Katha-1-3-14-uttiṣṭhata

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

Having thus merged into the purusha, the âtman, all the three, i.e., name, form and karma which are produced by false knowledge and are of the nature of action, agents and fruits, by a knowledge of the true nature of his âtman, as the water in the mirage, the serpent in the rope and the colour of the sky, disappear by seeing the true nature of the mirage, rope and the sky, one becomes free from anxiety and calm, his purpose accomplished. Therefore to know that, arise, Oh, living beings sleeping in beginningless ignorance, i.e., turn towards the acquisition of the knowledge of the âtman; and awake, i.e., put an end to the sleep of ignorance, horrible in form and the seed of all misery. How? Having approached excellent preceptors who know that, realise the âtman taught by them, the innermost and in all, thus ‘I am he.’ This is not to be neglected. Thus, the sruti, like a mother, says from compassion; because the object to be known can be realised only by very subtle intelligence. Why is it stated ‘by subtle in-tellect’’? The edge of a razor is pointed, i.e., made sharp and impassable, i.e., passable with difficulty; as that cannot be walked over by the feet, similarly hard to attain, the intelligent say, is the road of the knowledge of truth. The meaning is that because the object to be known is very subtle, they say the road of knowledge leading to that is not easily attainable.

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