Katha-2-1-11-manasaivedamāptavyaṃ

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

Before attaining the knowledge of the oneness, by the mind purified by the sâstrâs and by the preceptor, this unalloyed essence of the Brahman should be attained in the form ‘There is the Brahman alone, nothing else exists.’ When attained, ignorance, the cause of the perception of difference being removed, there is not even the slightest difference here, i.e., in the Brahman. But he who does not give up……*

* NOTE: The following page is missing from the book

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