Frost uses the rigidity of the sonnet form to present a formal philosophical problem. We are introduced, in the course of the octave, to 'Assorted characters of death and blight', three things the narrator happened to come across once: 'a dimpled spider, fat and white', a white flower, and, held up by the flower, 'a moth / Like a white piece of rigid satin cloth.' The three are introduced separately, assembled in synthesis to demonstrate the incongruity of their relationship, and then re-described in the last two lines of the octave for emphasis:
A snow-drop spider, a flower like a froth, And dead wings carried like a paper kite.
Up to this point, the scientist-poet has only permitted himself the emotional shock of the elements presented for his examination and he accepts them as specimens at random. In the sestet, however, he tries to solve the problems they pose and, as he does so, the tension suddenly breaks, along with the rhyme-scheme. In a series of negatives and outraged rhetorical questions, he demands reasons for the strange combinations of existence. What is the 'design' behind all this, he asks. All he can summon up, by way of an answer, is the following:
What but design of darkness to appal? -- If design govern in a thing so small.
Far from solving the problem, this conclusion only exacerbates it. For the alternatives are either that the 'design' reflects some vast malevolent joke, or that the concept of 'design' is absurdly irrelevant -- in which case, the process of questioning in the sestet is itself called into question. This, in effect, is the irresolution of 'For Once, Then, Something' returned with a vengeance, since on the borders of it now hovers a sense of fear. It is bad enough to believe that we are condemned to abide amidst uncertainties; it is even worse to suspect that those uncertainties harbour danger, that the universe is not only unknowable but treacherous.
However, like so much in Frost's poetry, this remains only a suspicion. Fear lurks beneath the surface of a poem like this, certainly: but, in other poems, Frost's playfulness, his willingness to entertain all kinds of doubts and possibilities leads him in the contrary direction -- not to transcendence of facts, perhaps, but to a wondering, joyful apprehension of their potential, to the sense that nature might after all be whispering secret, sympathetic messages to us.
From American Poetry of the Twentieth Century. Copyright © 1990 by Longman Group UK, Limited.