... Which is the soul? The person here ... who is the light in the heart...
This person (purusha) here in the heart is made of mind, is of the nature of light, is like a little grain of rice, is a grain of barley. This very one is ruler of everything, is lord of everything, governs this whole universe, whatsoever there is.
(p. 68, quoting from the Brihad Aranyaka Upanishad)