Sri Aurobindo translates Katha Upanishad, Second Cycle: First Chapter, Verse 3: “By the Self one knows form and taste and smell, by the Self one knows sound and touch and the joy of man with woman: what is there left in this world of which the Self not knows? This is That thou seekest.”
This verse explores who the “Knower” is in the process of knowing. We experience the outer world through the perceptions of the senses which convert the impressions into nervous impulses which are received by the brain. This is all physical instrumentation and process of action, but does not tell us “who knows” what has been experienced. Western philosophers, such as Rene Descartes, explored this process and determined “I think, therefore I am.” They thereby considered the ego-consciousness and the mind to be the “knower”. The sages of the Upanishads took this process further. They recognized that there must be something beyond the action of the mind and the ego that is essential to the ability to “know”, as these are secondary tools, not primary, in the larger creation. Where do the mind and the ego come from? Where do they get their powers? How are they born? How do they exist? What happens when they dissolve with the death of the individual? Is there something that survives the death of the body? It is this type of questions that led the sages to go further.
The existence of a Soul, Self, or Atman has been explored by seekers, through spiritual and religious experience and through deep internal self-examination throughout the ages. Sages such as Ramana Maharshi posed the question “Who am I?” When we eliminate the body; when we eliminate the life force; when we eliminate the mind; when we eliminate the ego-consciousness, what remains and knows? The Upanishads speak of the two birds on a common tree, or as in this verse, the Knower who experiences all the sensory stimuli of the world. The seeking of the Self and identification with the Self occurs when the individual turns his attention inward, away from the outer world, to identify who really exists and knows. Eventually the Self is identified with the universal Knower: “The spirit who is here in man, and the spirit who is there in the sun, it is one spirit and there is no other.”
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129