the pupil of the eye: the dark opening at its center, named as the one spot a person shields without thinking
The word names one small thing: the dark opening at the center of the eye. In the Bible it never turns up in a plain description of the body. Every time it carries this sense it sits inside a comparison, and always the same comparison: guard this the way you guard the pupil of your eye.
What the picture appeals to is reflex rather than sentiment. The pupil is the most exposed spot on a person and the least protected by bone or skin, so the body shields it before the mind has time to decide. Moses uses that instinct to describe how God kept Israel alive in the wilderness. The psalmist asks for exactly the same keeping. Proverbs then turns the image on a father's teaching, telling a son to hold the instruction as carefully as he holds his own sight.
Hebrew had other ways to say this. Psalm 17:8 stacks two of them together, setting "daughter of the eye" right after this word, and Zechariah uses a different word again (בָּבָה) when God warns that whoever touches his people touches the tender center of his own eye. The overlap between these expressions is real, but they are not simple twins, which is part of why Psalm 17:8 has been argued over for centuries.
How the two eye expressions stacked together in Psalm 17:8, אִישׁוֹן followed by בַּת־עָיִן, are meant to be read.
- majorityThey are near synonyms placed side by side for emphasis, naming the very center of the eye twice over.; David Kimhi (Radak), commentary on Psalm 17:8; Peter C. Craigie, Psalms 1-50 (Word Biblical Commentary 19); Frank-Lothar Hossfeld and Erich Zenger, Psalms (Hermeneia)
- minorityThe two words denote different parts of the eye, so the request is that God guard the psalmist as the eyelid or eyelash guards the eye; the traditional reading treats as redundant what is really a pairing of two organs.; Shimon Garti and Nathan Wasserman, "The Riddle of בבת עין," ZDMG 167 (2017): 71-79; Rashbam on Deuteronomy 32:10