"Now, that golden Person who is within the sun, who looks down upon this earth from his golden palace, is even He who dwells within the lotus of the heart and eats food." --Maitri Upanishad, VI. 1
The Maitri Upanishad is the last of what are known as the principal Upanishads. It recommends meditation upon the soul ("atman") and life ("prana"). It says that the body is like a chariot without intelligence but it is driven by an intelligent being, who is pure, tranquil, breathless, selfless, undying, unborn, steadfast, independent and endless. The charioteer is the mind, the reins are the five organs of perception, the horses are the organs of action, and the soul is unmanifest, imperceptible, incomprehensible, selfless, steadfast, stainless and self-abiding. It also tells the story of a king, Brihadratha, who realized that his body is not eternal, and went into the forest to practice austerity, and sought liberation from reincarnating existence.