Let us be grateful to the mirror for revealing to us our appearance only.

Butler’s aphorism wryly suggests that a mirror’s limitation is a mercy: it shows only the surface. The joke depends on an implied contrast between outward appearance and the less flattering realities a “truer” mirror might reveal—character, motives, vanity, self-deception, or moral blemishes. Gratitude is therefore ironic: we are thankful not because mirrors tell the whole truth, but because they do not. In Butler’s typical satirical mode, the line punctures self-regard and hints that human beings might be far less pleased if confronted with an equally clear reflection of their inner lives.