The Mukhya Upaniṣads

Kena · Khaṇḍa 2 · Known in Every Cognition

Mantra 3

यस्यामतं तस्य मतं मतं यस्य न वेद सः ।अविज्ञातं विजानतां विज्ञातमविजानताम् ॥ ३ ॥

In plain words

It is understood by the one to whom It is not an object of thought; the one to whom It is an object does not know It. It is unknown as an object to those who truly know, and treated as known by those who do not know.