In the evolution of consciousness, there is an ascending ladder, from Matter to Life and from Life to Mind. Neither the material consciousness nor the vital consciousness were able to determine the nature of the unknown that did not respond to the corresponding powers of these two stages of evolutionary development. Next comes the evolution of mind and the mental power. The mind brings a new level of power and insight with the rise of sensory perception, reflection, analysis and the logical reasoning intellect. Yet it too is unable to encompass the understanding of the unknown. When it tries to grasp the vastness, the complexity and the infinity of the manifestation through Time, it comes up with nothing and falls into a state of silence, drifting without anchor.
The Kena Upanishad, 3rd part, verse 11: “Then they said to Indra, ‘Master of plenitudes, get thou the knowledge, what is this mighty Daemon.’ He said, ‘So be it.’ He rushed upon That. That vanished before him.”
Sri Aurobindo observes: “Indra is the power of the Mind; the senses which the Life uses for enjoyment, are operations of Indra which he conducts for knowledge and all things that Agni has upbuilt and supports and destroys in the universe are Indra’s field and the subject of his functioning. If then this unknown Existence is something that the senses can grasp or, if it is something that the mind can envisage, Indra shall know it and make it part of his opulent possessions. But it is nothing that the senses can grasp or the mind envisage, for as soon as Indra approaches it, it vanishes. The mind can only envisage what is limited by Time and Space and this Brahman is that which, as the Rig-Veda has said, is neither today nor tomorrow and though it moves and can be approached in the conscious being of all conscious existences, yet when the mind tries to approach it and study it in itself, it vanishes from the view of the mind. The Omnipresent cannot be seized by the senses, the Omniscient cannot be known by the mentality.”
Sri Aurobindo, The Upanishads, Kena Upanishad and analysis, pp. 104-106, 171-176