Katha-1-2-20-aṇoraṇīyānmahato

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

How then does one know the âtman is explained? Subtler than the subtle, i.e., subtler than grain, etc.; greater than the great, i.e., greater than things of great dimensions, such as the earth (whatever thing is in the world, that is known to exist only by virtue of the eternal âtman; divorced from the âtman it becomes a non-entity; therefore, this âtman alone is subtler than the subtle and greater than the great, because all names, forms and actions are only conditions imposed upon it). This âtman is seated, as the âtman, in the heart of every living creature, irons Brahma down to the worm. That âtman to whose realisation, hearing, thought and meditation are indicated as aids; one free from desire, i.e., one whose intelligence has been diverted from all external objects, either of this world or of the world to come (when he is so,—the mind and the senses which are called Dhâtus.—because they suppprt the body, become composed); sees, i.e., directly realises, in the form ‘I am he’, the glory of the âtman, devoid of increase or diminution due to Karma; and, therefore, he becomes absolved from grief.

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