The Kaha Upanishad is one of the older, "primary" Upanishads commented upon by Shankara. It is associated with the Taittiriya school of the Black Yajurveda. It figures as number 3 in the Muktika canon of 108 Upanishads.
Know the Self as Lord of the chariot, the body as the chariot itself, the discriminating intellect as the charioteer, and the mind as the reins. The senses, say the wise, are the horses; selfish desires are the roads they travel.
The self-existent Lord pierced the senses to turn outward. Thus we look to the world outside and see not the Self within us. A sage withdrew his senses from the world of change and, seeking immortality, looked within and beheld the deathless self.