevails upon the listener to "assume" the same thing that he assumes. What he is saying here is that, if the speaker is celebrating himself, the listener should be happy to assume that this is a broader celebration of humanity, because "every atom belonging to me" also effectively belongs to "you," the listener.
Whitman goes on to elaborate on this theme that a celebration of the self is effectively a celebration of humanity as a whole. He notes that every "atom" of his person is one with the soil, the air, the atoms which built up his parents, and so on and so forth.
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