Katha-1-2-12-taṃ durdarśaṃ

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

The âtman which you wish to know is hard to see, being extremely subtle, lodged in the inmost recess, being concealed by the modifications of consciousness, caused by worldly objects. Located in intelligence (being realised, as if there lodged) and seated amidst manifold miseries. Being lodged in the inmost recess and located in intelligence, he is seated amidst miseries; (being thus seated he is hard to see) dwelling on that ancient âtman, with a mind weaned from all external objects and concentrated on the âtman, the intelligent man renounces joy and grief, as there is neither superiority nor inferiority for the âtman.

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