"One must have a mind of winter"
This conditional clause that begins the poem is really the crux of the poem, though we do not fully understand it until the very end. If we achieve a mind of winter, then, and only then, can we observe winter (or anything in the world) free of the emotional connotations with which we always saddle nature.
What does it mean to have a mind of winter? We can only infer this from the overall philosophy of the poem: it is to have a pure, objective outlook, to see nature without the distorting glass of emotion or imagination—if such objectivity is possible. For winter in particular, to have this mindset is to view the snow and cold merely as a natural fact, not as a season to be lamented with misery.
"And have been cold a long time"
This clause adds on to the poem's first line as an additional condition that needs to be met: i.e., you have to have a mind of winter, and have been cold for a long time, to not view winter as misery. On one level, this line functions the same way as the first line: the "snow man" must be completely accustomed to winter, and view it as a default. However, this line tempts us to ascribe an additional sadness or emotional trauma to the observer who is so able to give himself to the nothingness. We are left wondering whether Stevens meant "cold" emotionally as well as physically. If so, then to be a snow man you would have to be deeply, even infinitely sad or embittered. Or, Stevens may be testing us with the word, suggesting to us that the emotional connotation of "cold" is another subjective perspective that we impose on the world.
"The spruces rough in the distant glitter
Of the January sun"
In possibly the poem's only moment of organic visual beauty, Stevens captures the allure of sun glimmering on snowy trees. However, this romantic view of nature is out of place in the poem. Indeed, this image is immediately followed by a turn in the poem, after which Stevens increasingly condenses the landscape into bareness and sameness. The beautiful spruces, then, are an example of how even positive emotional projections onto nature are subjective, like their negative counterparts, imagined by the human observer. The sunlight on the snow is only beautiful because our minds consider it so.
"and not to think
Of any misery in the sound of the wind"
The traditional view of winter as miserable is the poem's central example of a perspective that humans bring to nature. If we could move past our immediate negative reaction to cold, barren landscapes and seasons, we could understand that they are just facts of nature, like any other environment.
"Nothing that is not there and the nothing that is."
The poem's last line is deceptively ambiguous. On one level, it says simply that the world is nothingness, into which the observer would be absorbed if he were to lose his subjective perspectives that give it meaning. However, to behold "nothing that is not there" could also mean 'to see no thing that is not there', or 'to see only the things that are present in the scene.' That is, the observer is actually seeing the trees, snow, and ice purely and clearly, without distractions. This possibility suggests, paradoxically, that by seeing nature as 'nothingness', the observer actually gets the sharpest possible view of it. Likewise, the last clause, "the nothing that is" [emphasis added], does not necessarily indicate an absence of being or a totally nihilistic worldview: it positively asserts a nothing that does exist—i.e., a blank canvas onto which our perspectives are projected.
