Text:

यस्त्वविज्ञानवान्भवत्ययुक्तेन मनसा सदा ।
तस्येन्द्रियाण्यवश्यानि दुष्टाश्वा इव सारथेः ॥ ५ ॥

yastvavijñānavānbhavatyayuktena manasā sadā |
tasyendriyāṇyavaśyāni duṣṭāśvā iva sāratheḥ || 5 ||

5. But of him who is not possessed of discrimination, and whose mind is always uncontrolled, the senses are not controllable as vicious horses of a driver.

Shankara’s Commentary:

This being so, of that driver known as Buddhi, who is not capable of discrimination as to what is to be done or omitted to be done, as the other driver in the guiding of his chariot has a mind like the reins of a chariot not well-grasped by the driver, i.e., uncontrolled by the intellect; of that incompetent driver, i.e., intellect, the senses which stand in the place of the horses become unruly and uncontrollable, as the vicious horses of the other driver.