We descend upon you and all things—we arrest you all, We realize the Soul only by you, you faithful solids and fluids, Through you color, form, location, sublimity, ideality, Through you every proof, comparison, and all the suggestions and determinations of ourselves.
The speaker has become even more inclusive. He uses the plural "we" form instead of the individual "me" or "I."
One of the most difficult aspects of Whitman's poetry is figuring out to whom the pronouns refer. He wants to leave it ambiguous.
The message in these lines is that "we" – whoever that might be – achieve or "realize" the spiritual reality of the Soul only through the material reality of the things.
This thought poses a challenge to some forms of Christian theology, which claim that spiritual reality is all that matters, and that people shouldn't put faith in the world of material things. Whitman reverses the claim and says that we only know the spirit through Nature.
Like a chemist, Whitman boils the entire scene down into "solids and fluids." (Hey, what about gases?) These basic materials provide us with physical qualities like color, form, and location; spiritual qualities like the "sublime" and the "ideal" (fancy philosophical terms we won't get into here); and intellectual achievements like "proofs" and "comparisons."
Where we might see the muddy East River and a bunch of ships, the speaker sees the building blocks of the world.