The "Idea" in the title echoes Plato's use of the term in his discussion of Forms and Ideas. In Platonic Idealism, reality is found in ideas rather than in material objects. And in Immanuel Kant's Transcendental Idealism, the human mind actually comes with built-in categories for organizing experience. Thanks, brain!
She sang beyond the genius of the sea. The water never formed to mind of voice, Like a body wholly body, fluttering Its empty sleeves (1-4)
This description of the sea gives us an image of the natural world as something that is incomplete, lacking, on its own—in other words "empty." From the philosophical perspective of Idealism, the natural world (reality) needs the internal realm of the mind and imagination to make it whole.
If it was only the dark voice of the sea […] If it was only the outer voice of sky […] However clear, it would have been deep air, The heaving speech of air […] And sound alone […] The meaningless plungings of water and wind, (21-30)
The sound of nature, the external objects on their own, lack meaning. Nature, without the human mind to experience it, is meaningless. Reality is a function of the mind.Think about it!