Katha Upanishad Verse 2.2.14
The realization of the Self is synonymous with attaining the “indescribable highest bliss.” It is beyond common understanding, yet it can be known as the foundation of all awareness.
Sanskrit Verse
तदेतदिति मन्यन्तेऽनिर्देश्यं परमं सुखम् ।
कथं नु तद्विजानीयां किमु भाति विभाति वा ॥ १४ ॥
Transliteration
tadetaditi manyante’nirdeśyaṃ paramaṃ sukham |
kathaṃ nu tadvijānīyāṃ kimu bhāti vibhāti vā || 14 ||
Translation
They think of that as this—the indescribable highest bliss. How shall I know that, whether that which shines is seen clearly or not.

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