Katha-2-2-11-sūryo yathā

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

Seeing that if one be the âtman of all, he may be regarded as subject to the grief of samsâra, this is said. As the sun benefiting the eye by its rays and manifesting even unclean things such as urine, ordure, etc., though being the eye of all, the world seeing them is not tainted with sins and other blemishes due to seeing unclean things, etc., and by stains caused by physical contact with unclean objects so the one internal âtman is not tainted with the misery of the world, being outside that; for, the world by ignorance superposed on the âtman, suffers misery arising from desire and karma. But that is not really in the âtman, just as, the serpent, silver, water and dirt superposed on the rope, mother-of-pearl, barren spot and the sky, respectively, do not really exist as blemishes in the rope, etc.; from the superposition, by false notion, they are perceived as blemishes in the true objects connected (by the notion); the true objects are not tainted by such blemishes because they are external to the false notion so superposed. Thus the world having superposed on the âtman, the false notion of deed, agency and fruits like the notion of the serpent (on the rope), suffers the misery of birth, death, etc., due to that. But the âtman, though the âtman of all the world, is not tainted by the misery of the world arising from the superposition of a false notion. Why? being external. Because he is like the rope, etc., external to the false notion superposed on him.

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