आत्मानँ रथितं विद्धि शरीरँ रथमेव तु ।
बुद्धिं तु सारथिं विद्धि मनः प्रग्रहमेव च ॥ ३ ॥

ātmānam̐ rathitaṃ viddhi śarīram̐ rathameva tu |
buddhiṃ tu sārathiṃ viddhi manaḥ pragrahameva ca || 3 ||

3. Know the atman as the lord of the chariot, the body as only the chariot, know also intelligence as the driver; know the minds as the reins.

Shankara’s Commentary:

Here a chariot is imagined for the atman, conditioned in Samsara, entitled to acquire knowledge and perform Karma for attaining emancipation and for travelling in Samsara, as a means to reach both. Know the atman, who is the enjoyer of the fruits of Karma and is in the bondage of Samsara, to be the lord of the chariot. Know the body to be verily the chariot, because like a chariot the body is drawn by the senses occupying the place of horses. Know also the intelligence to be the driver, furnished with the capacity for determination, because the body is mainly guided by the intelligence, as the chariot is mainly guided by the driver; for, everything done by the body is generally done by the intelligence. Know the mind with its characteristics of volition, doubt, etc., to be the reins; for, the senses, such as the ear, perform their functions when grasped by the mind as horses by the reins.