The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com

16. And on account of the statement of the way of him who has heard the Upanishads.

The person placed in the eye is the highest lord for the following reason also. From sruti as well as smriti we are acquainted with the way of him who has heard the Upanishads or the secret knowledge, i. e. who knows Brahman. That way, called the path of the gods, is described (Pra. Up. I, 10), 'Those who have sought the Self by penance, abstinence, faith, and knowledge gain by the northern path the sun. This is the home of the spirits, the immortal, free from fear, the highest. From thence they do not return;' and also (Bha. Gîtâ VIII, 24), 'Fire, light, the bright fortnight, the six months of the northern progress of the sun, on that way those who know Brahman go, when they have died, to Brahman.' Now that very same way is seen to be stated, in our text, for him who knows the person within the eye. For we read (Kh. Up. IV, 15, 5), 'Now whether people perform obsequies for him or no he goes to light;' and later on, 'From the sun (he goes) to the moon, from the moon to lightning. There is a person not human, he leads them to Brahman. This is the path of the gods, the path that leads to Brahman. Those who proceed on that path do not return to the life of man.' From this description of the way which is known to be the way of him who knows Brahman we ascertain that the person within the eye is Brahman.