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.
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.