Katha-2-2-9-agniryathaiko

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

Com.—As the knowledge of the oneness of the âtman, though supported by authority and often reiterated, is not firmly grasped by the intellect of perverse-headed Brahmins, whose mind is shaken by the intellect (arguments) of many logicians, the sruti, being anxious to inculcate it, says again and again: as fire, though one, bright by nature, having entered the world (Bhuvanam meaning world, because all bhavanti, i.e., are born there) in respect of everything to be burnt assumes separate shapes; so, being one only, the internal âtrnan of all, having entered all bodies such as fire in logs, etc., being extremely subtle, assumes forms responsive and is also without them all, like the âkâs, in his own unmodified nature.

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