Thomas Aquinas, on Ps 16:8

“From them that resist thy right hand keep me, as the apple of thy eye. Protect me under the shadow of thy wings.”

“Next he sets forth the manner of deliverance, which is diligently and safely; therefore he says, "Guard me as the pupil of the eye." The pupil of the eye is guarded with diligence, because nothing that could harm it is permitted to approach; so also God does in guarding his servant: Deut. 32: "He led him about and taught him and guarded him as the pupil of his eye." Zech. 2: "He who touches you touches the pupil of my eye." Or according to the Gloss, the pupil of the eye is said to be Christ, who directs: Sir. 3 says the power of sight is in the pupil, by which we discern good from evil; and Christ discerns the faithful from the unfaithful and the evil from the good. To manifest this diligent guarding, he uses a twofold metaphor: namely, of shadow and of wings. For shadow refreshes from the heat, and so also the protection of God refreshes, giving security. Likewise, with her wings the hen guards her chicks against the hawk; so also God, with his wings, which are charity and mercy, defends the just from the rapacity of demons. Mt. 23: "How often I wished to gather you, as a hen gathers her chicks under her wings, and you would not!" With these wings, therefore, God lifts us to heavenly things: Ps. 88: "Mercy and truth shall go before your face; blessed is the people," etc. Jer. 31: "With everlasting love I have loved you; therefore I have drawn you, having mercy." Or the pupil of the eye is said to be the soul, because just as the pupil, which is in the middle of the eye, is surrounded by many membranes for its defense, and a man puts forth his hand and nearly everything he has lest it be harmed, so a man ought to do for his soul: Job 2: "Skin for skin," etc., because, as Mk. 8 says, "What does it profit a man if he gains the whole world and loses his soul?" etc. Or "Guard me as the pupil of the eye," that is, as Christ; "protect me under the shadow of your wings," that is, under the guardianship of the angels: Ps. 90: "He has given his angels charge over you," etc. Or the two wings are the two arms of Christ extended on the cross: Deut. 32: "He spread his wings and took them up and carried them on his shoulders."”

Imported from an open dataset — not yet checked against the printed edition.