Katha-1-2-21-āsīno dūraṃ

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

Otherwise, this âtman cannot be known by worldly men having desires, because sitting, i.e., not moving, he goes a great distance. Lying, he goes everywhere. Thus the âtman is both joyful and joyless. Thus he has properties mutually opposed; therefore it being impossible to know him, who else but me can know the âtman, who is joyful and joyless. It is only by persons like us of subtle intellect and learning that the âtman can be known. Being conditioned by conflicting attributes of fixity and movement, and of constancy and change, the âtman appears as if itself possessed conflicting attributes like Visvarupa, a sum of various forms, or (more properly) like Chintâmani (a gem which appears according to the fancy of the seer). Therefore, Death indicates the difficulty of knowing the âtman by the statement ‘who else but me can know the âtman.’ The cessation of the activity of the senses is ‘lying;’ in the person lying, there is a cessation of the partial knowledge produced by the senses. In this state the âtman seems to go everywhere, because its knowledge then is of a general character, i.e., unqualified by conditions; but, though fixed in its own nature, when it has special or qualified knowledge it seems to go a great distance, because it is conditioned by the motion of the mind and the rest, but really he is here alone, i.e., in this body.

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