Katha-1-3-15-aśabdam asparśam

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

How the object to be known is very subtle is explained. This earth produced by sound, touch, form, taste and scent, and the object of all the senses, is gross; so is the body. Here, by the elimination one by one, of these gunâs, i.e., attributes from earth and the rest, difference in respect of subtlety, greatness, purity and durability, has been found in the element, from water upwards to the âkâs. Therefore the sruti shows that little need be said of the unsurpassable subtlety, etc., of that in which smell and the rest up to sound inclusive, mere modifications being gross, do not exist; which is soundless, touchless, formless, undecaying, so tasteless eternal and scentless, Brahman thus explained is undecaying; for, what has sound, etc., decays.

But this having no sound, etc., does not decay or suffer diminution; therefore also, it is eternal; for what decays is ephemeral; but this does not decay. Therefore, it is eternal; and being eternal, it is beginningless; i.e., has no cause; what has a beginning, that being an effect, is not eternal and is absorbed into its cause as earth, etc. But this being the cause of all is not an effect and not being an effect, it is eternal. It has no cause into which it could be absorbed; similarly endless, i.e., that which has no end or anything to be done by it. As the ephemeral nature of plantains, etc., is seen, by the fact of their yielding fruit and other results; not even thus, is it seen that Brahman, has an end; therefore also eternal. Beyond the mahat, distinct in nature from the principle known as mahat called intelligence, for it is the witness of all, eternal knowledge being its nature, and Brahman being the âtman of all things. For, it has been already said ‘This âtman concealed in all living beings, etc.’ Constant, i.e., changeless and eternal. Its eternal nature is not relative like that of the earth, etc. Having realised Brahman thus described as the âtman, one releases the âtman from the mouth of Death, i.e., from what is incidental to Death, i.e., from ignorance, desire and karma.

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