Śvetāśvatara Upaniṣad · 4..4

य आत्मनि तिष्ठन् यमात्मा न वेद यस्यात्मा शरीरं य आत्मनोऽन्तरो यमयत्येष त आत्मान्तर्याम्यमृतः

ya ātmani tiṣṭhan yam ātmā na veda yasyātmā śarīraṃ yo ātmano'ntaro yamayaty eṣa ta ātmāntaryāmy amṛtaḥ

He who, dwelling in the Ātman, whom the Ātman does not know, whose body is the Ātman, who controls the Ātman from within —He is your Ātman, the Inner Ruler, the Immortal.

The culmination of the series: even the individual ātman —the consciousness we believe ourselves to be— is inhabited and governed by a deeper Ātman. The jīvātman does not know the paramātman that animates it. In our yoga practice, this is spiritual maturity: recognizing that even my sense of “I am conscious” is an object of knowledge, not the ultimate knower. The true antaryāmin is beyond all “I”, immortal not because it lives forever but because it was never born and will never die.