Sri Aurobindo translates Mundaka Upanishad, Chapter 3, Section 1, Verse 8: “Eye cannot seize, speech cannot grasp Him, nor these other godheads; not by austerity can he be held nor by works: only when the inner being is purified by a glad serenity of knowledge, then indeed, meditating, one beholds the Spirit indivisible.”
The sense organs perceive the external world and its forms, but do not capture the Spirit which pervades and permeates these forms. Similarly, the mind cannot grasp nor speech define the Spirit. The seeker cannot seize hold of the Spirit through any form of outer action, works or austerity. All of these things create ripples in the “mind stuff” (chitta). Patanjali’s Yoga Sutras indicate that the perception comes when the mind stuff becomes still and receptive. Swami Vivekananda in Raja Yoga describes the inner poise and its role in the yogic progression at some length. The Mundaka Upanishad calls this state “a glad serenity of knowledge”.
Just as the day focuses our perception on the forms and actions in the world, but the night opens up the vast starry universe to our contemplation, so the active mind and senses focus on the outer world and its activities, and the quiet, receptive mind is able to reflect the indivisible Spirit. The practice of meditation, which involves quieting the mind to this state of “glad serenity of knowledge” prepares the seeker to perceive and experience the spiritual Truth of all existence.
What is needed is an “effortless effort”, a stilling of all desire, ambition, struggle, straining or impatience in the inner poise of the being, without falling into torpor, sleep, lassitude or indifference. Tapasya is required, in the sense that Sri Aurobindo translates it as “concentration of conscious force”. This is a particular state of gathered awareness that is still, receptive and serene.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210