The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 49

यत्किंच विजिज्ञास्यं मनसस्तद्रूपं मनोहि विजिज्ञास्यं मन एनं तद्भूत्वावति ॥

In plain words

Whatever can be made an object of inquiry is a form of the mind; for the mind itself is that which is sought to be known. The mind, becoming that very object of inquiry, sustains and protects the seeker.

Close to the Sanskrit

Whatever is desired to be known, that is the form of the mind; for the mind indeed is what is desired to be known. The mind, having become that, protects this person.