Whitman is one of the few poets whose works are more difficult than they seem. He goes out of his way not to intimidate the reader with complicated formal devices and fancy-pants language. But we find two things particularly complex in this poem. First, he uses vague pronouns, where we don't know to whom "you" or "we" refers. Second, whenever he starts talking about philosophy, things get confusing. The important thing to remember is that Whitman believes the physical world is the foundation of the spiritual one, not the other way around.