The Kena Upanishad is among the shortest of the principle Upanishads. It asserts that the highest knowledge is the mystical knowledge of Brahma. The first section describes the nature of Brahma as that which is not of the empirical world, but instead "that which is the hearing of the ear, the thought of the mind, the voice of speech, as also the breathing of the breath, and the sight of the eye." From this, one understands the Brahma is inscrutable which is made clear in the second section of the Upanishad.
In the third section, Brahma is described as greater than all the gods through a charming account of challenges set before the gods. Agni (fire) is unable to burn straw, while Vayu (wind) is unable to carry it away. Finally, Indra, attempts to approach Brahma, but it eludes him; however, the beautiful woman, Uma (knowledge), informs him of Brahma's nature. The final section reveals that knowledge of Brahma is a mystical doctrine that comes to one as a flash of lightening.
In this Upanishad the three central ideas of the Upanishads are presented: (1) Brahma (or the Atman) is the knowing subject within us, (2) Brahma (or the Atman) is unknowable, and (3) Brahma (or the Atman) is the sole and supreme reality.