There is so much one can write. One could write about life experiences, or one could just write about nature and its splendor. Emerson basically looks at everything in this world as a symbol because each image has so many different interpretations and ways to express it. That s what makes poetry so great. What Emerson meant by all of that was the poet is the namer because whatever he sees he gives it a name for his poetry.
Some poets write from imagination to create a great poem.
Homer is a great example. He used imagination to help him create his famous epics. Some say that imagination intoxicates the poet and that is true to some extent. It is a great sense of freedom when one can write whatever is on his mind and depict it so well. Imagination to me is a great tool if used correctly.