Katha-2-2-13-nityo'nityānāṃ

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

Again, deathless among mortal things, conscious among the conscious, such as Brahma and other living beings. As the power of burning in water and the rest, which are not fire in themselves, is due to fire, so, the intelligence of others is due to the intelligence of the âtman; again, he, omniscient and lord over all, dispenses to those having desire, i.e., to those in samsâra, according to their respective karma, the fruits of karma and desired objects, according to his grace, himself one, to many, without effort. To such intelligent men as see him seated in their selves, eternal peace accrues, not to others, i.e., to those who do not see so.

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