Verse 2.523

Sanskrit text and transliteration:

निष्कृष्याविद्योत्सङ्गस्थात्तत्साक्षिणमनात्मनः ।
साक्षात्तेनैव तं विद्मः प्रात्यक्ष्यात्सोऽयमित्यतः ॥ ५२३ ॥

niṣkṛṣyāvidyotsaṅgasthāttatsākṣiṇamanātmanaḥ |
sākṣāttenaiva taṃ vidmaḥ prātyakṣyātso'yamityataḥ || 523 ||

English translation of verse 2.523:

Distinguishing the Witness-self from the not-Self which lies at the lap of avidyā, we directly know Him by means of (i.e., as identical with) Brahman alone. Since the Witness-self is immediately known, He is referred to as “this” (in the śruti text).

Notes:

The Upaniṣad purports to teach the truth of non-duality. We ascertain the purport of Scripture through the harmony between the initial and the concluding passages. In the beginning of this chapter, Brahman has been defined as real, knowledge, and infinite. If Brahman, the ultimate reality, is infinite in the real sense of the term, it must necessarily be one (ekam) and non-dual (advitīyaṃ). The śruti passage sa yaścāyam puruṣe, etc., which we are considering here, concludes in the same way. It says that Brahman-Ātman, which is in man and also in the sun, is one (sa ekaḥ). From the harmony between the initial and the concluding passages, we conclude that śruti purports to teach the truth of non-duality.

Since the Witness-self, being self-luminous, is directly known, it is spoken of as “this” one in the śruti text sa yaścāyaṃ puruṣe.