यथोदकं शुद्धे शुद्धमासिक्तं तादृगेव भवति । एवं मुनेर्विजानत आत्मा भवति गौतम ॥ १५॥ इति काठकोपनिषदि द्वितीयाध्याये प्रथमा वल्ली ॥

ॐ

English Translation

As pure water poured into pure water becomes one with it, so also, O Gautama, does the Self of the sage who knows. 

Sri Shankaracharya

ॐ

English Commentary

But how becomes the âtman of a thinker who has acquired knowledge, whose perception of difference due to conditions has been destroyed, and who knows the âtman pure, dense with knowledge unalloyed, and one without a second, is explained. As water pure poured into pure becomes of the same quality, not otherwise, the âtman also of the thinker becomes the same, Oh Gautama. Therefore, leaving the perception of difference induced by bad logic and the erroneous notion of no-here-after, the perception of the one-ness of the âtman inculcated by the Vêdâs, a better well-wisher than thousands of mothers and fathers should be anxiously respected by those whose pride has been quelled.

Sri Shankaracharya