Final answer:
The rhetorical device used by Thoreau in the sentence is parallelism.
Step-by-step explanation:
The rhetorical device used by Henry David Thoreau in the sentence, "I ask for, not at one no government, but at one a better government," is parallelism. Parallelism is the repetition of grammatical structures or patterns for emphasis or clarity. In this case, Thoreau repeats the structure "not at one...but at one" to emphasize his desire for improvement in government rather than its complete absence.