Kaṭha Upaniṣad · 2.3.10
यदा पञ्चावतिष्ठन्ते ज्ञानानि मनसा सह । बुद्धिश्च न विचेष्टते तामाहुः परमां गतिम् ॥ १० ॥
yadā pañcāvatiṣṭhante jñānāni manasā saha | buddhiśca na viceṣṭate tāmāhuḥ paramāṃ gatim || 10 ||
When the five instruments of knowledge stand still together with the mind, and the intellect does not stir, that they call the supreme state.
The pañca jñānāni (five instruments of knowledge—the senses) together with the manas (mind) must avatiṣṭhante (stand still, become quiet). The buddhi (intellect) must stop viceṣṭate (stirring, moving from side to side). Only then is the paramā gatiḥ (supreme state, final goal) revealed.
This description corresponds to nirvikalpa samādhi, the state of formless meditation where all mental movement has ceased. It is not an unconscious state; it is more conscious than any ordinary state, but consciousness is free from objects and fluctuations.
The yogi practices this gradually: first pratyāhāra (withdrawal of the senses), then dhāraṇā (concentration), dhyāna (meditation), until the buddhi becomes completely still. In that silence, the Ātman knows itself—not as something known by another, but as knowledge itself.