In "The Plain Sense of Things," Wallace Stevens deals with the relationship between what is real (plain) and what is created by thought or the imagination of man. He presents a scene in the winter of a house, probably in the country, by a pond to help us see the "plain" reality after all the beauty of fall has passed. In the first stanza, he uses the image of a tree without its leaves to show this idea. The beauty of the tree is gone, and we are left with just a "plain" tree. He says this is like being at the "end of the imagination" because this tree has not been enhanced in any way by man to make it pretty or inspiring or anything.
In the second stanza, he mentions the "blank cold" and the "sadness." This creates a sad, sorrowful feeling to go along with the image of a dull winter. The "great structure," referring to the nice country home, now looks older, less impressive, and therefore "minor" in the winter. When he says that "No turban walks across the lessened floors," I believe he is saying that no man, not even one with a creative mind, would be able to liven-up the dull environment.
He mentions that the greenhouse needs paint, suggesting that maybe man should come to the rescue and make things more lively. The chimney is leaning as if man's influence on the environment has failed as nature makes sure that nothing thrives in the lifeless winter. Man has failed because he cannot revitalize the plainess.
But Stevens ends the poem with an interesting idea: that even desolation must be imagined by man. The idea of desolation is the creation of man's imagination. Therefore, this poem, which at first seemed to try to minimize the value of imagination and the influence of man by presenting a scene which was absent of man, now ends up affirming it. The last two lines of the poems, "Had to be imagined as an inevitable knowledge,/ Required, as a necessity requires," are difficult to interpret, but I believe Stevens is saying that man's imagination is "required" to always be at work defining reality, and that it is through man's ideas about reality that reality is defined. In other words, man might view a scene as desolate and "nothingness," but he is still using his imagination to form this view, and he has no choice but to do so because his imagination is always at work defining reality.