"… When we think of a snowman, most of us visualize balls of snow placed on top of each other, coals for eyes, a carrot nose, and the like. I mention these details only to point out Stevens does not. His poem does not describe but merely invokes "The Snow Man" by mentioning him in the title; thereafter the snowman is involved in the poem only as a metaphor of a metaphor. He is a metaphor of a "mind of winter," and this, in turn, is a metaphor of something even more abstract: a mind that entertains nothingness. …
But it is easy to imagine that whoever speaks or thinks this poem is himself looking at a snowman. In this case the poem may be related to the descriptive-meditative tradition in English poetry that comes down to us from the eighteenth century and the Romantic period. … A convention organizing all such poems is that the poet finds himself at some place or views some prospect or object. The poem describes what is seen and, as it proceeds, enacts a train of thought and feeling occasioned immediately by the place or object and referring repeatedly to it. Stevens’ "The Snow Man" presents the meditation with the description omitted. As "meditation," its form is thinking, the mind in activity, and this is also in part its subject, The poem is one sentence. It proceeds by amplification, illustrating the inherent dynamism of the mind, its fertile power to proceed on its own impulse.
Still dwelling on "The Snow Man," we may note that the poem posits two types of listener. One would hear a "misery in the sound of the wind." Through his own imaginative creativity he would project a human emotion into the scene and locate it there. Thus, he would make the landscape one with which human beings can feel sympathy. The other listener would hear nothing more than the sound of the wind. He would exert none of this spontaneous and almost inevitable creativity. The poem embodies Stevens’ central theme, the relation between imagination and reality. Endless permutations of this theme were possible. Was reality the world seen without imagination? If so, was imagination the world seen without reality? That was a bitter truth, if it was the truth. But perhaps the snowman, who heard no "misery" in the wind, was projecting himself into the scene just as much as the other listener. Perhaps the snowman beheld nothing only because he was "nothing himself," since, to cite a later poem, whoever "puts a pineapple together" always sees it "in the tangent of himself."
From David Perkins, A History of Modern Poetry: From the 1890s to the High Modernist Mode (Cambridge: Harvard University Press, 1976), 542-544.