"The Idea of Order at Key West" establishes its setting, and its main theme, in its title. This will be a philosophical poem about "order" (as in the opposite of chaos), set in a place usually associated less with intellectual brooding than with sensuous enjoyment: the beautiful shoreline of Key West, Florida. This combination is fairly typical of Wallace Stevens, whose poetry often mixes philosophical inquiry with a hedonistic delight in everyday pleasures (from beaches to music to ice cream).
The poem begins with a pair of comparisons that deserve close attention:
She sang beyond the genius of the sea.
The water never formed to mind or voice,
Like a body wholly body, fluttering
Its empty sleeves;
Line 1 compares a nameless woman's singing to the sounds made by the sea, or by the "genius of the sea." The word "genius" here refers to the spirit of the ocean, as in the ancient Latin term "genius loci," meaning the spirit of a place. (Originally, the term described an individual, guardian spirit—a kind of minor god—but it evolved to mean the overall atmosphere of a particular place.)
The speaker's comparison favors the woman: she sings "beyond" what the sea can manage. To the speaker, at least, her voice is more pleasing and/or more powerful, at least in part because it's the product of the human spirit rather than whatever spirit dominates nature. (The poem may also be bringing in the modern sense of the word "genius," meaning virtuosity or brilliance. In other words, it may be implying that the woman's virtuosic art upstages the natural splendor of this scene.)
Then comes a complex and curious simile in lines 2-4. The speaker says that the ocean "water" never shaped itself into, or was shaped by, anything human beings would recognize as a "mind" or "voice." It remained thoughtless, formless, and chaotic. In this way, the speaker suggests, it resembled a "body" that was only a "body"—in other words, one that lacked something essential. What the water lacked was a mind, but the speaker evokes a body "fluttering / Its empty sleeves," as if it lacked arms instead.
This is a strange conceptual leap, but it conveys a sense of haunting absence: the way the sea (or nature) seems eerily devoid of human expression or meaning. Visually, the image of fluttering sleeves also conjures up the ocean's mindless, empty "gesture[s]" (see line 16: "That ever-hooded, tragic-gestured sea") as it flings wave after wave onto the shore. To humans seeking comfort and companionship in nature, the poem suggests, nature perpetually disappoints, like a ghost or scarecrow.