Mundaka-3-1-8-na cakṣuṣā

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Com.—Again, a special aid to the attainment of Brahman is explained. It is not seen by the eye of anybody, because it has no form; nor is it grasped by speech, because it cannot be the subject of words; nor by the other senses. Though tapas is an aid to the attainment of all, the Brahman cannot be reached by Karma enjoined by the Vedas, such as agnihotra and the rest though their greatness is well known. What, then, is the means by which it could be grasped is explained. Gnana prasadena, though the intellect in all men is by nature competent to know the Atman, still being polluted by such faults, as love for external objects, etc., and hence unclear and impure, it does not, like a stained mirror and muddy water, grasp the entity of the Atman though always near; but when, by removal of the polluting taint, such as desire, etc., produced by contact with the objects of the senses, it is made clear and calm like mirror and water, then the intellect becomes clear; by this clearness of the intellect the mind is purified and the man becomes competent to realize the Brahman. Therefore, he sees the Atman which has no parts, by meditation, having recourse to such helps such as truth, etc., having controlled his senses and with a concentrated mind.

Links
Various resources on the Mundakopanishad
Upanishad Series – Mundakopanishad Commentary by Swami Chinmayananda