The higher knowledge is the knowledge of the Immutable. The Mundaka Upanishad differentiates those who seek to gain knowledge of the world and power of effectuation in the development of body, life and mind from those who abandon the lower knowledge to seek the Immutable Brahman.
Sri Aurobindo translates Chapter 1, Section 2, Verses 11-13: “But they who in the forest follow after faith and self-discipline, calm and full of knowledge, living upon alms, cast from them the dust of their passions, and through the gate of the Sun they pass on there where is the Immortal, the Spirit, the Self undecaying and imperishable. The seeker of the Brahman, having put to the test the worlds piled up by works, arrives at world-distaste, for not by work done is reached He who is Uncreated. For the knowledge of That, let him approach, fuel in hand, a Guru, one who is learned in the Veda and is devoted to contemplation of the Brahman. To him because he has taken entire refuge with him, with a heart tranquillised and a spirit at peace, that man of knowledge declares in its principles the science of the Brahman by which one comes to know the Immutable Spirit, the True and Real.”
Here we find the justification for what Sri Aurobindo calls “the refusal of the ascetic”. The basic issue is that as long as one is enamoured of the world and its fruits of action, as long as one is moved by the force of desire, the higher knowledge is inaccessible. One must cultivate or experience an extreme distaste for worldly results in order to have sufficient motivation to renounce it all and live a life of peaceful tranquility. The mind becomes calm and reflective which permits the light of the Supreme to enter and permeate the mind. This path has its obvious dangers and the guidance of a Guru is required, someone who has achieved this realisation and who can manifest the vibratory pattern that guides the seeker along the path. The knowledge is not transmitted through verbal instruction but through a way of being and the tuning of the inner awareness to the truth of the Spirit. Once again, as we noted in the Isha Upanishad, there is a “brilliant golden lid” which must be overpassed, or, as the Mundaka Upanishad describes the process as going “through the gates of the Sun” to reach the Immortal. The day of the worldly individual, controlled by the influence of the sun, is focused on the life of the world; but at night, when the sun is no longer visible, the seeker can peer into the vastness of the universe of space and stars. Similarly the consciousness must move beyond the range of the brilliance of mind to the calm, vast, and unchanging awareness of the Brahman that is all and permeates all in stillness.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210