There are five steps of the ladder of spiritual evolution, described in the Upanishads. These five, steps are as follows:
1. According to the Brihadaranyaka Upanishad, in the first stage the enquirer, thinking himself separate from others, feels it inside him, through mystic intuition.
2. In the second stage, the enquirer experiences that he is really the very self and that he is neither physical nor vital nor intellectual nor emotional, but that he is in his essential nature entirely identical with the pure self. According to Brihadaranyaka Upanishad the being, which calls itself ‘I’ within us, must be identified with the self.
3. In the third stage, the enquirer realizes that the self which he has realized is identical with Brahman. The Brihadaranyaka Upanishad, along with Isa and its cognate Upanishads, has explained the identification of Atman and Brahman, of the individual spirit and the universal spirit, of the self and the Absolute.
Thus in the third stage, the enquirer sees no difference between the self and the Absolute. In the Isa Upanishad, it has been proclaimed that it must be regarded as verily Brahman, that the Atman is infinite in its nature, that the Atman derives its being from Brahman and that if we subtract the infinite of the Atman from the infinite of the Brahman, the residuum is infinite.
4. In the fourth stage, the enquirer realizes that since he is the self and the self is the Absolute, it follows syllogistically that he is the Absolute. In the Brihadaranyaka Upanishad, it has been said that one must identify the ‘I’ with the Absolute. In the Chandogya Upanishad the same doctrine is proclaimed where the self comes also to be ‘projectively’ identified with the Absolute.
5. In the fifth stage, it is realized that everything that we see in this world, man and nature, self and the not-self, equally constitute the Absolute. The Brahman, according to Chandogya Upanishad, is verily the ‘ALL’. This is the position of Absolute monism. The realization of this Absolute is not intellectual but mystic.