Muṇḍaka Upaniṣad · 3.1.5

यस्त्वविज्ञानवान् भवत्ययुक्तेन मनसा सदा । तस्येन्द्रियाण्यवश्यानि दुष्टास्व इव सारथेः

yas tv avijñānavān bhavaty ayuktena manasā sadā | tasya indriyāṇy avaśyāni duṣṭāśva iva sāratheḥ

But whoever is devoid of discrimination, with mind ever uncontrolled — his senses are ungovernable, as bad horses of the charioteer.

The situation of the undiscriminating: chaos and suffering.

Avijñānavān — devoid of discrimination. Without viveka, the capacity to distinguish the real from the unreal.

Ayuktena manasā — with uncontrolled, un-yoked mind. Without discipline, without union with the higher.

Sadā — always, constantly. The habitual state of the undiscriminating.

Avaśyāni — ungovernable, untamable. The senses out of control, following their own impulses.

Duṣṭāśva iva — as bad horses. Vicious steeds that do not obey, pulling in multiple directions.

The metaphor is vivid: an incapable charioteer, rebellious horses, runaway chariot, dangerous journey. Such is the life of one who does not practice viveka and saṃyama (control).

Suffering inevitably arises when the senses pursue their objects without conscious direction. Yoga offers the alternative: strengthen the charioteer, train the horses.