A man thinks that by mouthing hard words he understands hard things.

Melville is criticizing a common intellectual vanity: confusing the performance of learning with learning itself. “Mouthing hard words” suggests parroting technical or elevated language—jargon, abstractions, or grand rhetoric—without the patient work of grasping the underlying realities (“hard things”). The line implies that genuine understanding is tested by clarity, humility, and contact with experience, not by verbal display. It also fits Melville’s broader skepticism about systems, cant, and self-deception: language can become a mask that flatters the speaker into thinking complexity has been mastered simply because complexity has been named.