Metaphor of the chariot

After introducing jivātmā and paramātmā as shadow and light, and two paths namely preyas and śreyas which were passed down by the ritualist and the knower of brahman respectively; now in the next two mantra, jīvātmā is described as the traveler of the chosen path, by using the metaphor of the chariot for easy understanding.

ātmānam rathinam viddhi śarīram rathameva tu |

buddhim tu sārathim viddhi manaḥ pragrahameva ca ||1.3.3||

May you know the jīvātmā to be the master of the chariot (and) the physical body to be the chariot. May you know the intellect to be the charioteer and the mind to be the reins.

ātmānam rathinam viddhi - may you know ātmā to be the master of the chariot. Ātmā here is jīvātmā - individual (not paramātmā), because jivātmā is the one who identifies with body-mind-senses, just like the master of the chariot owned up the chariot. The body-mind-senses are visualised as components of the chariot. While jīvātmā is the master of the chariot, he doesn’t physically drive the chariot; his intellect which characterised by decision making is the charioteer - buddhim tu sārathim. Intellect is the main factor of the body that decides the activities that are taken up by the body, just like the charioteer is the main factor that decides which road to take for a chariot. śarīram rathameva tu - and the body is the chariot, because the body is dragged by the sense organs which are compared to the chariot dragged by the horses. manaḥ pragrahameva ca - and the mind as the reins, which regulate the sense organs, just like the reins controlling the horses.