Verse 4.15.1

य एषोऽक्षिणि पुरुषो दृश्यत एष आत्मेति होवाचैतदमृतमभयमेतद्ब्रह्मेति तद्यद्यप्यस्मिन्सर्पिर्वोदकं वा सिञ्चति वर्त्मनी एव गच्छति ॥ ४.१५.१ ॥

ya eṣo'kṣiṇi puruṣo dṛśyata eṣa ātmeti hovācaitadamṛtamabhayametadbrahmeti tadyadyapyasminsarpirvodakaṃ vā siñcati vartmanī eva gacchati || 4.15.1 ||

1. The teacher said: ‘The person seen in the eyes is the Self. It is immortal and fearless. It is Brahman. This is why, if anyone puts clarified butter or water in the eyes, it goes to the corners of the eyes’.

Word-for-word explanation:

Yaḥ eṣaḥ akṣiṇi puruṣaḥ dṛśyate, this person who is seen in the eyes; eṣaḥ ātmā, this is the Self; iti ha uvāca, he [the teacher] said; etat amṛtam abhayam, this is immortal and fearless; etat brahma iti, this is Brahman; tat, this is why; asmin, into this [eye]; yadi api sarpiḥ vā udakam vā siñcati, if anyone puts clarified butter or water; vartmanī eva gacchati, it goes into the comers [of the eye].

Commentary:

How do you know the Self? You have to have full self-control. You have to have a gentle nature, and you must withdraw your mind completely from the external world. Then you can see the Self in the eyes. That Self is immortal, fearless, and it is Brahman (literally, ‘the biggest’).

As water does not stick to a lotus leaf, so also, if water or something else is put in the eyes, it does not hurt them because the eyelids protect them. This is why the eyelids are called ‘lotuses.’ The eyes are the seat of the Self. What the Upaniṣad is saying is that if the seat of the Self is pure and not susceptible to any impurity, the Self is naturally the same.