In the secret cave of the heart, two are seated by life’s fountain. The separate ego drinks of the sweet and bitter stuff, liking the sweet, disliking the bitter, while the supreme Self drinks sweet and bitter, neither liking this nor disliking that. The ego gropes in darkness, while the Self lives in light. (I.3.1)
When the five senses are stilled, when the mind is stilled, when the intellect is stilled, that is called the highest state by the wise. They say yoga is this complete stillness in which one enters the unitive state, never to become separate again. (II.3.10-11)
Know thou the soul as riding in a chariot, the body as the chariot. Know thou the intellect as the chariot-driver, and the mind as the reins. The senses are the horses; the objects of sense, what they range over. The self combined with senses and mind wise men call “the enjoyer”.
He who has not understanding, whose mind is not constantly held firm – his senses are uncontrolled, like the vicious horses of a chariot-driver. He, however, who has understanding, whose mind is constantly held firm – his senses are under control, like the good horses of a chariot-driver.
(Kaṭha Upaniṣad, tr. Olivelle)