Katha-2-2-12-eko vaśī

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

Moreover, he, the lord of all, all-pervading, independent, is one (there is none other equal to him or greater than he); vasî ] under whose control all the universe is; because he is the internal âtman of all; for, he makes himself though one, of the nature of unalloyed pure knowledge, diverse by the differences of impure conditions of name, form, etc., by his mere existence, having unthinkable powers. Âtmastham ] clearly perceived in the form of knowledge, in the conditioned intellect, in the âkâsa of the heart, within the body; for, the body is not the supporter of the âtman, he being formless as the âkâsa. He is like the face reflected in the mirror. To those discerning persons who perceive this lord, this âtman, all their external activities being checked in accordance with the teaching of the preceptor and the âgamâs and realise him directly, to those who have become lords of all, belongs the eternal bliss, i.e., delight in self and not to the undiscerning others, whose intelligence is engrossed by external objects, though the bliss is their own âtman, which in the case of the latter is concealed by ignorance.

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