Sri Aurobindo translates Katha Upanishad, Second Cycle: Second Chapter, Verses 13-15: “The One Eternal in the transient, the One consciousness in many conscious beings, who being One orders the desires of many: the calm and strong who behold Him in their self as in a mirror, theirs is eternal peace and ’tis not for others. ‘This is He’ is all they can realise of Him, a highest felicity which none can point to nor any define it. How shall I know of Him whether He shines or reflects one light and another? There the sun cannot shine and the moon has no lustre: all the stars are blind: there our lightnings flash not, neither any earthly fire. For all that is bright is but the shadow of His brightness and by His shining all this shines.”
The Upanishad provides a clue for the seeker who aspires to the Eternal. All that we see around us in the world exists and acts solely due to the presence of the One consciousness. The seeker experiences the One through an awareness “as in a mirror”, in other words, not through active external perception or analytical action of the mind, but through reflection in a quiet mind. Those who gain this experience have peace. Others, pushed by the forces of desire and the ever-changing sensory stimuli, thoughts and emotions, cannot experience this peace. The Eternal is not able to be defined, labeled or categorised as it transcends every attempt to limit Him through the mind.
When we view the sun and the stars, we consider them to be “self-luminous”. The Upanishad disabuses us of this notion by pointing out that no light in the universe exists but for the light of the Eternal, which illuminates the entire creation. All the forms, all the forces, all the beings we can see or experience in the world are manifestations of the divine Presence and they radiate their energy and luminosity due solely to the luminosity and energy of the Divine.
The Katha Upanishad reinforces here the need to unify the two dicta “One without a second” and “All this is the Brahman.” There is no contradiction between these two when we recognise that everything that exists is the Brahman. Sri Aurobindo calls this “reality omnipresent” and it bridges the divide between the seeking of the renunciate, who focuses all his attention on the One, and the life of the man of the world, who focuses all his attention on the Many. The Many exist solely as manifested forms of the One, and thus, when we observe the radiance of any luminous energy or form, we are looking at the radiance of the One Eternal.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129