The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 157

यो मनसि तिष्ठन्मनसोऽन्तरो यं मनोन वेद यस्य मनः शरीरं यो मनोऽन्तरोयमयत्येष त आत्मान्तर्याम्यमृतः ॥ २० ॥

In plain words

The one who lives inside your mind, yet is nothing the mind can claim as its own, whom the mind itself never knows, whose body the mind is, who governs it from within -- that is your self, the ruler seated inside everything, and death does not touch him.

Close to the Sanskrit

He who, dwelling in the mind, is other than the mind, whom the mind does not know, whose body the mind is, who controls the mind from within -- he is your self, the inner controller, immortal.