Final answer:
Herbert's description of the borders of New Hampshire as "an idea" implies that they are conceptual and shaped by human understanding and agreements rather than just natural divisions.
Step-by-step explanation:
When Herbert describes the borders of New Hampshire as "an idea," they are likely referring to the concept that borders are human constructs rather than natural, objective realities. This perspective suggests that what defines a place like New Hampshire is not just the physical land but the ideas, policies, history, and cultural identity projected onto it by humans. This aligns with a broader theme in literature and social studies that examines how human perception and societal constructs shape our understanding of geography and political boundaries.