Namaskaar. We move on to the penultimate part 34 of the Kathopanishat Commentary series of 35 parts by Shri V. Subrahmanian on website of Advaita Aacademy
In the previous mantra (2.3.11) it was said that brahman/Self is to be realized by subduing the senses and concentrate on the Pure Consciousness that is bereft of any modification of the mind/intellect. A question would arise, then, that it would be possible to apprehend brahman if It were an object of the activity of the instruments such as the mind. Further, in the resolving of the mind-activity, there will be no apprehending of brahman and therefore one concludes that brahman is verily non-existent. For, in the world that which is available to the instruments (of knowledge, etc.) alone is admitted to be existent and the contrary is non-existent. Therefore, the method, yoga, prescribed to apprehend brahman is of no use. Also, since brahman is not available/apprehensible, It is to be known as non-existent. In the wake of such a contingency, the upaniShad clarifies:
नैव वाचा न मनसा प्राप्तुं शक्यो न चक्षुषा ।
अस्तीति ब्रुवतोऽन्यत्र कथं तदुपलभ्यते ॥ १२ ॥
न एव never वाचा by speech न मनसा nor by the mind प्राप्तुं शक्यः possible to attain/obtain न चक्षुषा not by eye (senses) अस्ति ‘It is’ इति thus ब्रुवतोऽन्यत्र other than who says कथं how तत् That उपलभ्यते is attained?