The relativity oL divinity is the discovery of the Upanishads.  And the relativity of happiness hereafter is the key-note of their religious philosophy.  Pious men are of three classes, according to the completed system.  Some are good men, but they do not know enough to appreciate, intellectually or spiritually, the highest.  Let this class meditate on the Vedas.  They desire wealth, not freedom.  The second class wish, indeed, to emancipate themselves; but to do so step by step; not to reach absolute brahma, but to live in bliss hereafter.  Let these worship the Spirit as physical life.  They will attain to the bliss of the realm of light, the realm of the personal creator.  But the highest class, they that wish to emancipate themselves at once, know that physical life is but a form of spiritual life; that the personal creator is but a form of the Spirit; that the Spirit is absolute brahma; and that in reaching this they attain to immortality.  These, then, are to meditate on spirit as the highest Spirit, that is, the absolute.  To fear heaven as much as hell, to know that knowledge is, after all, the key to brahma; that brahma is knowledge; this is the way to emancipation.  The gods are; but they are forms of the ego, and their heaven is mortal.  It is false to deny the gods.  Indra and the Father-god exist, just as men exist, as transient forms of brahma.  Therefore, according to the weakness or strength of a man’s mind and heart (desire) is he fitted to ignore gods and sacrifice.  To obtain brahma his desires must be weak, his knowledge strong; but sacrifice is not to be put away as useless.  The disciplinary teaching of the sacrifice is a necessary preparation for highest wisdom.  It is here that the Upanishads, which otherwise are to a great extent on the highway to Buddhism, practically contrast with it.  Buddhism ignores the sacrifice and the stadia in a priest’s life.  The Upanishads retain them, but only to throw them over at the end when one has learned not to need them.  Philosophically there is no place for the ritual in the Upanishad doctrine; but their teachers stood too much under the dominion of the Br[=a]hmanas to ignore the ritual.  They kept it as a means of perfecting the knowledge of what was essential.

So ‘by wisdom’ it is said ‘one gets immortality.’  The Spirit develops gradually in man; by means of the mortal he desires the immortal; whereas other animals have only hunger and thirst as a kind of understanding, and they are reborn according to their knowledge as beasts again.  Such is the teaching of another of the Upanishads, the [=A]itareya [=A]ranyaka.