By whom is the mind directed? By whom is the breath directed to breathe? By what do people speak? The Kena Upaniṣad begins with this sequence of questions and answers them: by the one that is not known by the mind, because it is the mind's own knowing-ground.
The title comes from the first word: kena — by whom? The inquiry begins with the most intimate possible question: what is the power behind the mind, behind speech, behind sight? The answer will not satisfy a person looking for a new object of knowledge, because the answer is: the thing you are looking for is what is looking.
The text has four sections. The first two are verse — a question-and-answer exchange that establishes the paradox of Brahman as the knower who cannot be known as an object. The third and fourth are prose — a story in which the gods mistake their own victory for Brahman's, and learn through Indra's encounter with a teacher that the self behind the victory was never the ego.
Śaṅkara wrote a detailed bhāṣya distinguishing two interpretations of verse 1.4: Brahman is the mind of the mind, the eye of the eye, the ear of the ear — not the gross faculty but the consciousness that enables each faculty to function. This verse becomes one of Advaita's clearest statements of the witness-consciousness doctrine.