Katha-2-3-5-yathā''darśe

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

How is explained. As one in the world sees his own image reflected in a mirror very distinctly, so the realisation of the âtman in one’s intelligence, when spotless like a mirror, becomes clear. As in dreams, the perception is indistinct and produced, i.e., (obscured) by the reminiscences of the waking state, so indistinct is the realisation of the âtman in the world of the manes, being engrossed in the enjoyment of the fruits of karma. As the image of one’s self reflected in water is seen indistinctly, with the various parts not defined, so is the realisation of the âtman in the world of the Gandharvâs certainly indistinct. Thus, it is inferred from the authority of the sâstrâs, even in other worlds, it is only in one, the Brahmalôka, that it is very distinct, as that of light and shadow and that (Brahmalôka) is hard to attain requiring as means, a combination of very special karma and knowledge, i.e., (worship). The drift, therefore, is that one should attempt to realise the âtman even while here (in this world).

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