But the power of instruction is seldom of much efficacy, except in those happy dispositions where it is almost superfluous.

Gibbon’s remark expresses a skeptical, Enlightenment-era view of pedagogy: formal instruction rarely transforms character or intellect unless the learner already possesses the temperament—curiosity, discipline, receptivity—that makes teaching nearly unnecessary. The line implies that education’s effectiveness depends less on the teacher’s “power” than on the student’s disposition, and it quietly shifts responsibility from institutions to individual nature and habit. It also carries an ironic edge: the best pupils profit most from teaching, while those most in need of guidance are least improved by it. In a broader sense, it reflects Gibbon’s interest in how human character and social formation limit grand schemes of reform.