Katha-2-3-13-astītyevopalabdha

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

Therefore, having abandoned the theory of those who argue for non-existence, the âtman should be known as existing, as productive of effects and conditioned by intelligence. But when the âtman is devoid of that and subject to no modification (an effect has no existence independent of the cause); as the sruti says ‘a modification is a mere matter of speech and name; that it is mud is alone true,’ then, is the true nature of the âtman unconditioned, devoid of indicative marks, and incapable of being thought of, as existent or non-existent. In that nature also, ‘the âtman should be known’ follows. Of these two] of the conditioned and the unconditioned, i.e., known as existence and its true nature; the genitive case has the froce of Nirdhârana, i.e., determining; of the âtman previously known as merely existent] of the âtman known by the belief in its existence produced by its limitations, i.e., its perceived effects. Afterwards, the real nature of the âtman subject to no condition, different from both the known and the unknown, i.e., the manifested universe and the prakriti, one without a second, and indicated by the srutis ‘ not this, not that, etc.,’ ‘not gross, not subtle, not short ‘in the invisible, bodiless, supportless, etc.,’ faces him who had previously realised it as existent.

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