Keep my commandments, and live (see on Proverbs 4:4). As the apple of thine eye; literally, the little man (ishon, diminutive of ish) of the eye; so called from the miniature reflection of objects seen in the pupil, specially of the person who looks into another's eye.
It is a proverbial expression for anything particularly precious and liable to be injured unless guarded with scrupulous care (comp. Psalms 17:8, Zechariah 2:8). Similarly the Greeks called this organ κόρη, "damsel" or "puppet," and the Latin, pupilla.