In Hinduism, man is the microcosm who contains within himself all the constituents and manifestations of the Universal Self. It is by looking into oneself, by knowing and understanding oneself, one can experience the Truth of the Supreme Self. In the Katha Upanishad, Lord Yama declares to Nachiketa that it is through self-contemplation (adhyatma-yogadhigamena) that the wise man realizes the Primal God and leaves behind him both joy and sorrow (mortal existence).
The mysteries of creation and the mysteries of the Universal Existence of Supreme Self can be known only by knowing the mysteries of ones own creation and ones own existence. All the divinities that exists in the universe have their corresponding divinities in the human personality.
The creation and evolution of man is same as the creation and evolution of the entire universe. It is based upon this concept and analogy that the Upanishads unfold to us the grand vision of the Universal Self and the secrets of creation.
The Virat ( manifest world) is the waking consciousness, the visible reality, the Vaishwanara mentioned in the Mandukya Upanishad. Hiranyagarbha (the World Spirit), the creator of this world and the various forms in it, is the dream state, the Taijasa, who has all the forms and ideas already existing in him and who manifests reality through his creative ability.
Iswara, the Creative Spirit, the Saguna Brahman, the first Being manifested by the Non-Being, "in the beginning when nothing existed", is the deep sleep state called pragna. Finally, at the apex, is Brahman, the Absolute, the One without attributes, the state that is beyond sleep, who is Atman , the immortal and transcendental aspect hidden in each of us.
According to Hindu scriptures at the center of man is Atman and encircling it are five different sheaths or bodies. First is the physical body, called variously as the gross body, sthula sarira or annamaya kosa. It is made of food or earth and contains the senses and the organs of action. From food verily are produced all creatures of earth. Food verily is the eldest born of beings (annam hi bhutanam jyeshtham). Food is eaten and eats things. (Taittiriya II.2.1).
The second is the vital body or the breath body, called pranamaya kosa. Air is the food for this body. Breath is the life of beings (prano hi bhutanam ayuh). It is called sarira atma (the embodies soul of the gross body). It is part of the subtle body, sukshma sarira in contrast to the gross body. The autonomous nervous system is under its control.
While a person can easily control the movements of his gross body, the same is not possible in case of his breath body unless he gains mastery over the movements of his breath. (It is interesting to note that the aim of hatha yoga is basically to establish this control over the movements of breath and achieve mastery over the breath body.)
The third is the mental body, called manomaya kosa which uses the five senses and the five organs of action (speech, hands, feet, excretory organs and sexual organs). The breath body and the mental body together constitute the subtle body, or sukhsma sareera. Thoughts are its food.
The fourth is the intelligence body, called vignanamayakosa or buddhi. It is the reasoning aspect of man, the discriminatory, regulatory, selecting and directing awareness in us, which provides direction to our activities and shapes our destinies and our very existence. It is also called the casual body, because it is the cause of an individual's karma. It directs the sacrifice as well as the deeds ( vignanam yagna tanute, karman tanute). The gods (senses) worship buddhi is the eldest Brahman (brahma jyeshtham). Sometimes buddhi is also described as a constituent of subtle body. But these distinctions do not effect our understanding of the different sheaths.
The fifth is the bliss body, called anandamaya kosa, which is transcendental and beyond ordinary human experience. Very few individuals are capable of knowing it or experiencing it, as it is beyond the sensory and mental fields. It is only through restraining of the senses, the mind and the buddhi one can gain access to it. It is the very essence (rasa or ether) of our existence for who can live in this world unless there is bliss in the space? (Taittiriya II.7.1). We further learn from the same Upanishad that the Non-Being who was alone in the beginning produced the Being who made itself a soul which was the Bliss Body called the well made.
The sixth is the Atman, the eternal soul, the real self, the very Brahman in Its pure microcosmic state. It is the First Being , the unchanging, imperishable self in man. It is beyond the senses,beyond all conscious human experience. It is also called Purusha. It is the Truth Body. Words return from it not attaining it along with the mind. He who attains it becomes freed from fear. He is not perplexed or tormented by conflicting thoughts. His mind becomes tranquil.
It may be noted that while Shri Sankaracharya considered the bliss body and Atman to be different, Shri Ramanuja regarded the bliss body and Atman together as the one pure transcendental state.
The first four sheaths, namely, the gross body, the two subtle bodies and the casual body (which is sometimes grouped together with the life and mental bodies as casual body), constitute the Jiva. Jiva is the living element, the product of Prakriti who comes under the influence of illusion and develops ahamkara or the ego consciousness which gives rise to feelings of separation and alienation from the rest of the creation and failure to perceive the omnipresence of God.
It is the Jiva which together with Atman goes through the chain of repeated births and deaths. While Atman is impervious to change and suffering during this process of evolution, the Jiva remains at the center of desire oriented sensory activity and suffers from its consequences. At the time of death it leaves behind the gross body and goes to the other worlds with his subtle and casual bodies, where after exhausting its karma it returns again to take birth in this world in accordance with its previous samskaras or residual memories of its past lives.
The Taittiriya and Katha Upanishads are important sources of information for our understanding of the constitution of man. In the Katha Upanishad says Lord Yama, " Beyond the senses are the objects and beyond the objects is the mind. Beyond the mind is buddhi and beyond buddhi is the great self (mahan atma)".
In the Taittiriya Upanishad we come across the description of the various stages of Brahman starting with matter and life (II.2.1), life and mind (II. 2.1), mind and buddhi (II. 2.1), buddhi and bliss (II.2.1) and Brahman (II.2.1). The Taittiriya Upanishad ends with a mystical chant which is a joyous expression of a liberated soul who has realized the true nature of his self.
In an outburst of pure joy he declares to the world:
I am food, I am food, I am food.
I am the eater of food, I am the eater of food, I am the eater of food.
I am the combining agent, I am the combining agent, I am the combining agent
I am the first born of the world order, before the gods, at the center of immortality.
I am food. I eat the eater of food.
I am the whole world.
I have conquered the whole world.
I am the resplendent golden light.