Chandogya Upanishad:

 There is a beautiful expression of this in the Chandogya Upanishad: 'There is this City of Brahman, (that is the body), and in this city there is a shrine, and in that shrine there is a small lotus, and in that lotus there is a small space, (akasa). Now what exists within that small space, that is to be sought, that is to be understood.' This is the great discovery of the Upanishads, this inner shrine, this guha, or cave of the heart, where the inner meaning of life, of all human existence, is to be found. — Bede Griffiths