Appearing in one of the earliest Upanishads is the important and influential mystical concept is five selves. The Taittiriya Upanishad is exceptional in that it is one of the first writings to present a systemmatic metaphysic or theory of first principles. It speaks of the individual as divided into five selves (atma, initially - as with all such terms - "breath", and then "self" or "soul"; the term elsewhere, and especially later, came to characterise that aspect of the self which is synonymous with the Absolute).
Five levels of self are referred to:
The spiritual aspirant, in the quest for Self- and God-realisation, passes under the guidance of the Master through each of these selves in turn, finally attaining to the Absolute or Brahman, which is synonymous with the highest or Bliss Self.
There is a strong resemblance here to the Hellenistic Hermetic and Neoplatonic scheme of more than a millenia later. According to this, the gross physical body is only the outermost of a number of "bodies" or "vehicles" - ochema - to use the Neoplatonic term. Death means the discarding, first of the physical, than of the successive subtle vehicles, until only the immortal Spirit, the Nous, remains. As expressed in the Corpus Hermetica:
"The mind (nous) has for its vehicle the soul (psyche); the soul has for its vehicle the vital spirit (pnuema); and the vital spirit, traversing the arteries with the blood, moves the body....
(W)hen men quit the body...(t)he soul ascends to its own place, and is separated from the vital spirit; and the mind is separated from the soul. Thus the mind, which is divine by nature, is freed from its integuments..." [pp.195-9]
Comparing the Indian with the Hermetic scheme, it is obvious that the Food self is more or less the same as the Soma (Physical body), the Life-force self to the Vital Spirit (Pneuma), the Mental self to the Psyche, and the Consciousness self to the Nous, the Divine mind or spirit principle.
There is however little or no similarity with the Egyptian, Chinese, and classical Kabbalistic conceptions, all of which spoke in terms of many souls, rather than a single gradational continuum from material body to divine spirit. (Later Kabbalisticwriters on the other hand did posit a five-fold continuum (Nefesh-Ruah-Neshamah-Hayyah-Yehidah) more or less analogous to the Taittiriya and Hellenistic Hermetic versions.
The Taittiriya Upanishad presents a very world-affirming philosophy, because each level of self is described in a positive way, and Brahman itself is referred to emphatically as the nature of Bliss (Ananda). Thus, one begins with Life (or "food", referring perhaps to the ecological web) and attains to Bliss.
In the world-affirming cosmology of the Taittiriya Upanishad, in which each level of self is described in a positive way, and the highest self-level, the Self made of Bliss, is equated with Brahman, the Absolute. Brahman is therefore also of the nature of Bliss (Ananda). Fifteen hundred years after the Taittiriya Upanishad was composed (i.e. around the seventh and eighth centuries), the sages Guadapada and his prolific sucessor Shankara, who saw the cosmos as ultimately illusory (or Maya) and only the Absolute as real. In the teachings of Gaudapada and Shankara, the world-affirming element of the original Taittiriya Upanishad was denied, and the five selves (atma) become five koshas or "sheaths" which veil the light of the True transcendent self or Atman. The Taittirya Upanishad's terminology was retained, but the concepts used were, for the most part, rather different. This of course is always the case: words change much more slowly than the ideas they are used to represent. Thus a polemicist can use an ancient and respected authority to mean something totally different, and at times even totally at variance to, the meaning intended by the original writer (the classic example being the contemporary Christian fundamentalist's use (or misuse) of the Hebraic religious corpus).
Rather clumsily, the koshas are also identified with the four states of consciousness of the Mandukya Upanishad. So for example the Bliss kosha is equated with dreamless sleep, but how can dreamless sleep be blissful (except in the negative sense of freedom from pain)?
This Advaitin interpretation became very popular, and is still taught by practically all the gurus from the East. It also had a strong influence on Theosophy. There, the Food kosha became the Gross Physical body, the Life-force kosha the subtle double or Etheric body, the Mental kosha the Astral/Emotional and Mental principles, and the higher koshas and the Atman the higher self and transcendent spirit.
4 Levels and 3 Domains of Consciousness by Swami Jnaneshvara Bharati
5 Sheaths or Koshas by Swami Jnaneshvara Bharati