Katha-1-2-5-avidyāyāmantare

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

Com.—But those men living in Samsâra in the midst of ignorance as in thick darkness, entangled in a hundred meshes formed by attachment for sons, cattle and the rest, regarding themselves as intelligent and well-versed in the Sâstras, go round and round, get into many crooked ways afflicted with miseries such as old age, death, disease, etc., devoid of discernment, just as the blind led by the blind in uneven paths come to great grief.

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