The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 210

कतम आत्मेति योऽयं विज्ञानमयः

In plain words

'So which one is really the Self?' The one made of pure understanding itself--

Close to the Sanskrit

'Which is the Self?' [The answer:] This one that consists of understanding--