KAṬHA UPANISHAD · 1.3.12

एष सर्वेषु भूतेषु गूढोऽऽत्मा न प्रकाशते ।
दृश्यते त्वग्र्यया बुद्ध्या सूक्ष्मया सूक्ष्मदर्शिभिः ॥ १२॥

eṣa sarveṣu bhūteṣu gūḍho’’tmā na prakāśate ।
dṛśyate tvagryayā buddhyā sūkṣmayā sūkṣmadarśibhiḥ ॥ 12॥

The secret Self in all existences does not manifest Himself to the vision: yet is He seen by the seers of the subtle by a subtle and perfect understanding.