THis entire poem is singing the beauty of humanity and of each person's occupation. That every person is important as is every job. The "song" of each job (hammers, human voices, other occupational noises) is what creates the beauty of every day life.
In the lines you've given, it seems that Whitman is speaking of guiltless humanity--clear conscience and souls--must know who they are within themselves before they can recognize the beauty in humanity outside of the self. The unseen (what you don't know, haven't experienced yet, the future of your existence) cannot exist with the seen identity (the you that you live with every day, the person you should know inside and out, all the facets of the self which helps you to understand your moods and how and why you react to life as it happens).
In these lines Whitman is blessing himself and everything else in the universe, asserting both are holy in themselves and that they are holy in being connected to each other. He suggests a dialectical view of the world in which opposites depend upon each other for meaning and as a result of this dependence shape an entire new and complete vision. I can be “I” only if you remain “you,” and in this way we give meaning to each other.