Katha-1-3-13-yacchedvāṅmanasī

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

The text states the means of attaining that Yatchêt, draw into. Prâjnah, one having discernment. What? Vâky, i.e., speech. The word ‘vâk,’ i.e, speech, is illustratively used to denote all the senses. Where? In the mind. The lengthening of the vowel in manasî is a Vedic license; and that mind, let him sink into gnâna, i.e., intellect bright by nature; ‘intellect’ is called âtman here; for, it pervades the mind and the other senses; therefore, it is their ‘Pratyagâtman,’ i.e., internal principle; let him sink the intellect into the âtman; ‘great,’ i.e., first born Hiranyagarbha. The meaning is, let him make his intelligence as clear in its nature as the first born; let him sink that great âtman also into the peaceful âtman, i.e., into the primary âtman whose nature does not admit of any conditions, which is unmodified, which is within all and which is the witness of all the modifications of the intellect.

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