Katha-2-3-6-indriyāṇāṃ

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

How is he to be known and what avails it to know him are explained. Of the senses, such as the ear, etc., separately originating from their causes, the âkâsa, etc., for perceiving their respective objects, knowing their distinctness, i.e., dissimilarity of their nature to the nature of the âtman extremely pure, untainted, and all intelligence; and also the rising and setting, i.e., the creation and absorption of the senses, to depend on the waking and sleeping states and that the âtman has neither beginning nor end, the intelligent man does not grieve. The eternally identical nature of the âtman never changing, there can be no cause of grief. So also another sruti says ‘The knower of the âtman crosses grief.’

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