The Mukhya Upaniṣads

Kena · Khaṇḍa 1 · The Light Behind Mind and Senses

Mantra 3

न तत्र चक्षुर्गच्छति न वाग्गच्छति नो मनः ।न विद्मो न विजानीमो यथैतदनुशिष्यात् ॥ ३ ॥

In plain words

Sight does not reach there; speech does not reach there, nor does the mind. We do not know It as an object, nor understand how one might teach It in the ordinary way.