BṚHADĀRAṆYAKA UPANISHAD · 1.5.9
यत् किञ्च विजिज्ञास्यं मनसस् तद् रूपम् ।
मनो हि विजिज्ञास्यम् ।
मन एनं तद् भूत्वावति ॥
yat kiñca vijijñāsyaṃ manasas tad rūpam ।
mano hi vijijñāsyam ।
mana enaṃ tad bhūtvāvati ॥
What is to be known, has the form of mind, for mind is what is to be known. Mind, having become this, protects man.
F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)