Sri Aurobindo translates Mundaka Upanishad, Chapter 3, Section 1, Verse 5 as follows: “The Self can always be won by truth, by self-discipline, by integral knowledge, by a life of purity, — this Self that is in the inner body, radiant, made all of light whom, by the perishing of their blemishes, the doers of askesis behold.”
There are those who tell us that the supreme realisation cannot be achieved through individual effort, and that it comes as a form of Grace. The truth behind this assertion is that the realisation cannot be forced, but must come naturally to the seeker. That does not mean that all preparation or effort should be disregarded or abandoned; rather it means that one should not undertake the efforts with any expectation or attachment to the result.
The prescription provided by the Upanishad in this verse represents psychological statuses that bring about a quieting of the mind and the emotions and the clamour of the vital and physical being with their desires and needs. In the calm psychological space that results from continuous practice in these areas, the radiance of the Self can be perceived and experienced.
Swami Vivekananda in his Raja Yoga spends considerable time explaining the need for quieting the “mind stuff” so that the true Self can be reflected. It is worthwhile to note that the practices known as Yamas and Niyamas, considered preliminary steps in Patanjali’s Yoga Sutras, all conduce towards a similar end of quieting the mind. These are not, in the end, moral rules but guidelines for true spiritual growth and development to allow the shift to the Divine standpoint that is required.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210