"center of the eye," early 15c. (in English in Latin form from late 14c.), from Old French pupille (14c.), from Latin pupilla, originally "little girl-doll," diminutive of pupa "girl; doll" (see pupil (n.1)), so called from the tiny image one sees of oneself reflected in the eye of another. Greek used the same word, korē (literally "girl;" see Kore), to mean both "doll" and "pupil of the eye;" and compare obsolete baby "small image of oneself in another's pupil" (1590s), source of 17c. colloquial expression to look babies "stare lovingly into another's eyes."

Self-knowledge can be obtained only by looking into the mind and virtue of the soul, which is the diviner part of a man, as we see our own image in another's eye. [Plato, "Alcibiades," I.133]