Emerson contrasts physical strength with the more durable power of ideas. Gibraltar, long associated with military security and formidable fortifications, represents material defence. Ideas, by contrast, are described as “impregnable” because they cannot be defeated in the same way as a fortress. A person guided by a powerful conviction may therefore possess a kind of resilience that physical strength alone cannot provide.

Within Civilization, the statement reflects Emerson’s wider belief that social progress depends on principles, knowledge and moral purpose as well as material resources. His reference to the “hero” suggests an individual whose courage is strengthened by commitment to an idea larger than personal interest. The language is deliberately figurative: ideas can be challenged, rejected or forgotten. Emerson’s point is that deeply held principles can outlast physical obstacles and give individuals unusual persistence and influence.