Kena Upanishad is among the ten principal Upanishads. It is one of the Upanishads under the Samaveda. With only thirty-five mantra-s it is tiny in size but very profound in its teaching.
All major Upanishads broadly discuss four aspects – jiva (the bring), jagat (the universe), Brahman (the Supreme Reality) and sadhana (the means to attain Brahman). Kena, however, focuses merely on 1) the nature of Brahman, 2) to what extent the body mind complex is equipped to know it and 3) how to know Brahman.
The Upanishad is in four parts. The first part poses the fundamental question that man has been asking since long. Are the sense and mind the final authority on knowing things or is there anything else which is behind them, enabling them to function? There is a more basic principle which is the ear of the ear, the mind of the mind and which enlivens all. This is called Brahman, but that Brahman cannot be known as an object of cognition. Whatever is objectified is, by definition, inferior to the mind.
The Upanishad makes a radical statement that whatever the human mind has conceived in the form of god, in whatever religion it may be, is only a conception of mind and hence cannot be the Absolute Reality.
यन्मनसा न मनुते येनाहुर्मनो मतम् ।
तदेव ब्रह्म त्वं विद्धि नेदं यदिदमुपासते ॥ 1.5
May you know that alone as Brahman which one cannot objectify by the mind but by which, people, say, the mind itself is known. Brahman is not that which people worship.
It is also ironical to note that thought Brahman cannot be objectified, it can still be experienced. This is the assertion of all Upanishads. This is validated by the experience of the seers.