Kaṭha Upaniṣad · 1.3.8
य इमं परमं गुह्यं शास्त्रमुक्तं पुरातनम् । एतद्विदित्वा मेधावी सुखी भवति नान्यथा ॥ ८॥
ya imam paramaṃ guhyaṃ śāstram uktaṃ purātanam | etad viditvā medhāvī sukhī bhavati nānyathā
Whoever knows this supreme and secret śāstra, ancient as time — knowing it, the sage becomes blissful, not otherwise.
The Kaṭha Upaniṣad presents itself as paramaṃ guhyaṃ (supremely secret) and purātana (ancient). True happiness (sukha) does not come from external sources, but from viditvā (direct knowledge) of this teaching. The phrase “not otherwise” (nānyathā) is emphatic: fullness is only possible through self-knowledge. For the practitioner of yoga, this is an invitation to deep introspection as the only path to lasting happiness.