Kaṭha Upaniṣad 6.12

6.12

naiva vācā na manasā prāptuṃ śakyo na cakṣuṣā /
astīti bruvato 'nyatra kathaṃ tad upalabhyate //

12. Not by speech, not by mind, Not by sight can He be apprehended. How can He be comprehended Otherwise than by one’s saying ‘He is’? 13. He can indeed be comprehended by the thought ‘He is’ (_asti_) And by [admitting] the real nature of both [his comprehensibility and his incomprehensibility]. When he has been comprehended by the thought ‘He is,’ His real nature manifests itself.